Article

The ALMA-PILS survey: Isotopic composition of oxygen-containing complex organic molecules toward IRAS 16293-2422B

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Abstract

Context. One of the important questions of astrochemistry is how complex organic molecules, including potential prebiotic species, are formed in the envelopes around embedded protostars. The abundances of minor isotopologues of a molecule, in particular the D- and ¹³ C-bearing variants, are sensitive to the densities, temperatures and timescales characteristic of the environment in which they form, and can therefore provide important constraints on the formation routes and conditions of individual species. Aims. The aim of this paper is to systematically survey the deuteration and the ¹³ C content of a variety of oxygen-bearing complex organic molecules on solar system scales toward the “B component” of the protostellar binary IRAS16293–2422. Methods. We have used the data from an unbiased molecular line survey of the protostellar binary IRAS16293−2422 between 329 and 363 GHz from the Atacama Large Millimeter/submillimeter Array (ALMA). The data probe scales of 60 AU (diameter) where most of the organic molecules are expected to have sublimated off dust grains and be present in the gas phase. The deuterated and ¹³ C isotopic species of ketene, acetaldehyde and formic acid, as well as deuterated ethanol, are detected unambiguously for the first time in the interstellar medium. These species are analysed together with the ¹³ C isotopic species of ethanol, dimethyl ether and methyl formate along with mono-deuterated methanol, dimethyl ether and methyl formate. Results. The complex organic molecules can be divided into two groups with one group, the simpler species, showing a D/H ratio of ≈2% and the other, the more complex species, D/H ratios of 4–8%. This division may reflect the formation time of each species in the ices before or during warm-up/infall of material through the protostellar envelope. No significant differences are seen in the deuteration of different functional groups for individual species, possibly a result of the short timescale for infall through the innermost warm regions where exchange reactions between different species may be taking place. The species show differences in excitation temperatures between 125 and 300 K. This likely reflects the binding energies of the individual species, in good agreement with what has previously been found for high-mass sources. For dimethyl ether, the ¹² C/ ¹³ C ratio is found to be lower by up to a factor of 2 compared to typical ISM values similar to what has previously been inferred for glycolaldehyde. Tentative identifications suggest that the same may apply for ¹³ C isotopologues of methyl formate and ethanol. If confirmed, this may be a clue to their formation at the late prestellar or early protostellar phases with an enhancement of the available ¹³ C relative to ¹² C related to small differences in binding energies for CO isotopologues or the impact of FUV irradiation by the central protostar. Conclusions. The results point to the importance of ice surface chemistry for the formation of these complex organic molecules at different stages in the evolution of embedded protostars and demonstrate the use of accurate isotope measurements for understanding the history of individual species.

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... we attribute this discrepancy in SVS 13 to the larger beam of those observations, probing scales of ∼100 au, as compared with our data that trace smaller scales of ∼30 au. On the other hand, methanol column densities of ∼10 19 cm −2 , similar to the value we obtain for SVS 13, have been reported in other hot corinos, even at larger scales of ∼60 au in IRAS 16293−2422B (Jørgensen et al. 2016(Jørgensen et al. , 2018 and ∼120 au in L483 (Jacobsen et al. 2019). ...
... Our value is lower than the value of 0.10 reported by Belloche et al. (2020b) in SVS 13, likely because the column density of methanol obtained by these authors is lower (see above). The CH 3 OCHO abundances relative to methanol reported for other hot corinos span two orders of magnitude (see Fig. 7 Our value, similar to the 0.026 value found in IRAS 16293−2422B by Jørgensen et al. (2016Jørgensen et al. ( , 2018, lies in the middle of the range. Additionally, from the CH 3 OH and CH 2 DOH column densities toward CB (Table 6) we obtain a deuteration fraction D/H = 0.05, similar to values found in other low-mass protostars (Jørgensen et al. 2018;Taquet et al. 2019). ...
... The CH 3 OCHO abundances relative to methanol reported for other hot corinos span two orders of magnitude (see Fig. 7 Our value, similar to the 0.026 value found in IRAS 16293−2422B by Jørgensen et al. (2016Jørgensen et al. ( , 2018, lies in the middle of the range. Additionally, from the CH 3 OH and CH 2 DOH column densities toward CB (Table 6) we obtain a deuteration fraction D/H = 0.05, similar to values found in other low-mass protostars (Jørgensen et al. 2018;Taquet et al. 2019). ...
Preprint
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We present VLA and ALMA observations of the close (0.3" = 90 au separation) protobinary system SVS 13. We detect two small circumstellar disks (radii $\sim$12 and $\sim$9 au in dust, and $\sim$30 au in gas) with masses of $\sim$0.004-0.009 $M_{sun}$ for VLA 4A (the western component) and $\sim$0.009-0.030 $M_{sun}$ for VLA 4B (the eastern component). A circumbinary disk with prominent spiral arms extending $\sim$500 au and a mass of $\sim$0.052 $M_{sun}$ appears to be in the earliest stages of formation. The dust emission is more compact and with a very high optical depth toward VLA 4B, while toward VLA 4A the dust column density is lower, allowing the detection of stronger molecular transitions. We infer rotational temperatures of $\sim$140 K, on scales of $\sim$30 au, across the whole source, and a rich chemistry. Molecular transitions typical of hot corinos are detected toward both protostars, being stronger toward VLA 4A, with several ethylene glycol transitions detected only toward this source. There are clear velocity gradients, that we interpret in terms of infall plus rotation of the circumbinary disk, and purely rotation of the circumstellar disk of VLA 4A. We measured orbital proper motions and determined a total stellar mass of 1 $M_{sun}$. From the molecular kinematics we infer the geometry and orientation of the system, and stellar masses of $\sim$0.26 $M_{sun}$ for VLA 4A and $\sim$0.60 $M_{sun}$ for VLA 4B.
... The situation has changed in the last years with the detection of D-enriched iCOMs (Coudert et al. 2013;Coutens et al. 2016;Jørgensen et al. 2018;Manigand et al. 2019), because, whatever is their formation route, iCOMs deuteration is not anymore directly connected to the enhanced (gaseous) H 2 D + /H + 3 abundance ratio but to the deuteration of their parent species. In this case, the question is whether the iCOM deuteration is directly inherited from their parent species without any alteration or whether the processes leading from the par-ent to the daughter species can induce an enrichment or a decrease in the deuteration degree. ...
... To derive the total abundance of deuterated ethanol (i.e. including the gauche rotamers), Jørgensen et al. (2018) suggested to apply a factor of 2.69 to the measured anti rotamers abundance (see also Manigand et al. 2020). This practically means that having (so far) only the abundance of the anti deuterated conformers, it is necessary to multiply by 2.69 to account for the entire abundance of monodeuterated ethanol. ...
... The new calculations reported in the present work allow us to test whether the observed deuteration of glycolaldehyde is compatible with its formation in the gas phase from ethanol, following the chain of reactions suggested by Skouteris et al. (2018) and schematically shown in Fig. 1. This is also possible thanks to the very sensitive ALMA observations towards the Solar-like protostar IRAS16293 B hot corino obtained by the PILS project , which detected various isotopomers of deuterated ethanol and glycolaldehyde (Jørgensen et al. , 2018. Table 2 summarises the measured D/H abundance ratios of the three deuterated ethanol isotopomers and the three isotopomers of deuterated glycolaldehyde that would originate from them according to the scheme illustrated in Fig. 2. Please note that the abundance of a-a and a-s CH 2 DCH 2 OH have been added because, even if they can be spectroscopically identified, they are chemically indistinguishable. ...
Preprint
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Despite the detection of numerous interstellar complex organic molecules (iCOMs) for decades, it is still a matter of debate whether they are synthesized in the gas-phase or on the icy surface of interstellar grains. In the past, molecular deuteration has been used to constrain the formation paths of small and abundant hydrogenated interstellar species. More recently, the deuteration degree of formamide, one of the most interesting iCOM, has also been explained in the hypothesis that it is formed by the gas-phase reaction NH$_2$ + H$_2$CO. In this article, we aim at using molecular deuteration to constrain the formation of another iCOM, glycolaldehyde, which is an important prebiotic species. More specifically, we have performed dedicated electronic structure and kinetic calculations to establish the glycolaldehyde deuteration degree in relation to that of ethanol, which is its possible parent species according to the suggestion of Skouteris et al. (2018). We found that the abundance ratio of the species containing one D-atom over the all-protium counterpart depends on the produced D isotopomer and varies from 0.9 to 0.5. These theoretical predictions compare extremely well with the monodeuterated isotopomers of glycolaldehyde and that of ethanol measured towards the Solar-like protostar IRAS 16293-2422, supporting the hypothesis that glycolaldehyde could be produced in the gas-phase for this source. In addition, the present work confirms that the deuterium fractionation of iCOMs cannot be simply anticipated based on the deuterium fractionation of the parent species but necessitates a specific study, as already shown for the case of formamide.
... Numerous detections of isotopologs containing 13 C were reported in recent years [8,11,17,18,12,19] and some even much earlier. Variations in the 12 C/ 13 C ratios within one source, such as in IRAS 16293−2422 [18], may provide clues on the formation pathways of these molecules. ...
... Numerous detections of isotopologs containing 13 C were reported in recent years [8,11,17,18,12,19] and some even much earlier. Variations in the 12 C/ 13 C ratios within one source, such as in IRAS 16293−2422 [18], may provide clues on the formation pathways of these molecules. Some of these observations benefited greatly from recent or concomitant laboratory investigations of 13 C containing isotopologs, such as ethanol [20], acetaldehyde [21], or ethyl cyanide with two 13 C [17]. ...
... The relative abundance of 18 O is less favorable; the terrestrial 16 O/ 18 O ratio is almost exactly 500 [7], and the ratio in the Galactic center of ∼200 [11,13,26,27] is only slightly lower. Nevertheless, some complex organic molecules containing 18 O have been detected in recent years, among them methyl formate [28] and formamide [29]. ...
Preprint
We studied the rotational spectrum of oxirane in a sample of natural isotopic composition in selected regions between 158 GHz and 1093 GHz. Investigations of the isotopologs with one $^{13}$C or one $^{18}$O were the primary focus in order to facilitate searches for them in space. We also examined the main isotopic species mainly to look into the performance of Watson's A and S reductions both in an oblate and in a prolate representation. Even though oxirane is a rather asymmetric oblate rotor, the A reduction in the III$^l$ representation did not yield a satisfactory fit, as was observed frequently earlier for other molecules. The other three combinations yielded satisfactory fits of similar quality among each other; the A reduction in the I$^r$ representation required two parameters less than both S reduction fits.
... The deuterated species detected in the PILS data are the mono-deuterated isotopomers of the oxygen-bearing organics glycolaldehyde (Jørgensen et al. 2016), ethanol, ketene, formic acid and of mono-deuterated acetaldehyde species CH 3 CDO (Jørgensen et al. 2018) and CH 2 DCHO (Coudert et al. 2019;Manigand et al. 2020), of the nitrogenbearing organics isocyanic acid DNCO and the monodeuterated isotopomers of formamide (Coutens et al. 2016) and the cyanamide isotopologue HDNCN (Coutens et al. 2018) and sulfur-containing species such as the hydrogen sulfide isotopologue HD 34 S (Drozdovskaya et al. 2018). Also, the PILS data reveal the presence of doubly-deuterated organics including the methyl cyanide species CHD 2 CN (Calcutt et al. 2018), the methyl formate species CHD 2 OCHO (Manigand et al. 2019) and the dimethyl ether species CHD 2 OCH 3 (Richard et al. 2021) and enable new and more accurate constraints on the doubly-and triply-deuterated variants of methanol in the warm gas close to the protostars (Drozdovskaya et al. 2022;Ilyushin et al. 2022). ...
... These systematic studies also enabled a more detailed comparison across the different species. It was seen for example that the degree of deuteration in a given molecule (referenced to one H atom) does not change for structurally different H atoms within uncertainties for several organics (e.g., Jørgensen et al. 2016Jørgensen et al. , 2018. It also appears that different types of organics can be categorised in groups according to their D/H ratios, which in turn may reflect their underlying formation mechanisms (Jørgensen et al. 2018). ...
... It was seen for example that the degree of deuteration in a given molecule (referenced to one H atom) does not change for structurally different H atoms within uncertainties for several organics (e.g., Jørgensen et al. 2016Jørgensen et al. , 2018. It also appears that different types of organics can be categorised in groups according to their D/H ratios, which in turn may reflect their underlying formation mechanisms (Jørgensen et al. 2018). ...
Preprint
We prepared a sample of mono-deuterated oxirane and studied its rotational spectrum in the laboratory between 490 GHz and 1060 GHz in order to improve its spectroscopic parameters and consequently the calculated rest frequencies of its rotational transitions. The updated rest frequencies were employed to detect $c$-C$_2$H$_3$DO for the first time in the interstellar medium in the Atacama Large Millimetre/submillimetre Array (ALMA) Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293$-$2422. Fits of the detected lines using the rotation diagrams yield a temperature of $T_{\rm rot} = 103 \pm 19$ K, which in turn agrees well with 125 K derived for the $c$-C$_2$H$_4$O main isotopologue previously. The $c$-C$_2$H$_3$DO to $c$-C$_2$H$_4$O ratio is found to be $\sim$0.15 corresponding to a D-to-H ratio of $\sim$0.036 per H atom which is slightly higher than the D-to-H ratio of species such as methanol, formaldehyde, ketene and but lower than those of the larger complex organic species such as ethanol, methylformate and glycolaldehyde. This may reflect that oxirane is formed fairly early in the evolution of the prestellar cores. The identification of doubly deuterated oxirane isotopomers in the PILS data may be possible judged by the amount of mono-deuterated oxirane and the observed trend that multiply deuterated isotopologues have higher deuteration rates than their mono-deuterated variants.
... Indeed, similarities in the organic compositions of hot corinos compared to cometary ices support the idea that comets formed at least in part from the icy material with a pre/ protostellar origin . Moreover, probing isotopic fractionation levels in these organics provides powerful constraints on their formation conditions (e.g., Coutens et al., 2016;Jørgensen et al., 2018). This is key to determining where along the star-formation sequence organic complexity is built up. ...
... It is important to consider how the performance of OASIS will compare to ALMA, the current state-of-the-art facility for studying complex chemistry in hot corinos. Given its subarcsecond spatial resolution, ALMA is better beam-matched to hot corinos than OASIS and is capable of detecting lower abundance species such as even larger molecules (e.g., glycolaldehyde; Jørgensen et al., 2016) and rarer isotopologues Jørgensen et al., 2018). Still, OASIS has several unique advantages. 1) Bandwidth: In order to obtain sufficient SNR on the HD line in Band 3, OASIS will observe many sources for ∼12 h. ...
Article
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Chemistry along the star- and planet-formation sequence regulates how prebiotic building blocks -- carriers of the elements CHNOPS -- are incorporated into nascent planetesimals and planets. Spectral line observations across the electromagnetic spectrum are needed to fully characterize interstellar CHNOPS chemistry, yet to date there are only limited astrochemical constraints at THz frequencies. Here, we highlight advances to the study of CHNOPS astrochemistry that will be possible with the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS). OASIS is a NASA mission concept for a space-based observatory that will utilize an inflatable 14-m reflector along with a heterodyne receiver system to observe at THz frequencies with unprecedented sensitivity and angular resolution. As part of a survey of H2O and HD towards ~100 protostellar and protoplanetary disk systems, OASIS will also obtain statistical constraints on the inventories of light hydrides including NH3 and H2S towards protoplanetary disks, as well as complex organics in protostellar hot corinos and envelopes. Line surveys of additional star-forming regions, including high-mass hot cores, protostellar outflow shocks, and prestellar cores, will also leverage the unique capabilities of OASIS to probe high-excitation organics and small hydrides, as is needed to fully understand the chemistry of these objects.
... Constraining which of the two ways to synthesize iCOMs is efficient and where the iCOMs formation happen, is not a simple task. Many methods have been used, from the comparison of the iCOMs measured abundances in hot cores/corinos with model predictions to their measured deuterium fractionation (Ceccarelli et al., 1998;Coutens et al., 2016;Jørgensen et al., 2018;Turner, 1990). ...
... Le modèle peut être constitué de réseaux de réaction simples, tels que la formation de méthanol ou d'eau, et comparer les résultats avec les expériences existantes et les simulations de Monte Carlo (Cuppen et al., 2009(Cuppen et al., , 2010. the iCOMs form is not a simple task. Many methods have been used, from the comparison of the iCOM measured abundances in hot cores and hot corinos with model predictions to their measured deuterium fractionation (Turner 1990;Ceccarelli et al. 1998;Coutens et al. 2016;Jørgensen et al. 2018). ...
Thesis
So far, Earth is the only known planet-hosting life based on organic chemistry. The Solar Systems small objects (e.g., comets and asteroids) are enriched with organic compounds, which raises the question of whether the first steps of the organic chemistry that led to terrestrial life started during the formation of the Solar System. Stars and planetary systems like our Solar System are formed continuously in the Milky Way. So, in principle, we can study chemistry in those objects to recover the first steps of the organic chemistry of the young Solar System. In this thesis, I worked on two main objectives, modeling the chemical evolution in star-forming regions with Grainoble+ and modeling the experimental ice with Labice.The first objective of the thesis is to understand the chemical processes that form and destroy interstellar Complex Organic Molecules (aka iCOMs) in Solar-like star-forming regions. For this purpose, I developed an astrochemistry code, Grarinoble+. The model is based on Grainoble, previously developed by our group (Taquet et al., 2012). Grainoble+ is a three-phase gas-grain multi-grain astrochemical code simulating the chemical evolution in star-forming regions. We included the latest binding energies and diffusion and reaction rates from quantum chemical calculations (see, e. g., Senevirathne et al. 2017; Song et al. 2017; and Ferrero et al. 2020).I followed two goals with Grainoble+, modeling iCOMs formation in the shocked regions of NGC 1333 IRAS 4A (De Simone et al., 2020) and modeling the ice composition in Taurus MCs (Witzel et al. 2022, submitted.).The second goal of the thesis is to simulate the layered structure of ices in experimental chemistry laboratories and simulate the thermal desorption of species based on Temperature Programmed Desorption (TPD) techniques. For this purpose, I developed Labice toy model that simulates the TPD experiments with the rate equation approach with a few input parameters. Labice is a simple analog of Grainoble+ that uses the three-phase approach to model the ice, water phase transition, and thermal desorption in an experimental setup. The goal is to show the impact of the various parameters, such as multi-binding energy or the trapping effect of water ice, that will be used in astrochemical models. I followed two goals with the Labice toy model, modeling the impact of the multi-binding energy approach on the sublimation of species (Ferrero et al. 2020) and modeling and benchmarking the water and CO composite ices using the CO trapped fraction (Witzel et al. 2022, in prep).
... Thus, the observed chemical differentiation is not well explained theoretically, and the chemical pathways/formation mechanisms responsible for the differentiation are still an open question. To step forward, the temperature structure around the protostar needs to be clarified in detail, because it is tightly related to the chemical structure (e.g., Jørgensen et al. 2018;Taquet et al. 2019;Oya & Yamamoto 2020;van Gelder et al. 2020;van't Hoff et al. 2020avan't Hoff et al. , 2020bAmbrose et al. 2021). Hence, we here investigate the molecular distributions and temperature structure around the low-mass protostellar source B335, which is rich in COMs. ...
... For all four molecules, the derived rotation temperature is the highest at the continuum peak position, which is in the range of 193−215 K, and decreases as an increasing distance from the protostar. Such a high temperature toward the continuum peak is often seen in hot corino sources (e.g., Watanabe et al. 2017;Jørgensen et al. 2018;Bianchi et al. 2020;De Simone et al. 2020;Oya & Yamamoto 2020). The temperatures are 154−195 K at the positions of ±0 03 for the four molecules. ...
Article
Full-text available
Resolving physical and chemical structures in the vicinity of a protostar is of fundamental importance for elucidating their evolution to a planetary system. In this context, we have conducted 1.2 mm observations toward the low-mass protostellar source B335 at a resolution of 0.″03 with the Atacama Large Millimeter/submillimeter Array. More than 20 molecular species including HCOOH, NH 2 CHO, HNCO, CH 3 OH, CH 2 DOH, CHD 2 OH, and CH 3 OD are detected within a few tens au around the continuum peak. We find a systematic chemical differentiation between oxygen-bearing and nitrogen-bearing organic molecules by using the principal component analysis for the image cube data. The distributions of the nitrogen-bearing molecules are more compact than those of the oxygen-bearing ones except for HCOOH. The temperature distribution of the disk/envelope system is revealed by a multiline analysis for each of HCOOH, NH 2 CHO, CH 3 OH, and CH 2 DOH. The rotation temperatures of CH 3 OH and CH 2 DOH at the radius of 0.″06 along the envelope direction are derived to be 150–165 K. On the other hand, those of HCOOH and NH 2 CHO, which have a smaller distribution, are 75–112 K, and are significantly lower than those for CH 3 OH and CH 2 DOH. This means that the outer envelope traced by CH 3 OH and CH 2 DOH is heated by additional mechanisms rather than protostellar heating. We here propose the accretion shock as the heating mechanism. The chemical differentiation and the temperature structure on a scale of a few au provide us with key information to further understand chemical processes in protostellar sources.
... Moreover, probing isotopic fractionation levels in these organics provides powerful constraints on their formation conditions (e.g. Coutens et al. 2016;Jørgensen et al. 2018). This is key to determining where along the starformation sequence organic complexity is built up. ...
... Given its sub-arcsecond spatial resolution, ALMA is better beam-matched to hot corinos compared to OASIS, and is capable of detecting lower-abundance species such as even larger molecules (e.g. glycolaldehyde Jørgensen et al. 2016) and rarer isotopologues Jørgensen et al. 2018). Still, OASIS has several unique advantages. ...
Preprint
Chemistry along the star- and planet-formation sequence regulates how prebiotic building blocks -- carriers of the elements CHNOPS -- are incorporated into nascent planetesimals and planets. Spectral line observations across the electromagnetic spectrum are needed to fully characterize interstellar CHNOPS chemistry, yet to date there are only limited astrochemical constraints at THz frequencies. Here, we highlight advances to the study of CHNOPS astrochemistry that will be possible with the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS). OASIS is a NASA mission concept for a space-based observatory that will utilize an inflatable 14-m reflector along with a heterodyne receiver system to observe at THz frequencies with unprecedented sensitivity and angular resolution. As part of a survey of H2O and HD towards ~100 protostellar and protoplanetary disk systems, OASIS will also obtain statistical constraints on the inventories of light hydrides including NH3 and H2S towards protoplanetary disks, as well as complex organics in protostellar hot corinos and envelopes. Line surveys of additional star-forming regions, including high-mass hot cores, protostellar outflow shocks, and prestellar cores, will also leverage the unique capabilities of OASIS to probe high-excitation organics and small hydrides, as is needed to fully understand the chemistry of these objects.
... In IRAS 16293-2422, the observed differences are interpreted as a result of the onion-like structure of the hot corino (physical effect). In particular, the NH 2 CHO rotational temperature is slightly higher (T ∼ 140-300 K) than what was derived for other iCOMs (T ∼ 100 K) (Jørgensen et al. 2016(Jørgensen et al. , 2018Manigand et al. 2020), suggesting that N-bearing species trace hotter gas, closer to the protostar. On the other hand, recent ALMA observations of other two Class 0 protostars, Perseus B1-c and Serpens S68N, do not show a significant difference in the excitations conditions of N-bearing and O-bearing species (van Gelder et al. 2020;Nazari et al. 2021), leaving the question open. ...
Article
Full-text available
We present ALMA high-angular-resolution (∼50 au) observations of the Class I binary system SVS13-A. We report images of SVS13-A in numerous interstellar complex organic molecules: CH 3 OH, ¹³ CH 3 OH, CH 3 CHO, CH 3 OCH 3 , and NH 2 CHO. Two hot corinos at different velocities are imaged in VLA4A ( V sys = +7.7 km s ⁻¹ ) and VLA4B ( V sys = +8.5 km s ⁻¹ ). From a non-LTE analysis of methanol lines, we derive a gas density of 3 × 10 ⁸ cm ⁻³ and gas temperatures of 140 and 170 K for VLA4A and VLA4B, respectively. For the other species, the column densities are derived from an LTE analysis. Formamide, which is the only N-bearing species detected in our observations, is more prominent around VLA4A, while dimethyl ether, methanol, and acetaldehyde are associated with both VLA4A and VLA4B. We derive in the two hot corinos abundance ratios of ∼1 for CH 3 OH, ¹³ CH 3 OH, and CH 3 OCH 3 ; ∼2 for CH 3 CHO; and ∼4 for NH 2 CHO. The present data set supports chemical segregation between the different species inside the binary system. The emerging picture is that of an onion-like structure of the two SVS13-A hot corinos, caused by the different binding energies of the species, also supported by ad hoc quantum chemistry calculations. In addition, the comparison between molecular and dust maps suggests that the interstellar complex organic molecules emission originates from slow shocks produced by accretion streamers impacting the VLA4A and VLA4B disks and enriching the gas-phase component.
... This can be conducted via theoretical modelling work and complementary observations across the star formation sequence. The mono-deuterated isotopologues, CH 2 DOH and CH 3 OD, are routinely observed around low-mass and high-mass protostars, [26][27][28][29][30][31] and di-and tri-deuterated methanol have also been detected around lowmass protostars. [32][33][34][35] D/H ratios in the range of a few % establish the idea that deuterated methanol observed in the warm gas around protostars is not formed in-situ, but inherited from prestellar stages, because high fractionation combined with abundant methanol formation is only achieveable in cold phases. ...
Preprint
Mono-deuterated methanol is thought to form during the prestellar core stage of star formation. Observed variations in the CH2DOH/CH3OD ratio suggest that its formation is strongly dependent on the surrounding cloud conditions. Thus, it is a potential tracer of the physical conditions before the onset of star formation. A single-point physical model representative of a typical prestellar core is coupled to chemical models to investigate potential formation pathways towards deuterated methanol at the prestellar stage. Simple addition reactions of H and D are not able to reproduce observed abundances. The implementation of an experimentally verified abstraction scheme leads to the efficient formation of methyl-deuterated methanol, but lacks sufficient formation of hydroxy-deuterated methanol. CH3OD is most likely formed at a later evolutionarymstage, potentially from H-D exchange reactions in warm ices between HDO (and D2O) and CH3OH. The CH2DOH/CH3OD ratio is not an appropriate tracer of the physical conditions during the prestellar stage, but might be better suited as a tracer of ice heating.
... The presence and distribution of complex, potentially prebiotic molecules within the interstellar medium have been extensively probed using various ground-and space-based observatories. Perhaps the most productive of these studies, however, have been those which have made use of the ALMA for molecular detections (Belloche et al., 2016(Belloche et al., , 2019Garrod et al., 2017;Jørgensen et al., 2018;Calcutt et al., 2019;Willis et al., 2020). The impact of ALMA observations on the detection of interstellar molecules has been massive: first detections of prebiotic molecules which were previously controversial or unconfirmed, such as urea (NH 2 CONH 2 ), have been achieved (Belloche et al., 2019), and more information has been gleaned on interstellar chemical processes such as deuteration (Belloche et al., 2016). ...
Article
Full-text available
Stellar systems are often formed through the collapse of dense molecular clouds which, in turn, return copious amounts of atomic and molecular material to the interstellar medium. An in-depth understanding of chemical evolution during this cyclic interaction between the stars and the interstellar medium is at the heart of astrochemistry. Systematic chemical composition changes as interstellar clouds evolve from the diffuse stage to dense, quiescent molecular clouds to star-forming regions and proto-planetary disks further enrich the molecular diversity leading to the evolution of ever more complex molecules. In particular, the icy mantles formed on interstellar dust grains and their irradiation are thought to be the origin of many of the observed molecules, including those that are deemed to be “prebiotic”; that is those molecules necessary for the origin of life. This review will discuss both observational (e.g., ALMA, SOFIA, Herschel) and laboratory investigations using terahertz and far-IR (THz/F-IR) spectroscopy, as well as centimeter and millimeter spectroscopies, and the role that they play in contributing to our understanding of the formation of prebiotic molecules. Mid-IR spectroscopy has typically been the primary tool used in laboratory studies, particularly those concerned with interstellar ice analogues. However, THz/F-IR spectroscopy offers an additional and complementary approach in that it provides the ability to investigate intermolecular interactions compared to the intramolecular modes available in the mid-IR. THz/F-IR spectroscopy is still somewhat under-utilized, but with the additional capability it brings, its popularity is likely to significantly increase in the near future. This review will discuss the strengths and limitations of such methods, and will also provide some suggestions on future research areas that should be pursued in the coming decade exploiting both space-borne and laboratory facilities.
... In IRAS16293-2422 the observed differences are interpreted as a result of the onion-like structure of the hot corino (physical effect). In particular, the NH 2 CHO rotational temperature is slightly higher (T ∼ 140 -300 K) than what derived for other iCOMs (T ∼ 100 K) (Jørgensen et al. 2016(Jørgensen et al. , 2018Manigand et al. 2019), suggesting that N-bearing species trace hotter gas, closer to the protostar. On the other hand, recent ALMA observations of other two Class 0 protostars, Perseus B1-c and Serpens S68N, do not show significant difference in the excitations conditions of N-bearing and O-bearing species (Nazari et al. 2021;van Gelder et al. 2020), leaving the question open. ...
Preprint
We present ALMA high-angular resolution ($\sim$ 50 au) observations of the Class I binary system SVS13-A. We report images of SVS13-A in numerous interstellar complex organic molecules: CH$_{\rm 3}$OH, $^{13}$CH$_{\rm 3}$OH, CH$_{\rm 3}$CHO, CH$_{\rm 3}$OCH$_{\rm 3}$, and NH$_{\rm 2}$CHO. Two hot corinos at different velocities are imaged in VLA4A (V$_{sys}$= +7.7 km s$^{-1}$) and VLA4B (V$_{sys}$= +8.5 km s$^{-1}$). From a non-LTE analysis of methanol lines we derive a gas density of 3 $\times$ 10$^8$ cm$^{-3}$, and gas temperatures of 140 K and 170 K for VLA4A and VLA4B, respectively. For the other species the column densities are derived from a LTE analysis. Formamide, which is the only N-bearing species detected in our observations, is more prominent around VLA4A, while dimethyl ether, methanol and acetaldehyde are associated with both VLA4A and VLA4B. We derive in the two hot corinos abundance ratios of $\sim$ 1 for CH$_{\rm 3}$OH, $^{13}$CH$_{\rm 3}$OH, and CH$_{\rm 3}$OCH$_{\rm 3}$, $\sim$ 2 for CH$_{\rm 3}$CHO, and $\sim$ 4 for NH$_{\rm 2}$CHO. The present dataset supports a chemical segregation between the different species inside the binary system. The emerging picture is that of an onion-like structure of the two SVS13-A hot corinos, caused by the different binding energies of the species, also supported by ad hoc quantum chemistry calculations. In addition, the comparison between molecular and dust maps suggests that the interstellar complex organic molecules emission originates from slow shocks produced by accretion streamers impacting the VLA4A and VLA4B disks and enriching the gas-phase component.
... Other star-forming regions have reported a lower 12 C/ 13 C compared to expectations based on Galactocentric distance (Daniel et al. 2013;Jørgensen et al. 2018;Magalhães et al. 2018) as well as planetary nebulae (Ziurys et al. 2020). This may hint at new chemical processes (Colzi et al. 2020). ...
Preprint
We present the first high spectral resolution mid-infrared survey in the Orion BN/KL region, covering 7.2 to 28.3 micron. With SOFIA/EXES we target the enigmatic source Orion IRc2. While this is in the most prolifically studied massive star-forming region, longer wavelengths and molecular emission lines dominated previous spectral surveys. The mid-infrared observations in this work access different components and molecular species in unprecedented detail. We unambiguously identify two new kinematic components, both chemically rich with multiple molecular absorption lines. The "blue clump" has vLSR = -7.1 \pm 0.7 km/s and the "red clump" 1.4 \pm 0.5 km/s. While the blue and red clumps have similar temperatures and line widths, molecular species in the blue clump have higher column densities. They are both likely linked to pure rotational H2 emission also covered by this survey. This work provides evidence for the scenario that the blue and red clumps are distinct components unrelated to the classic components in the Orion BN/KL region. Comparison to spectroscopic surveys towards other infrared targets in the region show that the blue clump is clearly extended. We analyze, compare, and present in depth findings on the physical conditions of C2H2, 13CCH2, CH4, CS, H2O, HCN, H13CN, HNC, NH3, and SO2 absorption lines and an H2 emission line associated with the blue and red clumps. We also provide limited analysis of H2O and SiO molecular emission lines towards Orion IRc2 and the atomic forbidden transitions [FeII], [SI], [SIII], and [NeII].
... In contrast to water, methanol shows higher D/H ratios of ∼10 −2 toward IRAS4A (Taquet et al. 2019), suggesting the formation of methanol ices in the cold prestellar core phase. Higher D/H ratios of methanol than water are also seen in other protostellar cores such as IRAS2A and IRAS 16293-2422 (Persson et al. 2014;Taquet et al. 2019;Jørgensen et al. 2018;Manigand et al. 2020). The D/H ratio of ammonia measured toward IRAS4A in this work ( 10 −1 ) is higher than that of methanol. ...
Preprint
The nitrogen chemical evolution during star and planet formation is still not fully understood. Ammonia (NH$_3$) is a key specie in the understanding of the molecular evolution in star-forming clouds and nitrogen isotope fractionation. In this paper, we present high spatial resolution observations of multiple emission lines of NH$_3$ toward the protobinary system NGC1333 IRAS4A with Karl G. Jansky Very Large Array (VLA). We spatially resolved the binary (hereafter 4A1 and 4A2) and detected compact emission of NH$_3$ transitions with high excitation energies ($\gtrsim$100 K) from the vicinity of the protostars, indicating the NH$_3$ ice has sublimated at the inner hot region. The NH$_3$ column density is estimated to be $\sim 10^{17}-10^{18}$ cm$^{-2}$. We also detected two NH$_2$D transitions, allowing us to constrain the deuterium fractionation of ammonia. The NH$_2$D/NH$_3$ ratios are as high as $\sim 0.3-1$ in both 4A1 and 4A2. From the comparisons with the astrochemical models in the literature, the high NH$_2$D/NH$_3$ ratios suggest that the formation of NH$_3$ ices mainly started in the prestellar phase after the formation of bulk water ice finished, and that the primary nitrogen reservoir in the star-forming cloud could be atomic nitrogen (or N atoms) rather than nitrogen-bearing species such as N$_2$ and NH$_3$. The implications on the physical properties of IRAS4A cores are discussed as well.
... The present results first show that MF is the overwhelmingly major photoproduct from methanol on ASW while neither EG nor GA are detected. As shown in Figure 5(a), the relative abundance of MF to its precursor, MM, in the present study has a good correlation with observations toward hot corinos of lowmass protostars (Jørgensen et al. 2018;Manigand et al. 2020) and high-mass star-forming regions (McGuire et al. 2017;El-Abd et al. 2019). This correlation would suggest that MF and MM are formed on dust grains in their parent clouds at low temperatures and released into the gas phase during warming up. ...
... Jørgensen, Belloche & Garrod (2020), obtained from the PILS (Jørgensen et al. 2016) and EMoCA surveys. Observational column density ratios with respect to methanol are provided for the low-mass source IRAS 16293-2422B and the high-mass source Sgr B2(N2), using methanol column densities of 1 × 10 19 cm −2 (Jørgensen et al. 2018) and 4 × 10 19 cm −2 , respectively. Model ratios correspond to the peak gas-phase abundance of each molecule divided by the peak gas-phase methanol abundance, for each of the final model warm-up timescales (see Table 17). ...
Preprint
Full-text available
A new, more comprehensive model of gas-grain chemistry in hot molecular cores is presented, in which nondiffusive reaction processes on dust-grain surfaces and in ice mantles are implemented alongside traditional diffusive surface/bulk-ice chemistry. We build on our nondiffusive treatments used for chemistry in cold sources, adopting a standard collapse/warm-up physical model for hot cores. A number of other new chemical model inputs and treatments are also explored in depth, culminating in a final model that demonstrates excellent agreement with gas-phase observational abundances for many molecules, including some (e.g.~methoxymethanol) that could not be reproduced by conventional diffusive mechanisms. Observed ratios of structural isomers methyl formate, glycolaldehyde and acetic acid are well reproduced by the models. The main temperature regimes are identified in which various complex organic molecules (COMs) are formed. Nondiffusive chemistry advances the production of many COMs to much earlier times and lower temperatures than in previous model implementations. Those species may form either as by-products of simple-ice production, or via early photochemistry within the ices while external UV photons can still penetrate. Cosmic ray-induced photochemistry is less important than in past models, although it affects some species strongly over long timescales. Another production regime occurs during the high-temperature desorption of solid water, whereby radicals trapped in the ice are released onto the grain/ice surface, where they rapidly react. Several recently-proposed gas-phase COM-production mechanisms are also introduced, but they rarely dominate. New surface/ice reactions involving CH and CH$_2$ are found to contribute substantially to the formation of certain COMs.
Article
Solar-type protostars have been shown to harbor highly deuterated complex organics, as evidenced, for instance, by the high relative abundances of doubly and triply deuterated isotopologs. While this degree of deuteration may provide important clues in studying the formation of these species, spectroscopic information on multiply deuterated isotopologs is often insufficient. In particular, searches for triply deuterated methanol, CD 3 OH, are hampered to a large extent by the lack of intensity information from a spectroscopic model. The aim of the present study is to develop a spectroscopic model of CD 3 OH in low-lying torsional states that is sufficiently accurate to facilitate further searches for CD 3 OH in space. We performed a new measurement campaign for CD 3 OH involving three spectroscopic laboratories that covers the 34 GHz−1.1 THz and the 20−900 cm ⁻¹ ranges. The analysis was performed using the torsion-rotation Hamiltonian model based on the rho-axis method. We determined a model that describes the ground and first excited torsional states of CD 3 OH, up to quantum numbers J ≤ 55 and K a ≤ 23, and we derived a line list for radio-astronomical observations. The resulting line list is accurate up to at least 1.1 THz and should be sufficient for all types of radio-astronomical searches for this methanol isotopolog. This line list was used to search for CD 3 OH in data from the Protostellar Interferometric Line Survey of IRAS 16293−2422 using the Atacama Large Millimeter/submillimeter Array. Specifically, CD 3 OH is securely detected in the data, with a large number of clearly separated and well-reproduced lines. We not only detected lines belonging to the ground torsional state, but also several belonging to the first excited torsional state. The derived column density of CD 3 OH and abundance relative to the non-deuterated isotopolog confirm the significant enhancement of this multiply deuterated variant. This finding is in line with other observations of multiply deuterated complex organic molecules and may serve as an important constraint on their formation models.
Article
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Context. Hot corinos are compact regions around solar-mass protostellar objects that are very rich in interstellar Complex Organic Molecules (iCOMs). How the abundance of these molecules is affected by the environmental physical conditions is still an open question. More specifically, addressing this point is key to understand our own chemical origins since the Solar System formed in a large cluster of low- to high-mass stars and was therefore subject to external heating and ultraviolet irradiation which may have shaped the chemistry of its early formation stages. Aims. The goal of this high resolution study is to determine the abundance ratios of iCOMs in HOPS-108, which is a Class 0 protostar and a hot corino candidate located in the nearest Solar System analogue, the protostellar cluster OMC-2 FIR 4, in Orion. We aim to compare the abundance ratios to those found in other hot corinos, which are all located in less crowded environments, in order to understand the impact of environmental conditions on hot corinos’ chemistry. Methods. We observed the OMC-2 FIR 4 proto-cluster using the Band 6 of the Atacama Large (sub-)Millimetre Array in Cycle 4 with an angular resolution of ~0.′′28 (110 au). We determined the abundances and temperature of the species using local thermodynamic equilibrium (LTE) and non-LTE analysis. Results. Our results reveal a rich organic chemistry towards HOPS-108, asserting that it is a hot corino where the following iCOMs are detected: CH 3 OH, HCOOCH 3 , CH 3 OCH 3 , CH 3 ¹⁸ OH, CH 2 DOH, CH 3 COCH 3 , CH 3 CHO, CH 3 CN, ¹³ CH 3 CN, C 2 H 5 CN, and NH 2 CHO. Remarkably, we find a possible enhancement in the HCOOCH 3 abundance with respect to other known hot corinos. Indeed, the [CH 3 OCH 3 ]/[HCOOCH 3 ] abundance ratio in this source is ~0.2 and, within the uncertainties, it deviates from the known correlation marginally where [CH 3 OCH 3 ]/[HCOOCH 3 ] ~1. A relatively low [CH 2 DOH]/[CH 3 OH] abundance ratio of ~0.02 is also obtained, which is in agreement with that found in another Orion source, HH212, suggesting a higher gas temperature during the early phases of ice mantle formation. Conclusions. The [CH 3 OCH 3 ]/[HCOOCH 3 ] and [CH 2 DOH]/[CH 3 OH] abundance ratios in HOPS-108 might result from different physical conditions in the Orion molecular complex compared to other regions. The former ratio cannot be reproduced with current chemical models, highlighting the importance of improving the chemical networks with theoretical calculations. More hot corinos located in heavily clustered regions such as Orion should be targeted in order to measure these ratios and evaluate whether they are an environmental product or whether HOPS-108 is an exceptional hot corino overall.
Article
We report the detection of more than 120 emission lines corresponding to eight complex organic molecules (COMs; CH 3 OH, CH 3 CH 2 OH, CH 3 OCH 3 , CH 3 OCHO, CH 3 COCH 3 , NH 2 CHO, CH 2 DCN, and CH 3 CH 2 CN) and three isotopologues (CH 2 DOH, ¹³ CH 3 CN, and CH 3 C ¹⁵ N) toward the western component of the Ser-emb 11 binary young stellar object using observations with the Atacama Large Millimeter/submillimeter Array at ∼1 mm. The complex organic emission was unresolved with a ∼0.″5 beam (∼220 au) in a compact region around the central protostar, and a population diagram analysis revealed excitation temperatures above 100 K for all COMs, indicating the presence of a hot corino. The estimated column densities were in the range of 10 ¹⁷ −10 ¹⁸ cm ⁻² for the O-bearing COMs, and three orders of magnitude lower for the N-bearing species. We also report the detection of H 2 CO and CH 3 OH emission in a nearby millimeter source that had not been previously cataloged. Ser-emb 11 is classified in the literature as a Class I source near the Class 0/I cutoff. The estimated COM relative abundances in Ser-emb 11 W and the other three Class I hot corino sources reported in the literature are consistent with those of Class 0 hot corinos, suggesting a continuity in the chemical composition of hot corinos during protostellar evolution.
Article
Context. Class I protostars are a bridge between Class 0 protostars (≤10 ⁵ yr old), and Class II (≥10 ⁶ yr) protoplanetary disks. Recent studies show gaps and rings in the dust distribution of disks younger than 1 Myr, suggesting that planet formation may start already at the Class I stage. To understand what chemistry planets will inherit, it is crucial to characterize the chemistry of Class I sources and to investigate how chemical complexity evolves from Class 0 protostars to protoplanetary disks. Aims. There are two goals: (i) to perform a census of the molecular complexity in a sample of four Class I protostars, and (ii) to compare the data with the chemical compositions of earlier and later phases of the Sun-like star formation process. Methods. We performed IRAM-30 m observations at 1.3 mm towards four Class I objects (L1489-IRS, B5-IRS1, L1455-IRS1, and L1551-IRS5). The column densities of the detected species were derived assuming local thermodynamic equilibrium (LTE) or large velocity gradients (LVGs). Results. We detected 27 species: C-chains, N-bearing species, S-bearing species, Si-bearing species, deuterated molecules, and interstellar complex organic molecules (iCOMs; CH 3 OH, CH 3 CN, CH 3 CHO, and HCOOCH 3 ). Among the members of the observed sample, L1551-IRS5 is the most chemically rich source. Different spectral profiles are observed: (i) narrow lines (~1 km s ⁻¹ ) towards all the sources, (ii) broader lines (~4 km s ⁻¹ ) towards L1551-IRS5, and (iii) line wings due to outflows (in B5-IRS1, L1455-IRS1, and L1551-IRS5). Narrow c-C 3 H 2 emission originates from the envelope with temperatures of 5–25 K and sizes of ~2′′−10′′. The iCOMs in L1551-IRS5 reveal the occurrence of hot corino chemistry, with CH 3 OH and CH 3 CN lines originating from a compact (~0.′′15) and warm ( T > 50 K) region. Finally, OCS and H 2 S seem to probe the circumbinary disks in the L1455-IRS1 and L1551-IRS5 binary systems. The deuteration in terms of elemental D/H in the molecular envelopes is: ~10−70% (D 2 CO/H 2 CO), ~5−15% (HDCS/H 2 CS), and ~1−23% (CH 2 DOH/CH 3 OH). For the L1551-IRS5 hot corino we derive D/H ~2% (CH 2 DOH/CH 3 OH). Conclusions. Carbon chain chemistry in extended envelopes is revealed towards all the sources. In addition, B5-IRS1, L1455-IRS1, and L1551-IRS5 show a low-excitation methanol line that is narrow and centered at systemic velocity, suggesting an origin from an extended structure, plausibly UV-illuminated. The abundance ratios of CH 3 CN, CH 3 CHO, and HCOOCH 3 with respect to CH 3 OH measured towards the L1551-IRS5 hot corino are comparable to that estimated at earlier stages (prestellar cores, Class 0 protostars), and to that found in comets. The deuteration in our sample is also consistent with the values estimated for sources at earlier stages. These findings support the inheritance scenario from prestellar cores to the Class I phase when planets start forming.
Article
Context. Di-deuterated molecules are observed in the earliest stages of star formation at abundances of a few percent relative to their nondeuterated isotopologs, which is unexpected considering the scarcity of deuterium in the interstellar medium. With sensitive observations leading to the detection of a steadily increasing number of di-deuterated species, it is becoming possible to explore successive deuteration chains. Aims. The accurate quantification of the column density of di-deuterated methanol is a key piece of the puzzle that is missing in the otherwise thoroughly constrained family of D-bearing methanol in the deeply embedded low-mass protostellar system and astrochemical template source IRAS 16293-2422. A spectroscopic dataset for astrophysical purposes was built for CHD 2 OH and made publicly available to facilitate the accurate characterization of this species in astrochemical surveys. Methods. The newly computed line list and partition function were used to search for CHD 2 OH toward IRAS 16293-2422 A and B in data from the Atacama Large Millimeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS). Only nonblended, optically thin lines of CHD 2 OH were used for the synthetic spectral fitting. Results. The constructed spectroscopic database contains line frequencies and strengths for 7417 transitions in the 0–500 GHz frequency range. ALMA-PILS observations in the 329–363 GHz range were used to identify 105 unique, nonblended, optically thin line frequencies of CHD 2 OH for synthetic spectral fitting. The derived excitation temperatures and column densities yield high D/H ratios of CHD 2 OH in IRAS 16293-2422 A and B of 7.5 ± 1.1% and 7.7 ± 1.2%, respectively. Conclusions. Deuteration in IRAS 16293-2422 is not higher than in other low-mass star-forming regions (L483, SVS13-A, NGC 1333-IRAS2A, -IRAS4A, and -IRAS4B). Di-deuterated molecules consistently have higher D/H ratios than their mono-deuterated counterparts in all low-mass protostars, which may be a natural consequence of H–D substitution reactions as seen in laboratory experiments. The Solar System’s natal cloud, as traced by comet 67P/Churyumov–Gerasimenko, may have had a lower initial abundance of D, been warmer than the cloud of IRAS 16293-2422, or been partially reprocessed. In combination with accurate spectroscopy, a careful spectral analysis, and the consideration of the underlying assumptions, successive deuteration is a robust window on the physicochemical provenance of star-forming systems.
Article
The complex organic molecules (COMs) detected in star-forming regions are the precursors of the prebiotic molecules that can lead to the emergence of life. By studying COMs in more evolved protoplanetary disks we can gain a better understanding of how they are incorporated into planets. This paper presents ALMA band 7 observations of the dust and ice trap in the protoplanetary disk around Oph IRS 48. We report the first detection of dimethyl ether (CH 3 OCH 3 ) in a planet-forming disk and a tentative detection of methyl formate (CH 3 OCHO). We determined column densities for the detected molecules and upper limits on non-detected species using the CASSIS spectral analysis tool. The inferred column densities of CH 3 OCH 3 and CH 3 OCHO with respect to methanol (CH 3 OH) are of order unity, indicating unusually high abundances of these species compared to other environments. Alternatively, the ¹² CH 3 OH emission is optically thick and beam diluted, implying a higher CH 3 OH column density and a smaller emitting area than originally thought. The presence of these complex molecules can be explained by thermal ice sublimation, where the dust cavity edge is heated by irradiation and the full volatile ice content is observable in the gas phase. This work confirms the presence of oxygen-bearing molecules more complex than CH 3 OH in protoplanetary disks for the first time. It also shows that it is indeed possible to trace the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.
Article
We studied the rotational spectrum of oxirane in a sample of natural isotopic composition in selected regions between 158 GHz and 1093 GHz. Investigations of the isotopologs with one ¹³C or one ¹⁸O were the primary focus in order to facilitate searches for them in space. We also examined the main isotopic species mainly to look into the performance of Watson’s A and S reductions both in an oblate and in a prolate representation. Even though oxirane is a rather asymmetric oblate rotor, the A reduction in the IIIl representation did not yield a satisfactory fit, as was observed frequently earlier for other molecules. The other three combinations yielded satisfactory fits of similar quality among each other; the A reduction in the Ir representation required two parameters less than both S reduction fits.
Article
A new, more comprehensive model of gas–grain chemistry in hot molecular cores is presented, in which nondiffusive reaction processes on dust-grain surfaces and in ice mantles are implemented alongside traditional diffusive surface/bulk-ice chemistry. We build on our nondiffusive treatments used for chemistry in cold sources, adopting a standard collapse/warm-up physical model for hot cores. A number of other new chemical model inputs and treatments are also explored in depth, culminating in a final model that demonstrates excellent agreement with gas-phase observational abundances for many molecules, including some (e.g., methoxymethanol) that could not be reproduced by conventional diffusive mechanisms. The observed ratios of structural isomers methyl formate, glycolaldehyde, and acetic acid are well reproduced by the models. The main temperature regimes in which various complex organic molecules (COMs) are formed are identified. Nondiffusive chemistry advances the production of many COMs to much earlier times and lower temperatures than in previous model implementations. Those species may form either as by-products of simple-ice production, or via early photochemistry within the ices while external UV photons can still penetrate. Cosmic ray-induced photochemistry is less important than in past models, although it affects some species strongly over long timescales. Another production regime occurs during the high-temperature desorption of solid water, whereby radicals trapped in the ice are released onto the grain/ice surface, where they rapidly react. Several recently proposed gas-phase COM-production mechanisms are also introduced, but they rarely dominate. New surface/ice reactions involving CH and CH 2 are found to contribute substantially to the formation of certain COMs.
Article
Full-text available
Context. During the process of star formation, the dense gas undergoes significant chemical evolution leading to the emergence of a rich variety of molecules associated with hot cores and hot corinos. However, the physical conditions and the chemical processes involved in this evolution are poorly constrained; the early phases of emerging hot cores in particular represent an unexplored territory. Aims. We provide here a full molecular inventory of a massive protostellar core that is proposed to represent a precursor of a hot core. We investigate the conditions for the molecular richness of hot cores. Methods. We performed an unbiased spectral survey towards the hot core precursor associated with clump G328.2551-0.5321 between 159 GHz and 374 GHz, covering the entire atmospheric windows at 2 mm, 1.2 mm, and 0.8 mm. To identify the spectral lines, we used rotational diagrams and radiative transfer modelling assuming local thermodynamical equilibrium. Results. We detected 39 species plus 26 isotopologues, and were able to distinguish a compact (~2″), warm inner region with a temperature, T, of ~100 K, a colder, more extended envelope with T ~ 20 K, and the kinematic signatures of the accretion shocks that have previously been observed with ALMA. We associate most of the emission of the small molecules with the cold component of the envelope, while the molecular emission of the warm gas is enriched by complex organic molecules (COMs). We find a high abundance of S-bearing molecules in the cold gas phase, including the molecular ions HCS ⁺ and SO ⁺ . The abundance of sulphur-bearing species suggests a low sulphur depletion, with a factor of ≥1%, in contrast to low-mass protostars, where the sulphur depletion is found to be stronger. Similarly to other hot cores, the deuterium fractionation of small molecules is low, showing a significant difference compared to low-mass protostars. We find a low isotopic ratio in particular for ¹² C/ ¹³ C of ~30, and ³² S/ ³⁴ S of ~12, which are about two times lower than the values expected at the galactocentric distance of G328.2551-0.5321. We identify nine COMs (CH 3 OH, CH 3 OCH 3 , CH 3 OCHO, CH 3 CHO, HC(O)NH 2 , CH 3 CN, C 2 H 5 CN, C 2 H 3 CN, and CH 3 SH) in the warm component of the envelope, four in the cold gas, and four towards the accretion shocks. Conclusions. The presence of numerous molecular ions and high abundance of sulphur-bearing species originating from the undisturbed gas may suggest a contribution from shocked gas at the outflow cavity walls. The molecular composition of the cold component of the envelope is rich in small molecules, while a high abundance in numerous species of COMs suggests an increasing molecular complexity towards the warmer regions. The molecular composition of the warm gas is similar to that of both hot cores and hot corinos, but the molecular abundances are closer to the values found towards hot corinos than to values found towards hot cores. Considering the compactness of the warm region and its moderate temperature, we suggest that thermal desorption has not been completed towards this object yet, representing an early phase of the emergence of hot cores.
Article
Context. Linking atmospheric characteristics of planets to their formation pathways is a central theme in the study of extrasolar planets. Although the ¹² C/ ¹³ C isotope ratio shows little variation in the Solar System, the atmosphere of a super-Jupiter was recently shown to be rich in ¹³ CO, possibly as a result of dominant ice accretion beyond the CO snow line during its formation. Carbon isotope ratios are therefore suggested to be a potential tracer of formation pathways of planets. Aims. In this work, we aim to measure the ¹² CO/ ¹³ CO isotopologue ratio of a young, isolated brown dwarf. While the general atmospheric characteristics of young, low-mass brown dwarfs are expected to be very similar to those of super-Jupiters, their formation pathways may be different, leading to distinct isotopologue ratios. In addition, such objects allow high-dispersion spectroscopy at high signal-to-noise ratios. Methods. We analysed archival K -band spectra of the L dwarf 2MASS J03552337+1133437 taken with NIRSPEC at the Keck telescope. A free retrieval analysis was applied to the data using the radiative transfer code petitRADTRANS coupled with the nested sampling tool PyMultiNest to determine the isotopologue ratio ¹² CO/ ¹³ CO in its atmosphere. Results. The isotopologue ¹³ CO is detected in the atmosphere through the cross-correlation method at a signal-to-noise of ~8.4. The detection significance is determined to be ~9.5 σ using a Bayesian model comparison between two retrieval models (including or excluding ¹³ CO). We retrieve an isotopologue ¹² CO/ ¹³ CO ratio of 97 −18 ⁺²⁵ (90% uncertainty), marginally higher than the local interstellar standard. Its C/O ratio of ~0.56 is consistent with the solar value. Conclusions. Although only one super-Jupiter and one brown dwarf now have a measured ¹² CO/ ¹³ CO ratio, it is intriguing that they are different, possibly hinting to distinct formation pathways. Regardless of spectroscopic similarities, isolated brown dwarfs may experience a top-down formation via gravitational collapse, which resembles star formation, while giant exoplanets favourably form through core accretion, which potentially alters isotopologue ratios in their atmospheres depending on the material they accrete from protoplanetary disks. This further emphasises atmospheric carbon isotopologue ratio as a tracer of the formation history of exoplanets. In the future, analyses such as those presented here should be conducted on a wide range of exoplanets using medium-to-high-resolution spectroscopy to further assess planet formation processes.
Article
Context. Peptide-like bond molecules, which can take part in the formation of proteins in a primitive Earth environment, have been detected only towards a few hot cores and hot corinos up to now. Aims. We present a study of HNCO, HC(O)NH 2 , CH 3 NCO, CH 3 C(O)NH 2 , CH 3 NHCHO, CH 3 CH 2 NCO, NH 2 C(O)NH 2 , NH 2 C(O)CN, and HOCH 2 C(O)NH 2 towards the hot core G31.41+0.31. The aim of this work is to study these species together to allow a consistent study among them. Methods. We have used the spectrum obtained from the ALMA 3 mm spectral survey GUAPOS, with a spectral resolution of ~0.488 MHz (~1.3–1.7 km s ⁻¹ ) and an angular resolution of 1.′′2 × 1.′′2 (~4500 au), to derive column densities of all the molecular species presented in this work, together with 0.′′2 × 0.′′2 (~750 au) ALMA observations from another project to study the morphology of HNCO, HC(O)NH 2 , and CH 3 C(O)NH 2 . Results. We have detected HNCO, HC(O)NH 2 , CH 3 NCO, CH 3 C(O)NH 2 , and CH 3 NHCHO, but no CH 3 CH 2 NCO, NH 2 C(O)NH 2 , NH 2 C(O)CN, or HOCH 2 C(O)NH 2 . This is the first time that these molecules have been detected all together outside the Galactic centre. We have obtained molecular fractional abundances with respect to H 2 from 10 ⁻⁷ down to a few 10 ⁻⁹ and abundances with respect to CH 3 OH from 10 ⁻³ to ~4 × 10 ⁻² , and their emission is found to be compact (~2′′, i.e. ~7500 au). From the comparison with other sources, we find that regions in an earlier stage of evolution, such as pre-stellar cores, show abundances at least two orders of magnitude lower than those in hot cores, hot corinos, or shocked regions. Moreover, molecular abundance ratios towards different sources are found to be consistent between them within ~1 order of magnitude, regardless of the physical properties (e.g. different masses and luminosities), or the source position throughout the Galaxy. Correlations have also been found between HNCO and HC(O)NH 2 as well as CH 3 NCO and HNCO abundances, and for the first time between CH 3 NCO and HC(O)NH 2 , CH 3 C(O)NH 2 and HNCO, and CH 3 C(O)NH 2 and HC(O)NH 2 abundances. These results suggest that all these species are formed on grain surfaces in early evolutionary stages of molecular clouds, and that they are subsequently released back to the gas phase through thermal desorption or shock-triggered desorption.
Article
Context. Complex organic molecules (COMs) are often observed toward embedded Class 0 and I protostars. However, not all Class 0 and I protostars exhibit COM emission. Aims. The aim is to study variations in methanol (CH 3 OH) emission and use this as an observational tracer of hot cores to test if the absence of CH 3 OH emission can be linked to source properties. Methods. A sample of 148 low-mass and high-mass protostars is investigated using new and archival observations with the Atacama Large Millimeter/submillimeter Array (ALMA) that contain lines of CH 3 OH and its isotopologues. Data for an additional 36 sources are added from the literature, giving a total of 184 different sources. The warm ( T ≳ 100 K) gaseous CH 3 OH mass, M CH3OH , is determined for each source using primarily optically thin isotopologues and is compared to a simple toy model of a spherically symmetric infalling envelope that is passively heated by the central protostar. Results. A scatter of more than four orders of magnitude is found for M CH3OH among the low-mass protostars, with values ranging between 10 ⁻⁷ M ⊙ and ≲10 ⁻¹¹ M ⊙ . On average, Class I protostellar systems seem to have less warm M CH3OH (≲10 ⁻¹⁰ M ⊙ ) than younger Class 0 sources (~10 ⁻⁷ M ⊙ ). High-mass sources in our sample show more warm M CH3OH , up to ~10 ⁻⁷ −10 ⁻³ M ⊙ . To take into account the effect of the source’s overall mass on M CH3OH , a normalized CH 3 OH mass is defined as M CH3OH / M dust,0 , where M dust,0 is the cold plus warm dust mass in the disk and inner envelope within a fixed radius measured from the ALMA dust continuum. A correlation between M CH3OH / M dust,0 and L bol is found. Excluding upper limits, a simple power-law fit to the normalized warm CH 3 OH masses results in M CH3OH / M dust,0 ∝ L bol 0.70±0.05 over an L bol range of 10 ⁻¹ −10 ⁶ L ⊙ . This is in good agreement with the toy model, which predicts that the normalized M CH3OH increases with L bol 0.70 due to the snow line moving outward. Sources for which the size of the disk is equivalent to or smaller than the estimated 100 K radius fall within the 3 σ range of the best-fit power-law model, whereas sources with significantly larger disks show normalized warm CH 3 OH masses that are up to two orders of magnitude lower. Conclusions. The agreement between sources that are rich in CH 3 OH with the toy model of a spherically symmetric infalling envelope implies that the thermal structure of the envelopes in these sources is likely not strongly affected by a disk. However, based on the disagreement between the toy model and sources that show less warm CH 3 OH mass, we suggest that source structure such as a disk can result in colder gas and thus fewer COMs in the gas phase. Additionally, optically thick dust can hide the emission of COMs. Advanced modeling is necessary to quantify the effects of a disk and/or continuum optical depth on the presence of gaseous COMs in young protostellar systems.
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Characterizing the molecular composition of solar-type protostars is useful for improving our understanding of the physico-chemical conditions under which the Sun and its planets formed. In this work, we analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) data of the Protostellar Interferometric Line Survey (PILS), an unbiased spectral survey of the solar-type protostar IRAS 16293–2422, and we tentatively detected 3-hydroxypropenal (HOCHCHCHO) for the first time in the interstellar medium towards source B. Based on the observed line intensities and assuming local thermodynamic equilibrium, its column density is constrained to be ∼10 ¹⁵ cm ⁻² , corresponding to an abundance of 10 ⁻⁴ relative to methanol, CH 3 OH. Additional spectroscopic studies are needed to constrain the excitation temperature of this molecule. We included HOCHCHCHO and five of its isomers in the chemical network presented in Manigand et al. (2021, A&A, 645, A53) and we predicted their chemical evolution with the Nautilus code. The model reproduces the abundance of HOCHCHCHO within the uncertainties. This species is mainly formed through the grain surface reaction CH 2 CHO + HCO → HCOCH 2 CHO, followed by the tautomerization of HCOCH 2 CHO into HOCHCHCHO. Two isomers, CH 3 COCHO and CH 2 COHCHO, are predicted to be even more abundant than HOCHCHCHO. Spectroscopic studies of these molecules are essential in searching for them in IRAS 16293–2422 and other astrophysical sources.
Article
Investigating the physical and chemical structures of massive star-forming regions is critical for understanding the formation and the early evolution of massive stars. We performed a detailed line survey toward six dense cores named as MM1, MM4, MM6, MM7, MM8, and MM11 in G9.62+0.19 star-forming region resolved in ALMA band 3 observations. Toward these cores, about 172 transitions have been identified and attributed to 16 species including organic Oxygen-, Nitrogen-, Sulfur-bearing molecules and their isotopologues. Four dense cores MM7, MM8, MM4, and MM11 are line rich sources. Modeling of these spectral lines reveals the rotational temperature in a range of 72−115 K, 100−163 K, 102−204 K, and 84−123 K for the MM7, MM8, MM4, and MM11, respectively. The molecular column densities are 1.6 × 1015–9.2 × 1017 cm−2 toward the four cores. The cores MM8 and MM4 show chemical difference between Oxygen- and Nitrogen-bearing species, i.e. MM4 is rich in oxygen-bearing molecules while nitrogen-bearing molecules especially vibrationally excited HC3N lines are mainly observed in MM8. The distinct initial temperature at accretion phase may lead to this N/O differentiation. Through analyzing column densities and spatial distributions of O-bearing Complex Organic Molecules (COMs), we found that C2H5OH and CH3OCH3 might have a common precursor, CH3OH. CH3OCHO and CH3OCH3 are likely chemically linked. In addition, the observed variation in HC3N and HC5N emission may indicate that their different formation mechanism at hot and cold regions.
Article
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Context. The presence of many interstellar complex organic molecules (COMs) in the gas phase in the vicinity of protostars has long been associated with their formation on icy dust grain surfaces before the onset of protostellar activity, and their subsequent thermal co-desorption with water, the main constituent of the grains’ ice mantles, as the protostar heats its environment to ~100 K. Aims. Using the high angular resolution provided by the Atacama Large Millimetre/submillimetre Array (ALMA), we want to resolve the COM emission in the hot molecular core Sagittarius B2 (N1) and thereby shed light on the desorption process of COMs in hot cores. Methods. We used data taken as part of the 3 mm spectral line survey Re-exploring Molecular Complexity with ALMA (ReMoCA) to investigate the morphology of COM emission in Sagittarius B2 (N1). We also used ALMA continuum data at 1 mm taken from the literature. Spectra of ten COMs (including one isotopologue) were modelled under the assumption of local thermodynamic equilibrium (LTE) and population diagrams were derived for these COMs for positions at various distances to the south and west from the continuum peak. Based on this analysis, we produced resolved COM rotation temperature and column density profiles. H 2 column density profiles were derived from dust continuum emission and C ¹⁸ O 1–0 emission and used to derive COM abundance profiles as a function of distance and temperature. These profiles are compared to astrochemical models. Results. Based on the morphology, a rough separation into O- and N-bearing COMs can be done. The temperature profiles span a range of 80–300 K with power-law indices from −0.4 to −0.8, which is in agreement with expectations of protostellar heating of an envelope with optically thick dust. Column density and abundance profiles reflect a similar trend as seen in the morphology. While abundances of N-bearing COMs peak only at the highest temperatures, those of most O-bearing COMs peak at lower temperatures and remain constant or decrease towards higher temperatures. Many abundance profiles show a steep increase at ~100 K. To a great extent, the observed results agree with results of astrochemical models that, besides the co-desorption with water, predict that O-bearing COMs are mainly formed on dust-grain surfaces at low temperatures, while at least some N-bearing COMs and CH 3 CHO are substantially formed in the gas phase at higher temperatures. Conclusions. Our observational results, in comparison with model predictions, suggest that COMs that are exclusively or, to a great extent, formed on dust grains desorb thermally at ~100 K from the grain surface, likely alongside water. A dependence on the COM binding energy is not evident from our observations. Non-zero abundance values below ~100 K suggest that another desorption process of COMs is at work at these low temperatures: either non-thermal desorption or partial thermal desorption related to the lower binding energies experienced by COMs in the outer, water-poor ice layers. In either case, this is the first time that the transition between two regimes of COM desorption has been resolved in a hot core.
Article
Previously, the Coriolis coupled ν6 and ν8 bands of trans-DCOOH were analysed using a high-resolution infrared spectrum that only contained lines from the ν6 fundamental; the ν8 fundamental was too weak to be directly observed [Goh et al., Spectrochim. Acta A: Mol. Biomol. Spectrosc. 1999, 55, 1309]. Using a long pathlength multireflection cell and highly brilliant synchrotron radiation, we were able to observe the ν8 fundamental with excellent signal-to-noise (in addition to the ν6 fundamental). Analysis of the spectra using PGOPHER and SPFIT resulted in determination of an extensive set of molecular parameters, including two a-axis and four b-axis Coriolis coupling constants. The set features a significant refinement of the ν8=1 constants, with a band origin at 873.3848778(83) cm⁻¹, which is ∼0.72 cm⁻¹ higher than previously determined.
Article
We prepared a sample of mono-deuterated oxirane and studied its rotational spectrum in the laboratory between 490 and 1060 GHz in order to improve its spectroscopic parameters and consequently the calculated rest frequencies of its rotational transitions. The updated rest frequencies were employed to detect c-C2H3DO for the first time in the interstellar medium in the Atacama Large Millimetre/submillimetre Array Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293−2422. Fits of the detected lines using the rotation diagrams yield a temperature of Trot = 103 ± 19 K, which in turn agrees well with 125 K derived for the c-C2H4O main isotopologue previously. The c-C2H3DO to c-C2H4O ratio is found to be ∼0.15 corresponding to a D-to-H ratio of ∼0.036 per H atom, which is slightly higher than the D-to-H ratio of species such as methanol, formaldehyde, and ketene but lower than those of the larger complex organic species such as ethanol, methyl formate, and glycolaldehyde. This may reflect that oxirane is formed fairly early in the evolution of the prestellar cores. The identification of doubly deuterated oxirane isotopomers in the PILS data may be possibly judged by the amount of mono-deuterated oxirane and the observed trend that multiply deuterated isotopologues have higher deuteration rates than their mono-deuterated variants.
Article
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Recent astrochemical models and experiments have explained that complex organic molecules (COMs; molecules composed of six or more atoms) are produced on the dust grain mantles in cold and dense gas in prestellar cores. However, the detailed chemical processes and the roles of physical conditions on chemistry are still far from understood. To address these questions, we investigated 12 high-mass star-forming regions using Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations. They are associated with 44/95 GHz class I and 6.7 GHz class II CH 3 OH masers, indicative of undergoing active accretion. We found 28 hot cores with COM emission among 68 continuum peaks at 1.3 mm and specified 10 hot cores associated with 6.7 GHz class II CH 3 OH masers. Up to 19 COMs are identified including oxygen- and nitrogen-bearing molecules and their isotopologues in cores. The derived abundances show a good agreement with those from other low- and high-mass star-forming regions, implying that the COM chemistry is predominantly set by the ice chemistry in the prestellar core stage. One clear trend is that the COM detection rate steeply grows with the gas column density, which can be attributed to the efficient formation of COMs in dense cores. In addition, cores associated with a 6.7 GHz class II CH 3 OH maser tend to be enriched with COMs. Finally, our results suggest that the enhanced abundances of several molecules in our hot cores could be originated by the active accretion as well as different physical conditions of cores.
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Context. Several sugar-like molecules have been found in the interstellar medium (ISM). The molecule studied in this work, 2-hydroxyprop-2-enal, is among the candidates to be searched for, as it is a dehydration product of C 3 sugars and contains structural motifs that are typical for some interstellar molecules. Furthermore, it has recently been predicted that it is more abundant in the ISM than its tentatively detected isomer 3-hydroxypropenal. Aims. So far, only low-frequency microwave data of 2-hydroxyprop-2-enal have been published. The aim of this work is to deepen our knowledge about the millimetre-wave spectrum of 2-hydroxyprop-2-enal, enabling its detailed search towards astronomical objects. In particular, we target the solar-type protostar IRAS 16293-2422 and the star-forming region Sagittarius (Sgr) B2(N). Methods. The rotational spectrum of 2-hydroxyprop-2-enal was measured and analysed in the frequency regions of 128-166 GHz and 285-329 GHz. The interstellar exploration towards IRAS 16293-2422 was based on the Atacama Large Millimeter/submillimeter Array (ALMA) data of the Protostellar Interferometric Line Survey (PILS). We also used the imaging spectral line survey ReMoCA performed with ALMA towards Sgr B2(N) to search for 2-hydroxyprop-2-enal in the ISM. We modelled the astronomical spectra under the assumption of local thermodynamic equilibrium (LTE). Results. We provide laboratory analysis of hundreds of rotational transitions of 2-hydroxyprop-2-enal in the ground state and the lowest lying excited vibrational state. We report its non-detection towards IRAS 16293 B. The 2-hydroxyprop-2-enal/3-hydroxypropenal abundance ratio is estimated to be ≲0.9–1.3, in agreement with the predicted value of ~1.4. We report the non-detection of 2-hydroxyprop-2-enal towards the hot molecular core Sgr B2(N1), and we did not detect the related aldehydes 2-hydroxypropanal and 3-hydroxypropenal either. We find that these three molecules are at least nine, four, and ten times less abundant than acetaldehyde in this source, respectively. Conclusions. Despite the non-detections of 2-hydroxyprop-2-enal, the results of this work represent a significant improvement on previous investigations in the microwave region and meet the requirements for further searches for this molecule in the ISM.
Article
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Context . The deuteration of molecules forming in the ices such as methanol (CH 3 OH) is sensitive to the physical conditions during their formation in dense cold clouds and can be probed through observations of deuterated methanol in hot cores. Aims . The aim is to determine the D/H ratio of methanol for a large sample of 99 high-mass protostars and to link this to the physical conditions during the formation of methanol in the prestellar phases. Methods . Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) containing transitions of CH 3 OH, CH 2 DOH, CHD 2 OH, ¹³ CH 3 OH, and CH 3 ¹⁸ OH are investigated. The column densities of CH 2 DOH, CHD 2 OH, and CH 3 OH are determined for all sources, where the column density of CH 3 OH is derived from optically thin ¹³ C and ¹⁸ O isotopologues. Consequently, the D/H ratio of methanol is derived taking statistical effects into account. Results . Singly deuterated methanol (CH 2 DOH) is detected at the 3σ level toward 25 of the 99 sources in our sample of the high-mass protostars. Including upper limits, the (D/H) CH 3 OH ratio inferred from N CH 2 DOH / N CH 3 OH was derived for 38 of the 99 sources and varies between ~10−3-10−2. Including other high-mass hot cores from the literature, the mean methanol D/H ratio is 1.1 ± 0.7 × 10−3. This is more than one order of magnitude lower than what is seen for low-mass protostellar systems (2.2 ± 1.2 × 10−2). Doubly deuterated methanol (CHD 2 OH) is detected at the 3σ level toward 11 of the 99 sources. Including upper limits for 15 sources, the (D/H) CH 2 DOH ratios derived from N CHD 2 OH / N CH 2 DOH are more than two orders of magnitude higher than (D/H) CH 3 OH with an average of 2.0 ± 0.8 × 10−1 which is similar to what is found for low-mass sources. Comparison with literature GRAINOBLE models suggests that the high-mass prestellar phases are either warm (>20 K) or live shorter than the free-fall timescale. In contrast, for low-mass protostars, both a low temperature of <15 K and a prestellar phase timescale longer than the free-fall timescale are necessary. Conclusions . The (D/H) CH 3 OH ratio drops by more than an order of magnitude between low-mass and high-mass protostars due to either a higher temperature during the prestellar phases or shorter prestellar phases. However, successive deuteration toward CHD 2 OH seems equally effective between low-mass and high-mass systems.
Article
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The chemical diversity of low-mass protostellar sources has so far been recognized, and environmental effects are invoked as its origin. In this context, observations of isolated protostellar sources without the influence of nearby objects are of particular importance. Here, we report the chemical and physical structures of the low-mass Class 0 protostellar source IRAS 16544−1604 in the Bok globule CB 68, based on 1.3 mm Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of ∼70 au that were conducted as part of the large program FAUST. Three interstellar saturated complex organic molecules (iCOMs), CH 3 OH, HCOOCH 3 , and CH 3 OCH 3 , are detected toward the protostar. The rotation temperature and the emitting region size for CH 3 OH are derived to be 131 ± 11 K and ∼10 au, respectively. The detection of iCOMs in close proximity to the protostar indicates that CB 68 harbors a hot corino. The kinematic structure of the C ¹⁸ O, CH 3 OH, and OCS lines is explained by an infalling–rotating envelope model, and the protostellar mass and the radius of the centrifugal barrier are estimated to be 0.08–0.30 M ⊙ and <30 au, respectively. The small radius of the centrifugal barrier seems to be related to the small emitting region of iCOMs. In addition, we detect emission lines of c-C 3 H 2 and CCH associated with the protostar, revealing a warm carbon-chain chemistry on a 1000 au scale. We therefore find that the chemical structure of CB 68 is described by a hybrid chemistry. The molecular abundances are discussed in comparison with those in other hot corino sources and reported chemical models.
Preprint
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Recent astrochemical models and experiments have explained that complex organic molecules (COMs; molecules composed of six or more atoms) are produced on the dust grain mantles in cold and dense gas in prestellar cores. However, the detailed chemical processes and the roles of physical conditions on chemistry are still far from understood. To address these questions, we investigated twelve high-mass star-forming regions using the ALMA band 6 observations. They are associated with 44/95GHz class I and 6.7 GHz class II CH$_{3}$OH masers, indicative of undergoing active accretion. We found 28 hot cores with COMs emission among 68 continuum peaks at 1.3 mm and specified 10 hot cores associated with 6.7 GHz class II CH$_{3}$OH masers. Up to 19 COMs are identified including oxygen- and nitrogen-bearing molecules and their isotopologues in cores. The derived abundances show a good agreement with those from other low- and high-mass star-forming regions, implying that the COMs chemistry is predominantly set by the ice chemistry in the prestellar core stage. One clear trend is that the COMs detection rate steeply grows with the gas column density, which can be attributed to the efficient formation of COMs in dense cores. In addition, cores associated with a 6.7 GHz class II CH$_{3}$OH maser tend to be enriched with COMs. Finally, our results suggest that the enhanced abundances of several molecules in our hot cores could be originated by the active accretion as well as different physical conditions of cores.
Article
We present Atacama Large Millimeter Array band 6/7 (1.3 mm/0.87 mm) and Very Large Array Ka-band (9 mm) observations toward NGC 2071 IR, an intermediate-mass star-forming region. We characterize the continuum and associated molecular line emission toward the most luminous protostars, i.e., IRS1 and IRS3, on ∼100 au (0.″2) scales. IRS1 is partly resolved in the millimeter and centimeter continuum, which shows a potential disk. IRS3 has a well-resolved disk appearance in the millimeter continuum and is further resolved into a close binary system separated by ∼40 au at 9 mm. Both sources exhibit clear velocity gradients across their disk major axes in multiple spectral lines including C ¹⁸ O, H 2 CO, SO, SO 2 , and complex organic molecules like CH 3 OH, ¹³ CH 3 OH, and CH 3 OCHO. We use an analytic method to fit the Keplerian rotation of the disks and give constraints on physical parameters with a Markov Chain Monte Carlo routine. The IRS3 binary system is estimated to have a total mass of 1.4–1.5 M ⊙ . IRS1 has a central mass of 3–5 M ⊙ based on both kinematic modeling and its spectral energy distribution, assuming that it is dominated by a single protostar. For both IRS1 and IRS3, the inferred ejection directions from different tracers, including radio jet, water maser, molecular outflow, and H 2 emission, are not always consistent, and for IRS1 these can be misaligned by ∼50°. IRS3 is better explained by a single precessing jet. A similar mechanism may be present in IRS1 as well but an unresolved multiple system in IRS1 is also possible.
Article
The ALMA interferometer, with its unprecedented combination of high-sensitivity and high-angular resolution, allows for (sub-)mm wavelength mapping of protostellar systems at Solar System scales. Astrochemistry has benefited from imaging interstellar complex organic molecules in these jet-disk systems. Here we report the first detection of methanol (CH3OH) and methyl formate (HCOOCH3) emission towards the triple protostellar system VLA1623–2417 A1+A2+B, obtained in the context of the ALMA Large Program FAUST. Compact methanol emission is detected in lines from Eu = 45 K up to 61 K and 537 K towards components A1 and B, respectively. LVG analysis of the CH3OH lines towards VLA1623–2417 B indicates a size of 0${_{.}^{\prime\prime}}$11–0${_{.}^{\prime\prime}}$34 (14-45 au), a column density N(CH3OH) = 1016–1017 cm−2, kinetic temperature ≥ 170 K, and volume density ≥ 108 cm−3. An LTE approach is used for VLA1623–2417 A1, given the limited Eu range, and yields Trot ≤ 135 K. The methanol emission around both VLA1623–2417 A1 and B shows velocity gradients along the main axis of each disk. Although the axial geometry of the two disks is similar, the observed velocity gradients are reversed. The CH3OH spectra from B shows two broad (4–5 km s−1) peaks, which are red- and blue-shifted by ∼ 6–7 km s−1 from the systemic velocity. Assuming a chemically enriched ring within the accretion disk, close to the centrifugal barrier, its radius is calculated to be 33 au. The methanol spectra towards A1 are somewhat narrower (∼ 4 km s−1), implying a radius of 12–24 au.
Article
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Context. Deuteration is a precious tool for investigating the origin and formation routes of interstellar complex organic molecules in the different stages of the star formation process. Methyl cyanide (CH 3 CN) is one of the most abundant interstellar complex organic molecules (iCOMs); it is of particular interest because it is among the very few iCOMs detected not only around protostars but also in protoplanetary disks. However, its formation pathways are not well known and only a few measurements of its deuterated isotopologue (CH 2 DCN) have been made to date. Aims. We studied the line emission from CH 3 CN and its deuterated isotopologue CH 2 DCN towards the prototypical Class I object SVS13-A, where the deuteration of a large number of species has already been reported. Our goal is to measure the CH 3 CN deuteration in a Class I protostar, for the first time, in order to constrain the CH 3 CN formation pathways and the chemical evolution from the early prestellar core and Class 0 to the evolved Class I stages. Methods. We imaged CH 2 DCN towards SVS13-A using the IRAM NOEMA interferometer at 3mm in the context of the Large Program SOLIS (with a spatial resolution of 1″.8 × 1″.2). The NOEMA images were complemented by the CH 3 CN and CH 2 DCN spectra collected by the IRAM-30m Large Program ASAI, which provided an unbiased spectral survey at 3 mm, 2 mm, and 1.3 mm. The observed line emission was analysed using local thermodynamic equilibrium (LTE) and non-LTE large velocity gradient (LVG) approaches. Results. The NOEMA/SOLIS images of CH 2 DCN show that this species emits in an unresolved area centred towards the SVS13-A continuum emission peak, suggesting that methyl cyanide and its isotopologues are associated with the hot corino of SVS13-A, previously imaged via other iCOMs. In addition, we detected 41 and 11 ASAI transitions of CH 3 CN and CH 2 DCN, respectively, which cover upper level energies ( E up ) from 13 to 442 K and from 18 K to 200 K. The non-LTE LVG analysis of the CH 3 CN lines points to a kinetic temperature of (140 ± 20) K, a gas density n H2 ≥ 10 ⁷ cm ⁻³ , and an emitting size of ~0″.3, in agreement with the hypothesis that CH 3 CN lines are emitted in the SVS13-A hot corino. The derived [CH 2 DCN]/[CH 3 CN] ratio is ~9%. This value is consistent with those measured towards prestellar cores and a factor 2–3 higher than those measured in Class 0 protostars. Conclusions. Contrarily to what expected for other molecular species, the CH 3 CN deuteration does not show a decrease in SVS13-A with respect to measurements in younger prestellar cores and Class 0 protostars. Finally, we discuss why our new results suggest that CH 3 CN was likely synthesised via gas-phase reactions and frozen onto the dust grain mantles during the cold prestellar phase.
Article
Methanol (CH 3 OH) is an abundant interstellar species and is known to be an important precursor of various interstellar complex organic molecules. Among the methanol isotopologues, CH 2 DOH is one of the most abundant isotopologues and it is often used to study the deuterium fractionation of CH 3 OH in interstellar medium. However, the emission lines of CH 2 DOH can sometimes be optically thick, making the derivation of its abundance unreliable. Therefore, observations of its presumably optically thin ¹³ C substituted species, ¹³ CH 2 DOH, are essential to overcome this issue. In this study, the rotational transitions of ¹³ CH 2 DOH have been measured in the millimeter-wave region from 216 GHz to 264 GHz with an emission-type millimeter- and submillimeter-wave spectrometer by using a deuterium and ¹³ C enriched sample. The frequency accuracy of measured ¹³ CH 2 DOH is less than a few kHz, and the relative line intensity error is less than 10% in most of the frequency range by taking advantage of the wide simultaneous frequency-coverage of the emission-type spectrometer. These results offer a good opportunity to detect ¹³ CH 2 DOH in space, which will allow us to study the deuterium fractionation of CH 3 OH in various sources through accurate determination of the CH 2 DOH abundance.
Preprint
We present ALMA band 6/7 (1.3 mm/0.87 mm) and VLA Ka band (9 mm) observations toward NGC 2071 IR, an intermediate-mass star forming region. We characterize the continuum and associated molecular line emission towards the most luminous protostars, i.e., IRS1 and IRS3, on ~100 au (0. 2") scales. IRS1 is partly resolved in millimeter and centimeter continuum, which shows a potential disk. IRS3 has a well resolved disk appearance in millimeter continuum and is further resolved into a close binary system separated by ~40 au at 9 mm. Both sources exhibit clear velocity gradients across their disk major axes in multiple spectral lines including C18O, H2CO, SO, SO2, and complex organic molecules like CH3OH, 13CH3OH and CH3OCHO. We use an analytic method to fit the Keplerian rotation of the disks, and give constraints on physical parameters with a MCMC routine. The IRS3 binary system is estimated to have a total mass of 1.4-1.5$M_\odot$. IRS1 has a central mass of 3-5$M_\odot$ based on both kinematic modeling and its spectral energy distribution, assuming that it is dominated by a single protostar. For both IRS1 and IRS3, the inferred ejection directions from different tracers, including radio jet, water maser, molecular outflow, and H2 emission, are not always consistent, and for IRS1, these can be misaligned by ~50$^{\circ}$. IRS3 is better explained by a single precessing jet. A similar mechanism may be present in IRS1 as well but an unresolved multiple system in IRS1 is also possible.
Article
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We present Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations of the close (0.″3 = 90 au separation) protobinary system SVS 13. We detect two small circumstellar disks (radii ∼12 and ∼9 au in dust, and ∼30 au in gas) with masses of ∼0.004–0.009 M ☉ for VLA 4A (the western component) and ∼0.009–0.030 M ☉ for VLA 4B (the eastern component). A circumbinary disk with prominent spiral arms extending ∼500 au and a mass of ∼0.052 M ☉ appears to be in the earliest stages of formation. The dust emission is more compact and with a very high optical depth toward VLA 4B, while toward VLA 4A the dust column density is lower, allowing the detection of stronger molecular transitions. We infer rotational temperatures of ∼140 K, on scales of ∼30 au, across the whole source, and a rich chemistry. Molecular transitions typical of hot corinos are detected toward both protostars, being stronger toward VLA 4A, with several ethylene glycol transitions detected only toward this source. There are clear velocity gradients, which we interpret in terms of infall plus rotation of the circumbinary disk, and pure rotation of the circumstellar disk of VLA 4A. We measured orbital proper motions and determined a total stellar mass of 1 M ☉ . From the molecular kinematics, we infer the geometry and orientation of the system, and stellar masses of ∼0.26 M ☉ for VLA 4A and ∼0.60 M ☉ for VLA 4B.
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Context. The detection of a branched alkyl molecule in the high-mass star forming protocluster Sagittarius (Sgr) B2(N) permitted by the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) revealed a new dimension of interstellar chemistry. Astrochemical simulations subsequently predicted that beyond a certain degree of molecular complexity, branched molecules could even dominate over their straight-chain isomers. Aims. More generally, we aim to probe further the presence in the interstellar medium of complex organic molecules with the capacity to exhibit both a normal and iso form, via the attachment of a functional group to either a primary or secondary carbon atom. Methods. We used the imaging spectral line survey ReMoCA performed with ALMA at high angular resolution and the results of a recent spectroscopic study of propanol to search for the iso and normal isomers of this molecule in the hot molecular core Sgr B2(N2). We analyzed the interferometric spectra under the assumption of local thermodynamical equilibrium. We expanded the network of the astrochemical model MAGICKAL to explore the formation routes of propanol and put the observational results in a broader astrochemical context. Results. We report the first interstellar detection of iso-propanol, ¿-C 3 H 7 OH, toward a position of Sgr B2(N2) that shows narrow linewidths. We also report the first secure detection of the normal isomer of propanol, n-C 3 H 7 OH, in a hot core. Iso-propanol is found to be nearly as abundant as normal-propanol, with an abundance ratio of 0.6 which is similar to the ratio of 0.4 that we obtained previously for iso- and normal-propyl cyanide in Sgr B2(N2) at lower angular resolution with our previous ALMA survey, EMoCA. The observational results are in good agreement with the outcomes of our astrochemical models, which indicate that the OH-radical addition to propylene in dust-grain ice mantles, driven by water photodissociation, can produce appropriate quantities of normal- and iso-propanol. The normal-to-iso ratio in Sgr B2(N2) may be a direct inheritance of the branching ratio of this reaction process. Conclusions. The detection of normal- and iso-propanol and their ratio indicate that the modest preference for the normal form of propyl cyanide determined previously may be a more general feature among similarly sized interstellar molecules. Detecting other pairs of interstellar organic molecules with a functional group attached either to a primary or secondary carbon may help in pinning down the processes that dominate in setting their normal-to-iso ratios. Butanol and its isomers would be the next obvious candidates in the alcohol family, but their detection in hot cores will be challenging.
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Context. The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In the particular case of simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions taking place on top of interstellar dust grains may explain the isomer interconversion at low temperatures. Aims. We studied the isomerization processes of formic and thioformic acid that are likely to take place on the surface of interstellar dust grains after being initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to expand our understanding of the key precursors of organic molecules in space. Methods. We determined the rate constants for the H abstraction reactions as well as the binding energies for the acids on water ice using ab initio calculations and the instanton method for calculating the rate constants, including quantum tunneling. In addition, we tested the viability of the deuteration of formic acid with tailored experiments and looked for it on the L1544 source. Results. For formic acid, there is a clear dependence of the H abstraction reactions on the isomer of the reactant, with rate constants at ~50 K that differ by five orders of magnitude. Correspondingly, we did not observe the trans-cis reaction in our experiments. In the case of thioformic acid, a very similar cis-trans reactivity is found for abstraction reactions at the thiol (-SH) group in contrast to a preferential reactivity that is found when abstractions take place at the -CH moiety. We found comparable binding energies for both isomers with average binding energies of around −6200 and −3100 K for formic and thioformic acid, respectively. Our binding energy calculations show that the reactions are precluded for specific orientations, affecting the overall isomerization rate. For H abstractions initiated by deuterium atoms, we found very similar trends, with kinetic isotope effects varying in most cases between 13 and 20. Conclusions. Our results support the cis-trans interconversion of cis-formic acid on dust grains, suggesting that such an acid should not withstand the conditions found on these objects. On the other hand, the trans isomer is very resilient. Both isomers of thioformic acid are much more reactive. A non-trivial chemistry is behind the apparent excess of its trans isomer that is found in cold molecular clouds and star-forming regions due to a subtle combination of preferential reactivity and binding with the surface. In light of our results, all the deuterated counterparts of thioformic acid are viable molecules to be present on the ISM. In contrast, only the trans isomer of deuterated formic acid is expected, for which we provide upper bounds of detection. Given the mechanisms presented in this paper, other mechanisms must be at play to explain the tiny fraction of cis-formic acid observed in interstellar cold environments, as well as the current trans-DCOOH and trans-HCOOD abundances in hot-corinos.
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Understanding the chemical past of our Sun and how life appeared on Earth is no mean feat. The best strategy we can adopt is to study newborn stars located in an environment similar to the one in which our Sun was born and assess their chemical content. In particular, hot corinos are prime targets because recent studies have shown correlations between interstellar complex organic molecules abundances from hot corinos and comets. The ORion ALMA New GEneration Survey aims to assess the number of hot corinos in the closest and best analog to our Sun’s birth environment, the OMC-2/3 filament. In this context, we investigated the chemical nature of 19 solar-mass protostars and found that 26% of our sample sources show warm methanol emission indicative of hot corinos. Compared to the Perseus low-mass star-forming region, where the PErseus ALMA CHEmistry Survey detected hot corinos in ∼60% of the sources, the hot corinos seem to be relatively scarce in the OMC-2/3 filament. While this suggests that the chemical nature of protostars in Orion and Perseus is different, improved statistics is needed in order to consolidate this result. If the two regions are truly different, this would indicate that the environment is likely playing a role in shaping the chemical composition of protostars.
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The presence of complex organic molecules (COMs) in the interstellar medium is of great interest since it may link to the origin and prevalence of life in the universe. Aiming to investigate the occurrence of COMs and their possible origins, we conducted a chemical census toward a sample of protostellar cores as part of the Atacama Large Millimeter/submillimeter Array Survey of Orion Planck Galactic Cold Clumps project. We report the detection of 11 hot corino sources, which exhibit compact emissions from warm and abundant COMs, among 56 Class 0/I protostellar cores. All of the hot corino sources discovered are likely Class 0, and their sizes of the warm region (>100 K) are comparable to 100 au. The luminosity of the hot corino sources exhibits positive correlations with the total number of methanol and the extent of its emissions. Such correlations are consistent with the thermal desorption picture for the presence of hot corinos and suggest that the lower-luminosity (Class 0) sources likely have a smaller region with COM emissions. With the same sample selection method and detection criteria being applied, the detection rates of the warm methanol in the Orion cloud (15/37) and the Perseus cloud (28/50) are statistically similar when the cloud distances and the limited sample size are considered. Observing the same set of COM transitions will bring a more informative comparison between the cloud properties.
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Deeply embedded protostars are actively fed from their surrounding envelopes through their protostellar disk. The physical structure of such early disks might be different from that of more evolved sources due to the active accretion. We present 1.3 and 3 mm ALMA continuum observations at resolutions of 6.5 au and 12 au respectively, towards the Class 0 source IRAS 16293-2422 B. The resolved brightness temperatures appear remarkably high, with Tb > 100 K within ∼30 au and Tb peak over 400 K at 3 mm. Both wavelengths show a lopsided emission with a spectral index reaching values less than 2 in the central ∼ 20 au region. We compare these observations with a series of radiative transfer calculations and synthetic observations of magnetohydrodynamic and radiation hydrodynamic protostellar disk models formed after the collapse of a dense core. Based on our results, we argue that the gas kinematics within the disk may play a more significant role in heating the disk than the protostellar radiation. In particular, our radiation hydrodynamic simulation of disk formation, including heating sources associated with gravitational instabilities, is able to generate the temperatures necessary to explain the high fluxes observed in IRAS 16293B. Besides, the low spectral index values are naturally reproduced by the high optical depth and high inner temperatures of the protostellar disk models. The high temperatures in IRAS 16293B imply that volatile species are mostly in the gas phase, suggesting that a self-gravitating disk could be at the origin of a hot corino.
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Searches for the prebiotically-relevant cyanamide (NH$_2$CN) towards solar-type protostars have not been reported in the literature. We here present the first detection of this species in the warm gas surrounding two solar-type protostars, using data from the Atacama Large Millimeter/Submillimeter Array Protostellar Interferometric Line Survey (PILS) of IRAS 16293-2422 B and observations from the IRAM Plateau de Bure Interferometer of NGC1333 IRAS2A. We furthermore detect the deuterated and $^{13}$C isotopologues of NH$_2$CN towards IRAS 16293-2422 B. This is the first detection of NHDCN in the interstellar medium. Based on a local thermodynamic equilibrium analysis, we find that the deuteration of cyanamide ($\sim$ 1.7%) is similar to that of formamide (NH$_2$CHO), which may suggest that these two molecules share NH$_2$ as a common precursor. The NH$_2$CN/NH$_2$CHO abundance ratio is about 0.2 for IRAS 16293-2422 B and 0.02 for IRAS2A, which is comparable to the range of values found for Sgr B2. We explored the possible formation of NH$_2$CN on grains through the NH$_2$ + CN reaction using the chemical model MAGICKAL. Grain-surface chemistry appears capable of reproducing the gas-phase abundance of NH$_2$CN with the correct choice of physical parameters.
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Context. The enhanced degrees of deuterium fractionation observed in envelopes around protostars demonstrate the importance of chemistry at low temperatures, relevant in pre- and protostellar cores. Formaldehyde is an important species in the formation of methanol and more complex molecules. Aims. Here, we aim to present the first study of formaldehyde deuteration on small scales around the prototypical low-mass protostar IRAS 16293–2422 using high spatial and spectral resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations. We determine the excitation temperature, abundances and fractionation level of several formaldehyde isotopologues, including its deuterated forms. Methods. Excitation temperature and column densities of formaldehyde in the gas close to one of the components of the binary were constrained through modeling of optically thin lines assuming local thermodynamical equilibrium. The abundance ratios were compared to results from previous single dish observations, astrochemical models and local ISM values. Results. Numerous isotopologues of formaldehyde are detected, among them H 2 C ¹⁷ O, and D 2 ¹³ CO for the first time in the ISM. The large range of upper energy levels covered by the HDCO lines help constrain the excitation temperature to 106 ± 13 K. Using the derived column densities, formaldehyde shows a deuterium fractionation of HDCO/H 2 CO = 6.5 ± 1%, D 2 CO/HDCO = 12.8 –4.1 +3.3 %, and D 2 CO/H 2 CO = 0.6(4) ± 0.1%. The isotopic ratios derived are ¹⁶ O/ ¹⁸ O = 805 –79 ⁺⁴³ , ¹⁸ O/ ¹⁷ O = 3.2 –0.3 +0.2 , and ¹² C/ ¹³ C = 56 –11 ⁺⁸ . Conclusions. The HDCO/H 2 CO ratio is lower than that found in previous studies, highlighting the uncertainties involved in interpreting single dish observations of the inner warm regions. The D 2 CO/HDCO ratio is only slightly larger than the HDCO/H 2 CO ratio. This is consistent with formaldehyde forming in the ice as soon as CO has frozen onto the grains, with most of the deuteration happening toward the end of the prestellar core phase. A comparison with available time-dependent chemical models indicates that the source is in the early Class 0 stage.
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As many organic molecules, formic acid (HCOOH) has two conformers (trans and cis). The energy barrier to internal conversion from trans to cis is much higher than the thermal energy available in molecular clouds. Thus, only the most stable conformer (trans) is expected to exist in detectable amounts. We report the first interstellar detection of cis-HCOOH. Its presence in ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation region), with a low trans-to-cis abundance ratio of 2.8+-1.0, supports a photoswitching mechanism: a given conformer absorbs a stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we specifically study with ab initio quantum calculations, was not considered in Space before but likely induces structural changes of a variety of interstellar molecules submitted to UV radiation.
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The laboratory work presented here simulates the chemistry on icy dust grains as typical for the ‘CO freeze-out stage’ in dark molecular clouds. It differs from previous studies in that solid-state hydrogenation and vacuum UV photoprocessing are applied simultaneously to co-depositing molecules. In parallel, the reactions at play are described for fully characterized laboratory conditions. The focus is on the formation of molecules containing both carbon and nitrogen atoms, starting with NO in CO-, H2CO-, and CH3OH-rich ices at 13 K. The experiments yield three important conclusions. (1) Without UV processing hydroxylamine (NH2OH) is formed, as reported previously. (2) With UV processing (energetic) NH2 is formed through photodissociation of NH2OH. This radical is key in the formation of species with an N–C bond. (3) The formation of three N–C bearing species, HNCO, OCN−, and NH2CHO, is observed. The experiments put a clear chemical link between these species; OCN− is found to be a direct derivative of HNCO and the latter is shown to have the same precursor as formamide (NH2CHO). Moreover, the addition of VUV competing channels decreases the amount of NO molecules converted into NH2OH by at least one order of magnitude. Consequently, this decrease in NH2OH formation yield directly influences the amount of NO molecules that can be converted into HNCO, OCN−, and NH2CHO.
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Formamide (NH2CHO) has previously been detected in several star-forming regions and is thought to be a precursor for different prebiotic molecules. Its formation mechanism is still debated, however. Observations of formamide, related species, and their isopotologues may provide useful clues to the chemical pathways leading to their formation. The Protostellar Interferometric Line Survey (PILS) represents an unbiased, high angular resolution and sensitivity spectral survey of the low-mass protostellar binary IRAS 16293–2422 with the Atacama Large Millimeter/submillimeter Array (ALMA). For the first time, we detect the three singly deuterated forms of NH2CHO (NH2CDO, cis- and trans-NHDCHO), as well as DNCO towards the component B of this binary source. The images reveal that the different isotopologues are all present in the same region. Based on observations of the 13C isotopologues of formamide and a standard 12C/ 13C ratio, the deuterium fractionation is found to be similar for the three different forms with a value of about 2%. The DNCO/HNCO ratio is also comparable to the D/H ratio of formamide (∼1%). These results are in agreement with the hypothesis that NH2CHO and HNCO are chemically related through grain-surface formation.
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Recent interferometer observations have found that the D2O/HDO abundance ratio is higher than that of HDO/H2O by about one order of magnitude in the vicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice has sublimated. Previous laboratory and theoretical studies show that the D2O/HDO ice ratio should be lower than the HDO/H2O ice ratio, if HDO and D2O ices are formed simultaneously with H2O ice. In this work, we propose that the observed feature, D2O/HDO > HDO/H2O, is a natural consequence of chemical evolution in the early cold stages of low-mass star formation: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient, and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced due to the drop of the ortho-para ratio of H2, the weaker UV radiation field, etc. Using a simple analytical model and gas-ice astrochemical simulations tracing the evolution from the formation of molecular clouds to protostellar cores, we show that the proposed scenario can quantitatively explain the observed HDO/H2O and D2O/HDO ratios. We also find that the majority of HDO and D2O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H2O ice is formed. This work demonstrates the power of the combination of the HDO/H2O and D2O/HDO ratios as a tool to reveal the past history of water ice formation in the early cold stages of star formation and when the enrichment of deuterium in the bulk of water occurred. Further observations are needed to explore if the relation, D2O/HDO > HDO/H2O, is common in low-mass protostellar sources.
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Over the past five decades, radio astronomy has shown that molecular complexity is a natural outcome of interstellar chemistry, in particular in star forming regions. However, the pathways that lead to the formation of complex molecules are not completely understood and the depth of chemical complexity has not been entirely revealed. In addition, the sulfur chemistry in the dense interstellar medium is not well understood. We want to know the relative abundances of alkanethiols and alkanols in the Galactic Center source Sagittarius B2(N2), the northern hot molecular core in Sgr B2(N), whose relatively small line widths are favorable for studying the molecular complexity in space. We investigated spectroscopic parameter sets that were able to reproduce published laboratory rotational spectra of ethanethiol and studied effects that modify intensities in the predicted rotational spectrum of ethanol. We used the Atacama Large Millimeter Array (ALMA) in its Cycles~0 and 1 for a spectral line survey of Sagittarius B2(N) between 84 and 114.4 GHz. These data were analyzed by assuming local thermodynamic equilibrium (LTE) for each molecule. Our observations are supplemented by astrochemical modeling; a new network is used for the first time that includes reaction pathways for alkanethiols. The column density ratios involving methanol, ethanol, and methanethiol in Sgr B2(N2) are similar to values reported for Orion KL, but those involving ethanethiol are significantly different and suggest that the detection of ethanethiol reported toward Orion KL is uncertain. Our chemical model presently does not permit the prediction of sufficiently accurate column densities of alkanethiols or their ratios among alkanethiols and alkanols. Therefore, additional observational results are required to establish the level of C2H5SH in the dense and warm interstellar medium with certainty.
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The deuterium fractionation of gas-phase molecules in hot cores is believed to reflect the composition of interstellar ices. The deuteration of methanol is a major puzzle, however, because the isotopologue ratio [CH2DOH]/[CH3OD], which is predicted to be equal to 3 by standard grain chemistry models, is much larger (~20) in low-mass hot corinos and significantly lower (~1) in high-mass hot cores. This dichotomy in methanol deuteration between low-mass and massive protostars is currently not understood. In this study, we report a simplified rate equation model of the deuterium chemistry occurring in the icy mantles of interstellar grains. We apply this model to the chemistry of hot corinos and hot cores, with IRAS 16293-2422 and the Orion~KL Compact Ridge as prototypes, respectively. The chemistry is based on a statistical initial deuteration at low temperature followed by a warm-up phase during which thermal hydrogen/deuterium (H/D) exchanges occur between water and methanol. The exchange kinetics is incorporated using laboratory data. The [CH2DOH]/[CH3OD] ratio is found to scale inversely with the D/H ratio of water, owing to the H/D exchange equilibrium between the hydroxyl (-OH) functional groups of methanol and water. Our model is able to reproduce the observed [CH2DOH]/[CH3OD] ratios provided that the primitive fractionation of water ice [HDO]/[H2O] is ~ 2% in IRAS 16293-2422 and ~0.6% in Orion~KL. We conclude that the molecular D/H ratios measured in hot cores may not be representative of the original mantles because molecules with exchangeable deuterium atoms can equilibrate with water ice during the warm-up phase.
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Context. The acetaldehyde molecule is ubiquitous in the interstellar medium of our galaxy, and due to its dense and complex spectrum, large dipole moment, and several low-lying torsional states, acetaldehyde is considered to be a "weed" molecule for radio astronomy observations. Mono-13C acetaldehydes 13CH3CHO and CH313CHO are likely to be identified in astronomical surveys, such as those available with the very sensitive ALMA telescope. Laboratory measurements and analysis of the millimeter and submillimeter-wave spectra are the prerequisites for the successful radioastronomical search for the new interstellar molecular species, as well as for new isotopologs of already detected interstellar molecules. Aims. In this context, to provide reliable predictions of 13CH3CHO and CH313CHO spectra in millimeter and submillimeter wave ranges, we study rotational spectra of these species in the frequency range from 50 to 945 GHz. Methods. The spectra of mono-13C acetaldehydes were recorded using the spectrometer based on Schottky-diode frequencymultiplication chains in the Lille laboratory. The rotational spectra of 13CH3CHO and CH313CHO molecules were analyzed using the Rho axis method. Results. In the recorded spectra we have assigned 6884 for the 13CH3CHO species and 6458 for CH313CHO species new rotational transitions belonging to the ground, first, and second excited torsional states. These measurements were fitted together with previously published data to the Hamiltonian models that use 91 and 87 parameters to achieve overall weighted rms deviations 0.88 for the 13CH3CHO species and 0.95 for CH313CHO. On the basis of the new spectroscopic results, predictions of transition frequencies in the frequency range up to 1 THz with J ≤ 60 and Ka ≤ 20 are presented for both isotopologs.
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This study focuses on the formation of two molecules of astrobiological importance – glycolaldehyde (HC(O)CH2OH) and ethylene glycol (H2C(OH)CH2OH) – by surface hydrogenation of CO molecules. Our experiments aim at simulating the CO freeze-out stage in interstellar dark cloud regions, well before thermal and energetic processing become dominant. It is shown that along with the formation of H2CO and CH3OH – two well-established products of CO hydrogenation – also molecules with more than one carbon atom form. The key step in this process is believed to be the recombination of two HCO radicals followed by the formation of a C–C bond. The experimentally established reaction pathways are implemented into a continuous-time random-walk Monte Carlo model, previously used to model the formation of CH3OH on astrochemical time-scales, to study their impact on the solid-state abundances in dense interstellar clouds of glycolaldehyde and ethylene glycol.
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Context. Dimethyl ether is found in high abundance in the interstellar medium. Owing to its strong and dense spectrum throughout the entire microwave and terahertz regime, it contributes to the spectral confusion in astronomical line surveys. The great sensitivity of new observatories like ALMA enhances the need for reliable spectroscopic data, especially for isotopic species of abundant molecules. In addition, the study of the interstellar C-12/C-13 isotopic ratio can be used as a tracer of the formation process of a molecular species, and thus it gives insight into the chemical evolution of the observed region. Aims. The interpretation of astronomical observations depends on the knowledge of accurate rest frequencies and intensities. The objective of this work is to provide spectroscopic data for the two C-13-isotopologues of dimethyl ether in the vibrational ground state. Methods. High-resolution rotational-torsional spectra of (CH3OCH3)-C-12-C-13 and ((CH3)-C-13)(2)O have been measured in the laboratory covering frequencies up to 1.5 THz. The analysis is based on an effective rotational Hamiltonian for molecules with two large-amplitude motions. Results. Predictions of the complete ground state rotational spectrum of dimethyl ether-C-13(1) and -C-13(2) up to 2 THz are presented with accuracies better than 1 MHz. Based on the laboratory work, transitions of (CH3OCH3)-C-12-C-13 dimethyl ether have been detected for the first time in a large submillimeter line survey of the high-mass star forming region G327.3-0.6 performed with the APEX telescope.
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Complex Organic Molecules (COMs) are considered crucial molecules, since they are connected with organic chemistry, at the basis of the terrestrial life. More pragmatically, they are molecules in principle difficult to synthetize in the harsh interstellar environments and, therefore, a crucial test for astrochemical models. Current models assume that several COMs are synthesised on the lukewarm grain surfaces ($\gtrsim$30-40 K), and released in the gas phase at dust temperatures $\gtrsim$100 K. However, recent detections of COMs in $\lesssim$20 K gas demonstrate that we still need important pieces to complete the puzzle of the COMs formation. We present here a complete census of the oxygen and nitrogen bearing COMs, previously detected in different ISM regions, towards the solar type protostar IRAS16293-2422. The census was obtained from the millimeter-submillimeter unbiased spectral survey TIMASSS. Six COMs, out of the 29 searched for, were detected: methyl cyanide, ketene, acetaldehyde, formamide, dimethyl ether, and methyl formate. The multifrequency analysis of the last five COMs provides clear evidence that they are present in the cold ($\lesssim$30 K) envelope of IRAS16293-2422, with abundances 0.03-2 $\times 10^{-10}$. Our data do not allow to support the hypothesis that the COMs abundance increases with increasing dust temperature in the cold envelope, as expected if COMs were predominately formed on the lukewarm grain surfaces. Finally, when considering also other ISM sources, we find a strong correlation over five orders of magnitude, between the methyl formate and dimethyl ether and methyl formate and formamide abundances, which may point to a link between these two couples of species, in cold and warm gas.
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Extremely large deuteration of several molecules has been observed towards prestellar cores and low-mass protostars for a decade. New observations performed towards low-mass protostars suggest that water presents a lower deuteration in the warm inner gas than in the cold external envelope. We coupled a gas-grain astrochemical model with a one-dimension model of collapsing core to properly follow the formation and the deuteration of interstellar ices as well as their subsequent evaporation in the low-mass protostellar envelopes with the aim of interpreting the spatial and temporal evolutions of their deuteration. The astrochemical model follows the formation and the evaporation of ices with a multilayer approach and also includes a state-of-the-art deuterated chemical network by taking the spin states of H$_2$ and light ions into account. Because of their slow formation, interstellar ices are chemically heterogeneous and show an increase of their deuterium fractionation towards the surface. The differentiation of the deuteration in ices induces an evolution of the deuteration within protostellar envelopes. The warm inner region is poorly deuterated because it includes the whole molecular content of ices while the deuteration predicted in the cold external envelope scales with the highly deuterated surface of ices. We are able to reproduce the observed evolution of water deuteration within protostellar envelopes but we are still unable to predict the super-high deuteration observed for formaldehyde and methanol. Finally, the extension of this study to the deuteration of complex organics (COMs), important for the prebiotic chemistry, shows a good agreement with the observations, suggesting that we can use the deuteration to retrace their mechanisms and their moments of formation.
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We present a comprehensive analysis of a broadband spectral line survey of the Orion Kleinmann-Low nebula (Orion KL), one of the most chemically rich regions in the Galaxy, using the HIFI instrument on board the Herschel Space Observatory. This survey spans a frequency range from 480 to 1907 GHz at a resolution of 1.1 MHz. These observations thus encompass the largest spectral coverage ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and include frequencies >1 THz, where the Earth's atmosphere prevents observations from the ground. In all, we detect emission from 39 molecules (79 isotopologues). Combining this data set with ground-based millimeter spectroscopy obtained with the IRAM 30 m telescope, we model the molecular emission from the millimeter to the far-IR using the XCLASS program, which assumes local thermodynamic equilibrium (LTE). Several molecules are also modeled with the MADEX non-LTE code. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. A reduced χ2 analysis indicates that models for most species reproduce the observed emission well. In particular, most complex organics are well fit by LTE implying gas densities are high (>106 cm–3) and excitation temperatures and column densities are well constrained. Molecular abundances are computed using H2 column densities also derived from the HIFI survey. The distribution of rotation temperatures, T rot, for molecules detected toward the hot core is significantly wider than the compact ridge, plateau, and extended ridge T rot distributions, indicating the hot core has the most complex thermal structure.
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A sensitive broadband molecular line survey of the Sagittarius B2(N) star-forming region has been obtained with the HIFI instrument on the Herschel Space Observatory, offering the first high-spectral resolution look at this well-studied source in a wavelength region largely inaccessible from the ground (625-157 um). From the roughly 8,000 spectral features in the survey, a total of 72 isotopologues arising from 44 different molecules have been identified, ranging from light hydrides to complex organics, and arising from a variety of environments from cold and diffuse to hot and dense gas. We present an LTE model to the spectral signatures of each molecule, constraining the source sizes for hot core species with complementary SMA interferometric observations, and assuming that molecules with related functional group composition are cospatial. For each molecule, a single model is given to fit all of the emission and absorption features of that species across the entire 480-1910 GHz spectral range, accounting for multiple temperature and velocity components when needed to describe the spectrum. As with other HIFI surveys toward massive star forming regions, methanol is found to contribute more integrated line intensity to the spectrum than any other species. We discuss the molecular abundances derived for the hot core, where the local thermodynamic equilibrium approximation is generally found to describe the spectrum well, in comparison to abundances derived for the same molecules in the Orion KL region from a similar HIFI survey.
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Based on new measurements carried out in the laboratory from 0.77 to 1.2 THz and on a line-frequency analysis of these new data, along with previously published data, we build a line list for HCOOCH2D that leads to its first detection in the Orion KL nebula. The observed lines, both in space and in the laboratory, involve the cis D-in-plane and trans D-out-of-plane conformations of HCOOCH2D and the two tunneling states arising from the large-amplitude motion connecting the two trans configurations. The model used in the line position calculation accounts for both cis and trans conformations, as well as the large-amplitude motion.
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We report the first detection of doubly-deuterated methanol (CHD2OH), as well as firm detections of the two singly-deuterated isotopomers of methanol (CH2DOH and CH3OD), towards the solar-type protostar IRAS 16293-2422. From the present multifrequency observations, we derive the following abundance ratios: [CHD2OH]/[CH3OH] = 0.2 +/- 0.1, [CH2DOH]/[CH3OH] = 0.9 +/- 0.3, [CH3OD]/[CH3OH] = 0.04 +/- 0.02. The total abundance of the deuterated forms of methanol is greater than that of its normal hydrogenated counterpart in the circumstellar material of IRAS 16293-2422, a circumstance not previously encountered. Formaldehyde, which is thought to be the chemical precursor of methanol, possesses a much lower fraction of deuterated isotopomers ( ~ 20%) with respect to the main isotopic form in IRAS 16293-2422. The observed fractionation of methanol and formaldehyde provides a severe challenge to both gas-phase and grain-surface models of deuteration. Two examples of the latter model are roughly in agreement with our observations of CHD2OH and CH2DOH if the accreting gas has a large (0.2-0.3) atomic D/H ratio. However, no gas-phase model predicts such a high atomic D/H ratio, and hence some key ingredient seems to be missing.
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Context. Dimethyl ether is one of the most abundant complex organic molecules (COMs) in star-forming regions. Like other COMs, its formation process is not yet clearly established, but the relative abundances of its deuterated isotopomers may provide crucial hints in studying its chemistry and tracing the source history. The mono-deuterated species (CH2DOCH3) is still a relatively light molecule compared to other COMs. Its spectrum is the most intense in the THz domain in the 100-150 K temperature regime, tracing the inner parts of the low-mass star-forming region. Therefore, it is necessary to measure and assign its transitions in this range in order to be able to compute accurate predictions required by astronomical observations, in particular with the telescope operating in the submm range, such as ALMA. Aims. We present the analysis of mono-deuterated dimethyl ether in its ground-vibrational state, based on an effective Hamiltonian for an asymmetric rotor molecules with internal rotors. The analysis covers the frequency range 150-990 GHz. Methods. The laboratory rotational spectrum of this species was measured with a submillimeter spectrometer (50-990 GHz) using solid-state sources. For the astronomical detection, we used the IRAM 30 m telescope to observe a total range of 27 GHz, in 4 frequency bands from 100 GHz to 219 GHz. Results. New sets of spectroscopic parameters have been determined by a least squares fit with the ERHAM code for both conformers. These parameters have permitted the first identification in space of both mono-deuterated DME isomers via detection of twenty transitions in the solar-type protostar IRAS 16293-2422 with the IRAM 30 m telescope. The DME deuteration ratio in this source appears as high as observed for methanol and formaldehyde, two species known to play an important role in the COMs formation history.
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Context. Astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict transition frequencies that were not measured in the laboratory very precisely. Aims: We present an experimental study and a theoretical analysis of two 18O-methyl formate isotopologues, which were subsequently detected for the first time in Orion KL. Methods: The experimental spectra of both methyl formate isotopologues recorded in the microwave and sub-mm range from 1 to 660 GHz. Both spectra were analysed by using the rho-axis method (RAM) which takes into account the CH3 internal rotation. Results: We obtained spectroscopic constants of both 18O- methyl formate with high accuracy. Thousands of transitions were assigned and others predicted, which allowed us to detect both species in the IRAM 30 m line survey of Orion KL. Full Tables A.1 et A.2 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/538/A119
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Context. Water is a primordial species in the emergence of life, and comets may have brought a large fraction to Earth to form the oceans. To understand the evolution of water from the first stages of star formation to the formation of planets and comets, the HDO/H2O ratio is a powerful diagnostic. Aims: Our aim is to determine precisely the abundance distribution of HDO towards the low-mass protostar IRAS 16293-2422 and learn more about the water formation mechanisms by determining the HDO/H2O abundance ratio. Methods: A spectral survey of the source IRAS 16293-2422 was carried out in the framework of the CHESS (Chemical Herschel Surveys of Star forming regions) Herschel key program with the HIFI (Heterodyne Instrument for the Far-Infrared) instrument, allowing detection of numerous HDO lines. Other transitions have been observed previously with ground-based telescopes. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of HDO by assuming an abundance jump. To determine the H2O abundance throughout the envelope, a similar study was made of the H218O observed lines, as the H2O main isotope lines are contaminated by the outflows. Results: It is the first time that so many HDO and H218O transitions have been detected towards the same source with high spectral resolution. We derive an inner HDO abundance (T ≥ 100 K) of about 1.7 × 10-7 and an outer HDO abundance (T < 100 K) of about 8 × 10-11. To reproduce the HDO absorption lines observed at 894 and 465 GHz, it is necessary to add an absorbing layer in front of the envelope. It may correspond to a water-rich layer created by the photodesorption of the ices at the edges of the molecular cloud. At a 3σ uncertainty, the HDO/H2O ratio is 1.4-5.8% in the hot corino, whereas it is 0.2-2.2% in the outer envelope. It is estimated at ~4.8% in the added absorbing layer. Conclusions: Although it is clearly higher than the cosmic D/H abundance, the HDO/H2O ratio remains lower than the D/H ratio derived for other deuterated molecules observed in the same source. The similarity of the ratios derived in the hot corino and in the added absorbing layer suggests that water formed before the gravitational collapse of the protostar, contrary to formaldehyde and methanol, which formed later once the CO molecules had depleted on the grains. Based on Herschel/HIFI observations. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
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Dimethyl ether (CH_3OCH_3) is one of the largest organic molecules detected in the interstellar medium. As an asymmetric top molecule with two methyl groups which undergo large amplitude motions and a dipole moment of μ=1.3 D, it conveys a dense spectrum throughout the terahertz region and contributes to the spectral line confusion in astronomical observations at these frequencies. In this paper, we present rotational spectra of dimethyl ether in its ground vibrational states, which have been measured in the laboratory and analyzed covering frequencies up to 2.1 THz. The analysis is based on an effective Hamiltonian for a symmetric two-top rotor and includes experimental data published so far. Frequency predictions are presented up to 2.5 THz for astronomical applications with accuracies better than 1 MHz. Table A.1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/504/635
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(Abridged) Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. This study aims to experimentally quantify the broad-band UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K under ultra-high vacuum conditions. The reaction products are mainly identified by RAIRS and TPD experiments. Complex organics are readily formed in all experiments, both during irradiation and during a slow warm-up of the ices to 200 K after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3, HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH3OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. The experiments show that ice photochemistry in CH3OH ices is efficient enough to explain the observed abundances of complex organics around protostars and that ratios of complex molecules can be used to constrain their formation pathway.
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Torsional subbands of CH2DOH are assigned in the 20–750cm-1 region using the results of a theoretical calculation accounting for the fact that this molecule displays a large amplitude internal rotation of a methyl group without threefold symmetry. Ab initio calculations and a harmonic expansion along a minimum energy path allow us to obtain a mass dependent effective potential energy function. This potential energy function is used to retrieve rotation–torsion energy levels making use of a flexible model and deriving a four dimensional Hamiltonian. Its matrix is set up evaluating numerically torsional matrix elements with weights and nodes appropriate for periodic functions. The rotation–torsion energy levels thus obtained are in good agreement with experimental ones in the case of the isotopic species with a symmetrical CH3 or CD3 group. In the case of CH2DOH, a synthetic spectrum is calculated and allows us to assign 35 torsional subbands in the recorded far infrared and mid infrared spectra. The discrepancies between observed and calculated Q-branch origins are on the order of 1cm-1.
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We present new observations of the H2CO emission around IRAS16293-2422, a low mass protostar in the ρ Ophiuchus complex. Bright H2CO and D2CO emission is detected up to 40″ from the center, corresponding to a linear distance of ∼5000 AU. The derived H2CO abundance profile has two jumps at r ≤ 150 AU and r ≤ 700 AU, where the dust temperature reaches 100 K and 50 K respectively. The measured [D2CO]/[H2CO] abundance ratio in the envelope is between 0.03 and 0.16, an extremely high value. We demonstrate that the present new observations can only be explained if the D2CO (and H2CO) are formed during the previous cold pre-collapse phase, stored in the grain mantles, and released in the gas phase during the pre-collapse phase. We consider the two main competing theories for mantle formation, i.e. pure accretion against grain surface chemistry, and we conclude that the former theory cannot explain the present observations, whereas grain active chemistry very naturally does. We found that the mantles are evaporated because of the thermal heating of the grains by the central source and that in the outer cold envelope H2CO and D2CO molecules are embedded in CO-rich mantles which sublimate when the dust is warmer than 25 K. Finally, the present day H2CO and D2CO abundances very probably reflect the mantle composition. We argue that mantles have likely formed in an onion-like structure with the innermost ice layers more enriched in H2CO molecules and we give estimates of the CO hydrogenation efficiency across the envelope and/or in different ices.
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Context. Hydroxylamine (NH 2 OH) and methylamine (CH 3 NH 2 ) have both been suggested as precursors to the formation of amino acids and are therefore, of interest to prebiotic chemistry. Their presence in interstellar space and formation mechanisms, however, are not well established. Aims. We aim to detect both amines and their potential precursor molecules NO, N 2 O, and CH 2 NH towards the low-mass protostellar binary IRAS 16293–2422, in order to investigate their presence and constrain their interstellar formation mechanisms around a young Sun-like protostar. Methods. ALMA observations from the unbiased, high-angular resolution and sensitivity Protostellar Interferometric Line Survey (PILS) are used. Spectral transitions of the molecules under investigation are searched for with the CASSIS line analysis software. Results. CH 2 NH and N 2 O are detected for the first time, towards a low-mass source, the latter molecule through confirmation with the single-dish TIMASSS survey. NO is also detected. CH 3 NH 2 and NH 2 OH are not detected and stringent upper limit column densities are determined. Conclusions. The non-detection of CH 3 NH 2 and NH 2 OH limits the importance of formation routes to amino acids involving these species. The detection of CH 2 NH makes amino acid formation routes starting from this molecule plausible. The low abundances of CH 2 NH and CH 3 NH 2 compared to Sgr B2 indicate that different physical conditions influence their formation in low- and high-mass sources.
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Context . Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. Their detection is crucial to understanding the chemical evolution of the Universe and exploring the link between the early stages of star formation and the formation of solar system bodies, where complex organic molecules have been found in abundance. In particular, molecules that contain nitrogen are interesting due to the role nitrogen plays in the development of life and the compact scales such molecules have been found to trace around forming protostars. Aims . The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a –C≡N functional group) towards two hot corino sources in the low-mass protostellar binary IRAS 16293–2422. This work explores the abundance differences between the two sources, the isotopic ratios, and the spatial extent derived from molecules containing the nitrile functional group. Methods . Using data from the Protostellar Interferometric Line Survey (PILS) obtained with ALMA, we determine abundances and excitation temperatures for the detected nitriles. We also present a new method for determining the spatial structure of sources with high line density and large velocity gradients – Velocity-corrected INtegrated emission (VINE) maps. Results . We detect methyl cyanide (CH 3 CN) as well as five of its isotopologues, including CHD 2 CN, which is the first detection in the interstellar medium (ISM). We also detect ethyl cyanide (C 2 H 5 CN), vinyl cyanide (C 2 H 3 CN), and cyanoacetylene (HC 3 N). We find that abundances are similar between IRAS 16293A and IRAS 16293B on small scales except for vinyl cyanide which is only detected towards the latter source. This suggests an important difference between the sources either in their evolutionary stage or warm-up timescales. We also detect a spatially double-peaked emission for the first time in molecular emission in the A source, suggesting that this source is showing structure related to a rotating toroid of material. Conclusions . With high-resolution observations, we have been able to show for the first time a number of important similarities and differences in the nitrile chemistry in these objects. These illustrate the utility of nitriles as potential tracers of the physical conditions in star-forming regions.