Article

# The ALMA-PILS survey: inventory of complex organic molecules towards IRAS 16293–2422 A

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## Abstract

Context. Complex organic molecules are detected in many sources in the warm inner regions of envelopes surrounding deeply embedded protostars. Exactly how these species form remains an open question. Aims. This study aims to constrain the formation of complex organic molecules through comparisons of their abundances towards the Class 0 protostellar binary IRAS 16293–2422. Methods. We utilised observations from the ALMA Protostellar Interferometric Line Survey of IRAS 16293–2422. The species identification and the rotational temperature and column density estimation were derived by fitting the extracted spectra towards IRAS 16293–2422 A and IRAS 16293–2422 B with synthetic spectra. The majority of the work in this paper pertains to the analysis of IRAS 16293–2422 A for a comparison with the results from the other binary component, which have already been published. Results. We detect 15 different complex species, as well as 16 isotopologues towards the most luminous companion protostar IRAS 16293–2422 A. Tentative detections of an additional 11 isotopologues are reported. We also searched for and report on the first detections of methoxymethanol (CH 3 OCH 2 OH) and trans-ethyl methyl ether (t-C 2 H 5 OCH 3 ) towards IRAS 16293–2422 B and the follow-up detection of deuterated isotopologues of acetaldehyde (CH 2 DCHO and CH 3 CDO). Twenty-four lines of doubly-deuterated methanol (CHD 2 OH) are also identified. Conclusions. The comparison between the two protostars of the binary system shows significant differences in abundance for some of the species, which are partially correlated to their spatial distribution. The spatial distribution is consistent with the sublimation temperature of the species; those with higher expected sublimation temperatures are located in the most compact region of the hot corino towards IRAS 16293–2422 A. This spatial differentiation is not resolved in IRAS 16293–2422 B and will require observations at a higher angular resolution. In parallel, the list of identified CHD 2 OH lines shows the need of accurate spectroscopic data including their line strength.

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... Complex organic molecules (COMs), consisting of more than six atoms with one carbon atom at least, are important species for understanding the molecular evolution in the universe. They have extensively been found in various environments of low-mass star formation: quiescent dense clouds, pre-stellar cores (e.g., Bacmann et al. 2012;Cernicharo et al. 2012;Ceccarelli et al. 2017;Soma et al. 2018;Scibelli & Shirley 2020;Jiménez-Serra et al. 2021;Scibelli et al. 2021), disk/envelope systems of protostellar cores (e.g., Cazaux et al. 2003;Bottinelli et al. 2004;Kuan et al. 2004;Pineda et al. 2012;Jørgensen et al. 2016;Oya et al. 2016Oya et al. , 2018Lee et al. 2019;Imai et al. 2019;Bianchi et al. 2020;Manigand et al. 2020;van Gelder et al. 2020;Martín-Doménech et al. 2021;Nazari et al. 2021;Tychoniec et al. 2021;Chahine et al. 2022), and outflow shock regions around protostars (e.g., Arce et al. 2008;Sugimura et al. 2011;Codella et al. 2020;De Simone et al. 2020). A chemical differentiation between nitrogen-bearing and oxygen-bearing species has long been recognized in high-mass protostellar clusters (e.g., Blake et al. 1987;Beuther et al. 2005;Fontani et al. 2007;Crockett et al. 2015;Feng et al. 2015;Wright et al. 1996;Wyrowski et al. 1999) and a low-mass binary source (Kuan et al. 2004;Caux et al. 2011;Calcutt et al. 2018). ...
... At offsets of ±0 03, the HCOOH/CH 3 OH and NH 2 CHO/CH 3 OH ratios are 0.02-0.03. These values are higher than those found in other sources L483, B1-c, and S68N, and IRAS 16293-2422 Source A : 10 −3 -10 −4 (Jacobsen et al. 2019;Manigand et al. 2020;van Gelder et al. 2020), as shown in Table 6. Interestingly, we note that these ratios of B335 are comparable to those reported by the chemical network calculation (Garrod et al. 2022): 10 −2 for HCOOH/CH 3 OH and (1-6) × 10 −3 for NH 2 CHO/CH 3 OH. ...
... c B1-c in the Perseus Barnard 1 cloud and SerpensS68N (van Gelder et al. 2020). d IRAS 16293-2422 Source A(Manigand et al. 2020). ...
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Article
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.
... From a chemical point of view, close binaries are unique laboratories as they are expected to originate from the same parent core, i.e., the same initial gas composition and similar gas conditions, in terms of temperature, density, and UV illumination. Observed differences in the chemistry of the two binary components are then expected to be the result of a chemical evolution inside the system (e.g., Manigand et al. 2020). SVS13-A is a perfect case study because it is a very well-studied Class I protostellar system in NGC 1333, hosting a rich chemistry including emission from several interstellar complex organic molecules (hereafter iCOMs; López-Sepulcre et al. 2015;Codella et al. 2016;Bianchi et al. 2017;De Simone et al. 2017;Bianchi et al. 2019;Belloche et al. 2020;Diaz-Rodriguez et al. 2021;Yang et al. 2021). ...
... In this case, the dichotomy is reproduced well by chemical models that assume that the formamide formation is dominated by gas-phase reactions (chemistry effect). A chemical differentiation has also been observed in the prototypical Class 0 binary system IRAS 16293-2422 (Manigand et al. 2020). The abundance of CH 3 OCH 3 relative to methanol is similar in the two components of the IRAS 16293-2422 system, while it differs by a factor of 2 for CH 3 CHO and 4 for NH 2 CHO, similarly to what was found in VLA4A and VLA4B. ...
... 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. ...
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Article
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.
... Blake et al. 1987 ;Csengeri et al. 2019 ;Law et al. 2021 ), hot corinos (e.g. Cazaux et al. 2003 ;Manigand et al. 2020 ;Yang et al. 2021 ;Chahine et al. 2022 ), protostellar molecular shocks (e.g. Lefloch et al. 2017 ;Codella et al. 2020 ;De Simone et al. 2020 ), and young discs (e.g. ...
... Ho we ver, the analysis of the emission lines of different species and their spatial distributions sometimes suggest that there is a differentiation in the sublimation of different species (e.g. Manigand et al. 2020 ;Bianchi et al. 2022 ). ...
Article
Acetaldehyde is one of the most common and abundant gaseous interstellar complex organic molecules, found in cold and hot regions of the molecular interstellar medium. Its presence in the gas-phase depends on the chemical formation and destruction routes, and its binding energy (BE) governs whether acetaldehyde remains frozen onto the interstellar dust grains or not. In this work, we report a combined study of the acetaldehyde BE obtained via laboratory TPD (Temperature Programmed Desorption) experiments and theoretical quantum chemical computations. BEs have been measured and computed as a pure acetaldehyde ice and as mixed with both polycrystalline and amorphous water ice. Both calculations and experiments found a BE distribution on amorphous solid water that covers the 4000–6000 K range, when a pre-exponential factor of 1.1 × 1018s−1 is used for the interpretation of the experiments. We discuss in detail the importance of using a consistent couple of BE and pre-exponential factor values when comparing experiments and computations, as well as when introducing them in astrochemical models. Based on the comparison of the acetaldehyde BEs measured and computed in the present work with those of other species, we predict that acetaldehyde is less volatile than formaldehyde, but much more than water, methanol, ethanol, and formamide. We discuss the astrochemical implications of our findings and how recent astronomical high spatial resolution observations show a chemical differentiation involving acetaldehyde, which can easily explained as due to the different BEs of the observed molecules.
... Blake et al. 1987;Csengeri et al. 2019;Law et al. 2021) and hot corinos (e.g. Cazaux et al. 2003;Manigand et al. 2020;Yang et al. 2021;Chahine et al. 2022), protostellar molecular shocks (e.g. Lefloch et al. 2017;De Simone et al. 2020;Codella et al. 2020) and young disks (e.g. ...
... However, the analysis of the emission lines of different species and their spatial distributions sometimes suggest that there is a differentiation in the sublimation of different species (e.g. Manigand et al. 2020;Bianchi et al. 2022). ...
Full-text available
Preprint
Acetaldehyde is one of the most common and abundant gaseous interstellar complex organic molecules, found in cold and hot regions of the molecular interstellar medium. Its presence in the gas-phase depends on the chemical formation and destruction routes, and its binding energy (BE) governs whether acetaldehyde remains frozen onto the interstellar dust grains or not. In this work, we report a combined study of the acetaldehyde BE obtained via laboratory TPD (Temperature Programmed Desorption) experiments and theoretical quantum chemical computations. BEs have been measured and computed as a pure acetaldehyde ice and as mixed with both polycrystalline and amorphous water ice. Both calculations and experiments found a BE distribution on amorphous solid water that covers the 4000--6000 K range, when a pre-exponential factor of $1.1\times 10^{18}s^{-1}$ is used for the interpretation of the experiments. We discuss in detail the importance of using a consistent couple of BE and pre-exponential factor values when comparing experiments and computations, as well as when introducing them in astrochemical models. Based on the comparison of the acetaldehyde BEs measured and computed in the present work with those of other species, we predict that acetaldehyde is less volatile than formaldehyde, but much more than water, methanol, ethanol, and formamide. We discuss the astrochemical implications of our findings and how recent astronomical high spatial resolution observations show a chemical differentiation involving acetaldehyde, which can easily explained as due to the different BEs of the observed molecules.
... 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. ...
... Horizontal dashed lines are observational relative abundances. The relative abundance toward IRAS16293-2422A is referred to (1)Manigand et al. (2020). For IRAS16293-2422B and NGC 6344I MM1, the abundances are based on (2) Jørgensen et al.(2018) and (3) El-Abd et al. (2019) for MF and on Manigand et al. (2020) and (4) McGuire et al. (2017) for MM, respectively. ...
... However, hints of a shift were also seen in high-resolution VLA continuum observations at 42 GHz reported by Rodríguez et al. (2005) and Hernández-Gómez et al. (2019b), unlike what is seen in the case of the highly inclined disk in HH212 . Furthermore, the line emission from complex organic molecules from the ALMA-PILS survey also appears to show a peak shifted to the west (Calcutt et al. 2018a,b;Manigand et al. 2020Manigand et al. , 2021. Likewise, the source does not show a significant gradient indicative of rotation in previous ALMA line observations from 100 au down to 70 au scales. ...
... For instance, in the distribution of the spectral index shown in Fig. 1, the minimum value is fairly close to the center of the overall structure, while the peak of the dust continuum emission is not. This difference may be due to the continuum peak tracing instead the position of a hot structure close to the protostar and not the protostar itself, which would also be supported by the assymetric distribution of the complex organic molecule emission (Calcutt et al. 2018a,b;Manigand et al. 2020Manigand et al. , 2021. This assymetry could also be associated with asymmetric accretion from the envelope onto the disk, based on the previous detection of infalling material in source B (Pineda et al. 2012;Zapata et al. 2013). ...
Preprint
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 $T_{\rm b} >$ 100\,K within $\sim$30\,au and $T_{\rm b}$ 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 $\sim$ 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.
... Most deuterated COMs have been detected towards the lowmass protostar IRAS 16293-2422 A and B (e.g. Coutens et al. 2016;Jørgensen et al. 2016Jørgensen et al. , 2018Manigand et al. 2019Manigand et al. , 2020 among which are those of MF (hereafter DMF). Using IRAM-30 m and JCMT data from the TIMASSS program (Caux et al. 2011), Demyk et al. (2010) reported tentatively the first detection of the mono-deuterated methyl formate DCOOCH 3 with a column density of 6 × 10 14 cm −2 in both IRAS 16293A and IRAS 16293B where the column density of MF is estimated to be 1 × 10 16 and 9 × 10 15 cm −2 in both cores, respectively. ...
... (1) HCOOCH 3 ࣠ 2.2 (-8) 1.4 (-8) All (1) DCOOCH 3 ࣠ 1.3 (-9) 9 (-10) M2, M12 (1) HCOOCH 2 D ࣠ 4.0 (-9) ≤ 4.0 (-11) All (2) HCOOCHD 2 ࣠ 9.1 (-10) Table 2 in Manigand et al. (2019). ‡ The observed ratio in IRAS 16293A is taken from Manigand et al. (2020) are then computed with respect to the total H following a simple expression n(X)/n H = 0.5 n(X)/n H2 (Pety et al. 2005). ...
Article
Methyl formate, HCOOCH3, and many of its isotopologues have been detected in astrophysical regions with considerable abundances. However, the recipe for the formation of this molecule and its isotopologues is not yet known. In this work, we attempt to investigate, theoretically, the successful recipe for the formation of interstellar HCOOCH3 and its deuterated isotopologues. We used the gas-grain chemical model, UCLCHEM, to examine the possible routes of formation of methyl formate on grain surfaces and in the gas-phase in low-mass star-forming regions. Our models show that radical-radical association on grains are necessary to explain the observed abundance of DCOOCH3 in the protostar IRAS 16293–2422. H-D substitution reactions on grains significantly enhance the abundances of HCOOCHD2, DCOOCHD2, and HCOOCD3. The observed abundance of HCOOCHD2 in IRAS 16293–2422 can only be reproduced if H-D substitution reactions are taken into account. However, HCOOCH2D remain underestimated in all of our models. The deuteration of methyl formate appears to be more complex than initially thought. Additional studies, both experimentally and theoretically, are needed for a better understanding of the interstellar formation of these species.
... 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. ...
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Preprint
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.
... 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 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). ...
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.
... The isotope 12 C/ 13 C ratio derived from fitting the observed positive correlation with a linear proportional function is four, an order of magnitude lower than the nominal local ISM value of ∼70 (Wirström et al. 2011) and ∼50 in the Orion cloud (Kahane et al. 2018). The low 12 C/ 13 C ratio for our hot corino sources most likely suggests high optical depths of the main CH 3 OH isotopologue, similar to those reported in the literature (Zapata et al. 2013 18 OH] value in G208N1 is significantly lower than 562 ± 221 in the hot corino source IRAS 16293A (Manigand et al. 2020), 181 in the hot core sources Sgr B2(N2) , and 560 ± 25 in the local ISM (Wilson & Rood 1994). Considering that CH 3 OH is indeed optically thick and factoring the 12 C/ 13 C ratio ranging between 50 and 77 (Wilson & Rood 1994;Wirström et al. 2011;Kahane et al. 2018), we scale the column density of 13 CH 3 OH accordingly and find that the 16 O/ 18 O ratio of methanol in G208N1 becomes 127-190. ...
Article
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.
... 3 OH] value in G208N1 is significantly lower than 562 ± 221 in the hot corino source IRAS 16293A (Manigand et al. 2020), 181 in the hot core sources Sgr B2(N2) , and 560 ± 25 in the local interstellar medium (ISM, Wilson & Rood 1994). Considering CH 3 OH being indeed optically thick and factoring the 12 C/ 13 C ratio ranging between 50 and 77 Wilson & Rood (1994); Wirström et al. (2011);Kahane et al. (2018), we scale the column density of 13 CH 3 OH accordingly and find that the 16 O/ 18 O ratio of methanol in G208N1 becomes 127 -190. ...
Full-text available
Preprint
The presence of complex organic molecules (COMs) in the interstellar medium (ISM) 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 ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP) 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 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 corino and suggest that the lower luminosity (Class 0) sources likely have a smaller region with COMs 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.
... In molecular clouds where stars are born, temperatures can be as low as 10 K, and water is the main component of the ice mantles coating micron-sized dust grains. 1 On the surface of these grains, a rich chemistry accounts for much of the chemical complexity of the known interstellar complex organic molecules (COMs). [2][3][4][5] At the core of COM synthesis in space is the formation of bonds to carbon atoms, which, in turn, depends upon the main reservoir of carbon. In the translucent stage of a molecular cloud or under influence of cosmic-ray irradiation, carbon is predominantly present in its atomic form C( 3 P 0 ). ...
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Preprint
We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H 2 CO). This simplest carbonylic compound is central to the formation of complex organics in cold in-terstellar clouds, and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H 2 CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer cat-alyzed by the water hydrogen bonding network. Consequently, the reactions 3 C + H 2 O −−→ 3 HCOH and 1 HCOH −−→ 1 H 2 CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers or absence thereof explain the very small kinetic isotope effect in our experiments when comparing the formation of H 2 CO to D 2 CO. Our results reconcile the disagreement found in the literature on the reaction route: C + H 2 O −−→ H 2 CO.
... In molecular clouds where stars are born, temperatures can be as low as 10 K, and water is the main component of the ice mantles coating micron-sized dust grains. 1 On the surface of these grains, a rich chemistry accounts for much of the chemical complexity of the known interstellar complex organic molecules (COMs). [2][3][4][5] At the core of COM synthesis in space is the formation of bonds to carbon atoms, which, in turn, depends upon the main reservoir of carbon. In the translucent stage of a molecular cloud or under influence of cosmic-ray irradiation, carbon is predominantly present in its atomic form C( 3 P 0 ). ...
Preprint
We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H$_2$CO). This simplest carbonylic compound is central to the formation of complex organics in cold interstellar clouds, and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H$_2$CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions $^3$C + H$_2$O -> $^3$HCOH and $^1$HCOH -> $^1$H$_2$CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers or absence thereof explain the very small kinetic isotope effect in our experiments when comparing the formation of H$_2$CO to D$_2$CO. Our results reconcile the disagreement found in the literature on the reaction route: C + H$_2$O -> H$_2$CO.
... [7][8][9] Most data pertaining to the physical and chemical-physical states of gaseous matter in the Universe come from atomic or molecular spectroscopy. For decades, most of the quantitative information was gained from the rotational lines of ground state molecules, thanks to the high precision, high specificity of rotational spectra in the cm to sub-mm spectral regions 10,11 . ...
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Rates of conversions of molecular internal energy to and from kinetic energy by means of molecular collision allows to compute collisional line shapes and transport properties of gases. Knowledge of ro-vibrational quenching rates is necessary to connect spectral observations to physical properties of warm astrophysical gasses, including exo-atmospheres. For a system of paramount importance in this context, the vibrational bending mode quenching of H2O by H2, we show here that exchange of vibrational to rotational and kinetic energy remains a quantum process, despite the large numbers of quantum levels involved and the large vibrational energy transfer. The excitation of the quantized rotor of the projectile is by far the most effective ro-vibrational quenching path of water. To do so, we use a fully quantum first principle computation, potential and dynamics, converging it at all stages, in a full coupled channel formalisms. We present here rates for the quenching of the first bendingmode of ortho-H2O by ortho H2, up to 500K, in a fully converged coupled channels formalism.
... Most deuterated COMs have been detected towards the low-mass protostar IRAS 16293-2422 A & B (e.g. Coutens et al. 2016;Jørgensen et al. 2016Jørgensen et al. , 2018Manigand et al. 2019Manigand et al. , 2020 among which are those of MF (hereafter DMF). E-mail:zma@sci.cu.edu.eg ...
Preprint
Methyl formate, HCOOCH$_3$, and many of its isotopologues have been detected in astrophysical regions with considerable abundances. However, the recipe for the formation of this molecule and its isotopologues is not yet known. In this work, we attempt to investigate, theoretically, the successful recipe for the formation of interstellar HCOOCH$_3$ and its deuterated isotopologues. We used the gas-grain chemical model, UCLCHEM, to examine the possible routes of formation of methyl formate on grain surfaces and in the gas-phase in low-mass star-forming regions. Our models show that radical-radical association on grains are necessary to explain the observed abundance of DCOOCH$_3$ in the protostar IRAS~16293--2422. H-D substitution reactions on grains significantly enhance the abundances of HCOOCHD$_2$, DCOOCHD$_2$, and HCOOCD$_3$. The observed abundance of HCOOCHD$_2$ in IRAS 16293--2422 can only be reproduced if H-D substitution reactions are taken into account. However, HCOOCH$_2$D remain underestimated in all of our models. The deuteration of methyl formate appears to be more complex than initially thought. Additional studies, both experimentally and theoretically, are needed for a better understanding of the interstellar formation of these species.
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.
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).
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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.
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Article
Formamide (NH 2 CHO) is considered an important prebiotic molecule because of its potential to form peptide bonds. It was recently detected in the atmosphere of the HH 212 protostellar disk on the solar system scale where planets will form. Here we have mapped it and its potential parent molecules HNCO and H 2 CO, along with other molecules CH 3 OH and CH 3 CHO, in the disk atmosphere, studying its formation mechanism. Interestingly, we find a stratified distribution of these molecules, with the outer emission radius increasing from ∼24 au for NH 2 CHO and HNCO, to 36 au for CH 3 CHO, to 40 au for CH 3 OH, and then to 48 au for H 2 CO. More importantly, we find that the increasing order of the outer emission radius of NH 2 CHO, CH 3 OH, and H 2 CO is consistent with the decreasing order of their binding energies, supporting that they are thermally desorbed from the ice mantle on dust grains. We also find that HNCO, which has much lower binding energy than NH 2 CHO, has almost the same spatial distribution, kinematics, and temperature as NH 2 CHO, and is thus more likely a daughter species of desorbed NH 2 CHO. On the other hand, we find that H 2 CO has a more extended spatial distribution with different kinematics from NH 2 CHO, thus questioning whether it can be the gas-phase parent molecule of NH 2 CHO.
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Article
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.
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Article
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.
Article
Unravelling the generation of complex organic molecules (COMs) on interstellar nanoparticles (grains) is essential in establishing predictive astrochemical reaction networks and recognizing evolution stages of molecular clouds and star-forming regions. The formation of COMs has been associated with the irradiation of interstellar ices by ultraviolet photons and galactic cosmic rays. Herein, we pioneer the first incorporation of synchrotron vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry (SVUV-PI-ReTOF-MS) in laboratory astrophysics simulation experiments to afford an isomer-selective identification of key COMs (ketene (H2C═CO); acetaldehyde (CH3CHO); vinyl alcohol (H2C═CHOH)) based on photoionization efficiency (PIE) curves of molecules desorbing from exposed carbon monoxide-methane (CO-CH4) ices. Our results demonstrate that the SVUV-PI-ReTOF-MS approach represents a versatile, rapid methodology for a comprehensive identification and explicit understanding of the complex organics produced in space simulation experiments. This methodology is expected to significantly improve the predictive nature of astrochemical models of complex organic molecules formed abiotically in deep space, including biorelated species linked to the origins-of-life topic.
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.
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Article
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
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
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.
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Article
Context. The primary alcohol n-propanol (i.e., normal-propanol or propan-1-ol; C 3 H 7 OH) occurs in five different conformers: Ga, Gg, Gg' , Aa, and Ag. All rotational spectra of the three conformers of the G family are well described, making astronomical search of their spectroscopic signatures possible, as opposed to those of the Aa and Ag conformers. Aims. Our goal is to facilitate the astronomical detection of Aa and Ag conformers of n-propanol by characterizing their rotational spectra. Methods. We recorded the rotational spectra of n-propanol in the frequency domain of 18-505 GHz. Additional double-modulation double-resonance (DM-DR) measurements were performed, more specifically with the goal to unambiguously assign weak transitions of the Aa conformer and to verify assignments of the Ag conformer. Results. We derived a spectroscopic quantum mechanical model with experimental accuracy (with J max = 70 and K a ,max = 6) for Aa n -propanol. Furthermore, we unambiguously assigned transitions (with J max = 69 and K a, max = 9) of Ag n-propanol; in doing so, we prove the existence of two tunneling states, Ag + and Ag ⁻ . Conclusions. The astronomical search of all five conformers of n-propanol is now possible via their rotational signatures. These are applied in a companion article on the detection of n-propanol toward the hot molecular core Sgr B2(N2).
Article
Context. The protostellar stage is known to be the richest star formation phase in emission from gaseous complex organic molecules. However, some protostellar systems show little or no millimetre (mm) line emission of such species. This can be interpreted as a low abundance of complex organic molecules. Alternatively, complex species could be present in the system, but are not seen in the gas. Aims. The goal is to investigate the second hypothesis for methanol as the most abundant complex organic molecule in protostellar systems. This work aims to determine how effective dust optical depth is in hiding methanol in the gas, and whether methanol can mainly reside in the ice due to the presence of a disk that lowers the temperatures. Hence, we attempt to answer the question whether the presence of a disk and optically thick dust reduce methanol emission even if methanol and other complex species are abundant in the ices and gas. Methods. Using the radiative transfer code RADMC-3D, we calculated methanol emission lines from an envelope-only model and from an envelope-plus-disk model. We compared the results with each other and with the observations. Methanol gas and ice abundances were parametrised inside and outside of the snow surfaces based on values from observations. Both models included either dust grains with low mm opacity or high mm opacity, and their physical parameters such as envelope mass and disk radius were varied. Results. Methanol emission from the envelope-only model is always stronger than from the envelope-plus-disk model by at least a factor ∼2 as long as the disk radius is larger than ∼30 au (for L = 8 L ⊙ ). In most cases, this is due to lower temperatures (disk shadowing), which causes the smaller amount of warm (≳70 K) methanol inside the snow surface of the envelope-plus-disk model. The intensities drop by more than an order of magnitude for models including high mm opacity dust grains and disk radii of at least ∼50 au (for L = 8 L ⊙ ) due to continuum over-subtraction. Conclusions. The line intensities from the envelope-only models match the observations moderately well when methanol emission is strong, but they overproduce the observations of protostars with lower methanol emission even with large dust optical depth effects. The envelope-plus-disk models can explain the bulk of the observations. However, they can only reproduce the observations of sources with high luminosities and very low methanol emission when the dust optical depth is significant in the envelope and continuum over-subtraction becomes effective in the disk (high mm opacity dust grains are used). Therefore, both the effects of disk and dust optical depth should be considered to explain the observations. In conclusion, it is important to take physical structure into account in future chemical studies of low-mass protostars: absence of gas-phase methanol emission does not imply absence of methanol molecules in either gas or ice.
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Article
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
In this short communication the selection rules for transitions involving asymmetry split energy levels belonging to various torsional-vibrational-rotational states are put forward for the first time in asymmetrically deuterated methanol species (e.g., CH2DOH,CHD2OH, etc.). Unlike the parent symmetrical methanol species these rules depend on the parity of the torsional vibration states, It has been discovered that each mixed torsional vibrational state could be treated as independent vibrational states connected by the b- or c-type dipole moments. Due to the complexity of torsion and rotation, the energies for the K-rotational ladder seem to vary randomly for various torsional-vibrational states. These often result in inverted and/or large asymmetry splitting. In these regards, the aspects in which caution is rendered in the information available in the literature are briefly discussed. The findings are supported by rigorous assignment studies in the very high-resolution synchrotron radiation-assisted Fourier transform spectrum.
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Article
Multiphase astrochemical modeling presents a numerical challenge especially for the simulation of objects with the wide range of physical parameters such as protoplanetary disks. We demonstrate an implementation of the analytical Jacobian for the numerical integration of the system of differential rate equations that govern chemical evolution in star-forming regions. The analytical Jacobian allowed us to greatly improve the stability of the code in protoplanetary disk conditions. We utilize the MONACO code to study the evolution of abundances of chemical species in protoplanetary disks. The chemical model includes 670 species and 6,015 reactions in the gas phase and on interstellar grains. The specific feature of the utilized chemical model is the inclusion of low-temperature chemical processes leading to the formation of complex organic molecules (COMs), included previously in the models of chemistry of COMs in prestellar clouds. To test the impact of analytical Jacobian on the stability of numerical simulations of chemical evolution in protoplanetary disks, we calculated the chemical composition of the disk using a two-phase model and four variants of the chemical reaction network, three values of the surface diffusion rates, and two types of the initial chemical composition. We also show a preliminary implementation of the analytical Jacobian to a three-phase model.
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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Context. In recent times, large organic molecules of exceptional complexity have been found in diverse regions of the interstellar medium. Aims. In this context, we aim to provide accurate frequencies of the ground vibrational state of two key aliphatic aldehydes, n -butanal and its branched-chain isomer, i-butanal, to enable their eventual detection in the interstellar medium. We also want to test the level of complexity that interstellar chemistry can reach in regions of star formation. Methods. We employ a frequency modulation millimeter-wave absorption spectrometer to measure the rotational features of n - and i -butanal. We analyze the assigned rotational transitions of each rotamer separately using the A -reduced semirigid-rotor Hamiltonian. We use the spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array to search for n - and i -butanal toward the star-forming region Sgr B2(N). We also search for both aldehydes toward the molecular cloud G+0.693−0.027 with IRAM 30 m and Yebes 40 m observations. The observational results are compared with computational results from a recent gas-grain astrochemical model. Results. Several thousand rotational transitions belonging to the lowest-energy conformers of two distinct linear and branched isomers have been assigned in the laboratory spectra up to 325 GHz. A precise set of the relevant rotational spectroscopic constants has been determined for each structure as a first step toward identifying both molecules in the interstellar medium. We report non-detections of n -and i-butanal toward both sources, Sgr B2(N1S) and G+0.693-0.027. We find that n - and i -butanal are at least 2-6 and 6-18 times less abundant than acetaldehyde toward Sgr B2(N1S), respectively, and that n -butanal is at least 63 times less abundant than acetaldehyde toward G+0.693−0.027. While propanal is not detected toward Sgr B2(N1S) either, with an abundance at least 5–11 lower than that of acetaldehyde, propanal is found to be 7 times less abundant than acetaldehyde in G+0.693−0.027. Comparison with astrochemical models indicates good agreement between observed and simulated abundances (where available). Grain-surface chemistry appears sufficient to reproduce aldehyde ratios in G+0.693−0.027; gas-phase production may play a more active role in Sgr B2(N1S). Model estimates for the larger aldehydes indicate that the observed upper limits may be close to the underlying values. Conclusions. Our astronomical results indicate that the family of interstellar aldehydes in the Galactic center region is characterized by a drop of one order of magnitude in abundance at each incrementation in the level of molecular complexity.
Article
Prebiotic sugars are thought to be formed on primitive Earth by the formose reaction. However, their formation is not fully understood and it is plausible that key intermediates could have formed in extraterrestrial environments and subsequently delivered on early Earth by cometary bodies. 1,2-Ethenediol, the enol form of glycolaldehyde, represents a highly reactive intermediate of the formose reaction and is likely detectable in the interstellar medium. Here, we report the identification and first characterization of (Z)-1,2-ethenediol by means of rotational spectroscopy. The title compound has been produced in the gas phase by flash vacuum pyrolysis of bis-exo-5-norbornene-2,3-diol at 750 °C, through a retro-Diels-Alder reaction. The spectral analysis was guided by high-level quantum-chemical calculations, which predicted spectroscopic parameters in very good agreement with the experiment. Our study provides accurate spectral data to be used for searches of (Z)-1,2-ethenediol in the interstellar space.
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
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
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.
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Article
Context. Methoxymethanol (CH 3 OCH 2 OH) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. Like several other C-, O-, and H-containing complex organic molecules (COMs), this molecule is expected to form upon hydrogen addition and abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products. Aims. The goal of this work is to experimentally and theoretically investigate the most likely solid-state methoxymethanol reaction channel – the recombination of CH 2 OH and CH 3 O radicals – for dark interstellar cloud conditions and to compare the formation efficiency with that of other species that were shown to form along the CO-hydrogenation line. We also investigate an alternative hydrogenation channel starting from methyl formate. Methods. Hydrogen atoms and CO or H 2 CO molecules were co-deposited on top of predeposited H 2 O ice to mimic the conditions associated with the beginning of “rapid” CO freeze-out. The formation of simple species was monitored in situ using infrared spectroscopy. Quadrupole mass spectrometry was used to analyze the gas-phase COM composition following a temperature-programmed desorption. Monte Carlo simulations were used for an astrochemical model comparing the methoxymethanol formation efficiency with that of other COMs. Results. The laboratory identification of methoxymethanol is found to be challenging, in part because of diagnostic limitations, but possibly also because of low formation efficiencies. Nevertheless, unambiguous detection of newly formed methoxymethanol has been possible in both CO+H and H 2 CO+H experiments. The resulting abundance of methoxymethanol with respect to CH 3 OH is about 0.05, which is about six times lower than the value observed toward NGC 6334I and about three times lower than the value reported for IRAS 16293B. Astrochemical simulations predict a similar value for the methoxymethanol abundance with respect to CH 3 OH, with values ranging between 0.03 and 0.06. Conclusions. We find that methoxymethanol is formed by co-deposition of CO and H 2 CO with H atoms through the recombination of CH 2 OH and CH 3 O radicals. In both the experimental and modeling studies, it is found that the efficiency of this channel alone is not sufficient to explain the observed abundance of methoxymethanol with respect to methanol. The rate of a proposed alternative channel, the direct hydrogenation of methyl formate, is found to be even less efficient. These results suggest that our knowledge of the reaction network is incomplete or involving alternative solid-state or gas-phase formation mechanisms.
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
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
Context. Observations of the different isomers of molecules in the interstellar medium (ISM) have revealed that both low- and high-energy isomers can be present in space despite the low temperature conditions. It has been shown that the presence of these isomers may be due to tunneling effects. Aims. We carried out a theoretical study of the cis–trans isomerization reactions of two astrophysically relevant acids, formic acid (HCOOH) and thioformic acid (HC(O)SH), where the latter has recently been discovered in space. We also searched for these molecules towards the hot core G31.41+0.31 to compare their abundances with the expected theoretical isomerization results. Methods. We employed high-level ab initio calculations to study the reaction rate constants of the isomerization reactions. We used the canonical variational transition state theory with the multidimensional small curvature tunneling approximation in the temperature range of 10–400 K. Moreover, we used the spectrum obtained from the ALMA 3mm spectral survey GUAPOS (GUAPOS: G31 Unbiased ALMA sPectral Observational Survey), with a spectral resolution of ~0.488 MHz and an angular resolution of 1.′′2×1.′′2 (~4500 au), to derive column densities of HCOOH and HC(O)SH towards G31.41+0.31. Results. Our results demonstrate that these isomerizations are viable in the conditions of the ISM due to ground-state tunneling effects, which allow the system to reach the thermodynamic equilibrium at moderately low temperatures. At very low temperatures ( T kin ~ 10 K), the reaction rate constants for the cis-to-trans isomerizations are very small, which implies that the cis isomers should not be formed under cold ISM conditions. This is in disagreement with observations of the cis/trans isomers of HCOOH in cold cores where the cis isomer is found to be ~5–6% the trans isomer. At high temperatures (~150–300 K), our theoretical data not only match the observed behavior of the trans/cis abundance ratios for HCOOH (the cis form is undetected), but they support our tentative detection of the trans and – for the first time in the insterstellar medium – the cis isomer of HC(O)SH towards the hot molecular core G31.41+0.31 (with a measured trans/cis abundance ratio of ~3.7). Conclusions. While the trans/cis ratio for HC(O)SH in the ISM depends on the relative stability of the isomers, the trans/cis ratio for HCOOH cannot be explained by isomerization, and is determined by other competitive chemical processes.
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Article
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
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
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.
Article
Rates of conversions of molecular internal energy to and from kinetic energy by means of molecular collision allow us to compute collisional line shapes and transport properties of gases. Knowledge of ro-vibrational quenching rates is necessary to connect spectral observations to physical properties of warm astrophysical gasses, including exo-atmospheres. For a system of paramount importance in this context, the vibrational bending mode quenching of H2O by H2, we show here that the exchange of vibrational to rotational and kinetic energy remains a quantum process, despite the large numbers of quantum levels involved and the large vibrational energy transfer. The excitation of the quantized rotor of the projectile is by far the most effective ro-vibrational quenching path of water. To do so, we use a fully quantum first-principles computation, potential and dynamics, converging it at all stages, in a full coupled channel formalism. We present here rates for the quenching of the first bending mode of ortho-H2O by ortho-H2, up to 500 K, in a fully converged coupled channel formalism.
Article
Context. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation. Aims. The aim is to determine which physical components are traced by different molecules at subarcsecond scales (<100–400 au). Methods. We used a suite of Atacama Large Millimeter/submillimeter Array (ALMA) datasets in band 6 (1 mm), band 5 (1.8 mm), and band 3 (3 mm) at spatial resolutions 0.″5–3″ for 16 protostellar sources. For a subset of sources, Atacama Compact Array (ACA) data at band 6 with a spatial resolution of 6″ were added. The availability of low- and high-excitation lines and data on small and larger scales, is important to understand the full picture. Results. The protostellar envelope is well traced by C ¹⁸ O, DCO ⁺ , and N 2 D ⁺ , which stems from the freeze-out of CO governing the chemistry at envelope scales. Molecular outflows are seen in classical shock tracers such as SiO and SO, but ice-mantle products such as CH 3 OH and HNCO that are released with the shock are also observed. The molecular jet is a key component of the system. It is only present at the very early stages, and it is prominent not only in SiO and SO, but occasionally also in H 2 CO. The cavity walls show tracers of UV-irradiation such as C 2 H, c-C 3 H 2 and CN. In addition to showing emission from complex organic molecules (COMs), the hot inner envelope also presents compact emission from small molecules such as H 2 S, SO, OCS, and H ¹³ CN, which most likely are related to ice sublimation and high-temperature chemistry. Conclusions. Subarcsecond millimeter-wave observations allow us to identify these (simple) molecules that best trace each of the physical components of a protostellar system. COMs are found both in the hot inner envelope (high-excitation lines) and in the outflows (lower-excitation lines) with comparable abundances. COMs can coexist with hydrocarbons in the same protostellar sources, but they trace different components. In the near future, mid-infrared observations with JWST–MIRI will provide complementary information about the hottest gas and the ice-mantle content, at unprecedented sensitivity and at resolutions comparable to ALMA for the same sources.
Article
Context. Deuterated molecules are good tracers of the evolutionary stage of star-forming cores. During the star formation process, deuterated molecules are expected to be enhanced in cold, dense pre-stellar cores and to deplete after protostellar birth. Aims. In this paper, we study the deuteration fraction of formaldehyde in high-mass star-forming cores at different evolutionary stages to investigate whether the deuteration fraction of formaldehyde can be used as an evolutionary tracer. Methods. Using the APEX SEPIA Band 5 receiver, we extended our pilot study of the J = 3 →2 rotational lines of HDCO and D 2 CO to eleven high-mass star-forming regions that host objects at different evolutionary stages. High-resolution follow-up observations of eight objects in ALMA Band 6 were performed to reveal the size of the H 2 CO emission and to give an estimate of the deuteration fractions HDCO/H 2 CO and D 2 CO/HDCO at scales of ~6″ (0.04–0.15 pc at the distance of our targets). Results. Our observations show that singly and doubly deuterated H 2 CO are detected towards high-mass protostellar objects (HMPOs) and ultracompact H II regions (UC H II regions), and the deuteration fraction of H 2 CO is also found to decrease by an order of magnitude from the earlier HMPO phases to the latest evolutionary stage (UC H II ), from ~0.13 to ~0.01. We have not detected HDCO and D 2 CO emission from the youngest sources (i.e. high-mass starless cores or HMSCs). Conclusions. Our extended study supports the results of the previous pilot study: the deuteration fraction of formaldehyde decreases with the evolutionary stage, but higher sensitivity observations are needed to provide more stringent constraints on the D/H ratio during the HMSC phase. The calculated upper limits for the HMSC sources are high, so the trend between HMSC and HMPO phases cannot be constrained.
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Context. In 2013, we published the first rotational analysis and detection of mono-deuterated dimethyl ether in the solar-type protostar IRAS 16293-2422 with the IRAM 30 m telescope. Dimethyl ether is one of the most abundant complex organic molecules in star-forming regions, and its D-to-H (D/H) ratios are important to understand its chemistry and trace the source history. Aims. We present the first analysis of doubly deuterated dimethyl ether (methoxy- d 2 -methane, 1,1-dideuteromethylether) in its ground-vibrational state, based on an effective Hamiltonian for an asymmetric rotor molecule with internal rotors. The analysis covers the frequency range 0.15–1.5 THz. Methods. The laboratory rotational spectrum of this species was measured between 150 and 1500 GHz with Lille’s submillimeter spectrometer. For the astronomical detection, we used the Atacama Large Millimeter/submillimeter Array observations from the Protostellar Interferometric Line Survey. Results. New sets of spectroscopic parameters have been determined by a least squares fit with the ERHAM code for both symmetric and asymmetric conformers. As for the mono-deuterated species, these parameters have permitted the first identification in space of both conformers of a doubly deuterated dimethyl ether via detection near the B component of the Class 0 protostar IRAS 16293-2422.
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Context. Formic acid (HCOOH) and carbon dioxide (CO 2 ) are simple species that have been detected in the interstellar medium. The solid-state formation pathways of these species under experimental conditions relevant to prestellar cores are primarily based off of weak infrared transitions of the HOCO complex and usually pertain to the H 2 O-rich ice phase, and therefore more experimental data are desired. Aims. Here, we present a new and additional solid-state reaction pathway that can form HCOOH and CO 2 ice at 10 K “non-energetically” in the laboratory under conditions related to the “heavy” CO freeze-out stage in dense interstellar clouds, i.e., by the hydrogenation of an H 2 CO:O 2 ice mixture. This pathway is used to piece together the HCOOH and CO 2 formation routes when H 2 CO or CO reacts with H and OH radicals. Methods. Temperature programmed desorption – quadrupole mass spectrometry (TPD-QMS) is used to confirm the formation and pathways of newly synthesized ice species as well as to provide information on relative molecular abundances. Reflection absorption infrared spectroscopy (RAIRS) is additionally employed to characterize reaction products and determine relative molecular abundances. Results. We find that for the conditions investigated in conjunction with theoretical results from the literature, H + HOCO and HCO + OH lead to the formation of HCOOH ice in our experiments. Which reaction is more dominant can be determined if the H + HOCO branching ratio is more constrained by computational simulations, as the HCOOH:CO 2 abundance ratio is experimentally measured to be around 1.8:1. H + HOCO is more likely than OH + CO (without HOCO formation) to form CO 2 . Isotope experiments presented here further validate that H + HOCO is the dominant route for HCOOH ice formation in a CO-rich CO:O 2 ice mixture that is hydrogenated. These data will help in the search and positive identification of HCOOH ice in prestellar cores.
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Context. 1-propanol (CH 3 CH 2 CH 2 OH) is a three carbon-bearing representative of the primary linear alcohols that may have its origin in the cold dark cores in interstellar space. To test this, we investigated in the laboratory whether 1-propanol ice can be formed along pathways possibly relevant to the prestellar core phase. Aims. We aim to show in a two-step approach that 1-propanol can be formed through reaction steps that are expected to take place during the heavy CO freeze-out stage by adding C 2 H 2 into the CO + H hydrogenation network via the formation of propanal (CH 3 CH 2 CHO) as an intermediate and its subsequent hydrogenation. Methods. Temperature programmed desorption-quadrupole mass spectrometry (TPD-QMS) was used to identify the newly formed propanal and 1-propanol. Reflection absorption infrared spectroscopy (RAIRS) was used as a complementary diagnostic tool. The mechanisms that can contribute to the formation of solid-state propanal and 1-propanol, as well as other organic compounds, during the heavy CO freeze-out stage are constrained by both laboratory experiments and theoretical calculations. Results. Here it is shown that recombination of HCO radicals formed upon CO hydrogenation with radicals formed via C 2 H 2 processing – H 2 CCH and H 3 CCH 2 – offers possible reaction pathways to solid-state propanal and 1-propanol formation. This extends the already important role of the CO hydrogenation chain to the formation of larger complex organic molecules. The results are compared with ALMA observations. The resulting 1-propanol:propanal ratio concludes an upper limit of <0.35−0.55, which is complemented by computationally derived activation barriers in addition to the experimental results.
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Nitrogen oxides are thought to play a significant role as a nitrogen reservoir and to potentially participate in the formation of more complex species. Until now, only NO, NO, and HNO have been detected in the interstellar medium. We report the first interstellar detection of nitrous acid (HONO). Twelve lines were identified towards component B of the low-mass protostellar binary IRAS 16293-2422 with the Atacama Large Millimeter/submillimeter Array, at the position where NO and NO have previously been seen. A local thermodynamic equilibrium model was used to derive the column density (∼9 × 1014 cm in a 0 .″5 beam) and excitation temperature (∼100 K) of this molecule. HNO, NO, NO+, and HNO3 were also searched for in the data, but not detected. We simulated the HONO formation using an updated version of the chemical code Nautilus and compared the results with the observations. The chemical model is able to reproduce satisfactorily the HONO, NO, and NO abundances, but not the NO, HNO, and NHOH abundances. This could be due to some thermal desorption mechanisms being destructive and therefore limiting the amount of HNO and NHOH present in the gas phase. Other options are UV photodestruction of these species in ices or missing reactions potentially relevant at protostellar temperatures.
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We investigate the chemical segregation of complex O-bearing species (including the largest and most complex ones detected to date in space) towards Orion KL, the closest high-mass star-forming region. The molecular line images obtained using the ALMA science verification data reveal a clear segregation of chemically related species depending on their different functional groups. We map the emission of ¹³CH3OH, HCOOCH3, CH3OCH3, CH2OCH2, CH3COOCH3, HCOOCH2CH3, CH3CH2OCH3, HCOOH, OHCH2CH2OH, CH3COOH, CH3CH2OH, CH3OCH2OH, OHCH2CHO, and CH3COCH3 with ∼1.5″ angular resolution and provide molecular abundances of these species toward different gas components of this region. We disentangle the emission of these species in the different Orion components by carefully selecting lines free of blending and opacity effects. Possible effects in the molecular spatial distribution due to residual blendings and different excitation conditions are also addressed. We find that while species containing the C-O-C group, i.e. an ether group, exhibit their peak emission and higher abundance towards the compact ridge, the hot core south is the component where species containing a hydroxyl group (-OH) bound to a carbon atom (C-O-H) present their emission peak and higher abundance. This finding allows us to propose methoxy (CH3O-) and hydroxymethyl (-CH2OH) radicals as the major drivers of the chemistry in the compact ridge and the hot core south, respectively, as well as different evolutionary stages and prevailing physical processes in the different Orion components.
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Context . Intermediate-mass (IM) protostars provide a bridge between the low- and high-mass protostars. Despite their relevance, little is known about their chemical diversity. Aims . We want to investigate the molecular richness towards the envelope of I-M protostars and to compare their properties with those of low- and high-mass sources. Methods . We have selected the isolated IM Class 0 protostar Cep E-mm to carry out an unbiased molecular survey with the IRAM 30 m telescope between 72 and 350 GHz with an angular resolution lying in the range 7–34″. Our goal is to obtain a census of the chemical content of the protostellar envelope. These data were complemented with NOEMA observations of the spectral bands 85.9–89.6 GHz and 216.8–220.4 GHz at angular resolutions of 2.3″ and 1.4″, respectively. Results . The 30 m spectra show bright emission of O- and N-bearing complex organic molecules (COMs): CH 3 OH and its rare isotopologues CH 2 DOH and ¹³ CH 3 OH, CH 3 CHO, CH 3 OCH 3 , CH 3 COCH 3 , HCOOH, HCOOCH 3 , H 2 CCO, NH 2 CHO, CH 3 CN, C 2 H 3 CN, C 2 H 5 CN, HNCO and H 2 CO. We identify up to three components in the spectral signature of COMs: an extremely broad line (eBL) component associated with the outflowing gas ( FWHM > 7kms ⁻¹ ), a narrow line (NL) component ( FWHM < 3kms ⁻¹ ) associated with the cold envelope, and a broad line (BL) component ( FWHM ≃ 5.5kms ⁻¹ ) which traces the signature of a hot corino. The eBL and NL components are detected only in molecular transitions of low excitation and dominate the emission of CH 3 OH. The BL component is detected in highly excited gas ( E up > 100 K). The NOEMA observations reveal Cep E-mm as a binary protostellar system, whose components, Cep E-A and Cep E-B, are separated by ≈1.7″. Cep E-A dominates the core continuum emission and powers the long-studied, well-known, high-velocity jet associated with HH377. The lower flux source Cep E-B powers another high-velocity molecular jet, reaching velocities of ≈80 km s ⁻¹ , which propagates in a direction close to perpendicular with respect to the Cep E-A jet. Our interferometric maps show that the emission of COMs arises from a region of ≈0.7″ size around Cep E-A, and corresponds to the BL component detected with the IRAM 30 m telescope. On the contrary, no COM emission is detected towards Cep E-B. We have determined the rotational temperature ( T rot ) and the molecular gas column densities from a simple population diagram analysis or assuming a given excitation temperature. Rotational temperatures of COMs emission were found to lie in the range 20−40 K with column densities ranging from a few times 10 ¹⁵ cm ⁻² for O-bearing species, down to a few times 10 ¹⁴ cm ⁻² for N-bearing species. Molecular abundances are similar to those measured towards other low- and intermediate-mass protostars. Ketene (H 2 CCO) appears as an exception, as it is found significantly more abundant towards Cep E-A. High-mass hot cores are significantly less abundant in methanol and N-bearing species are more abundant by two to three orders of magnitude. Conclusions . Cep E-mm reveals itself as a binary protostellar system with a strong chemical differentiation between both cores. Only the brightest component of the binary is associated with a hot corino. Its properties are similar to those of low-mass hot corinos.
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The formation of asteroids, comets, and planets occurs in the interior of protoplanetary disks during the early phase of star formation. Consequently, the chemical composition of the disk might shape the properties of the emerging planetary system. In this context, it is crucial to understand whether and what organic molecules are synthesized in the disk. In this Letter, we report the first detection of formic acid (HCOOH) toward the TW Hydrae protoplanetary disk. The observations of the trans-HCOOH 6_((1,6)–5(1,5)) transition were carried out at 129 GHz with Atacama Large Millimeter/Submillimeter Array (ALMA). We measured a disk-averaged gas-phase t-HCOOH column density of ~(2–4) × 10^(12) cm^(−2), namely as large as that of methanol. HCOOH is the first organic molecule containing two oxygen atoms detected in a protoplanetary disk, a proof that organic chemistry is very active, albeit difficult to observe, in these objects. Specifically, this simplest acid stands as the basis for synthesis of more complex carboxylic acids used by life on Earth.
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IRAS 16293-2422 is a very well studied young stellar system seen in projection towards the L1689N cloud in the Ophiuchus complex. However, its distance is still uncertain with a range of values from 120 pc to 180 pc. Our goal is to measure the trigonometric parallax of this young star by means of H$_2$O maser emission. We use archival data from 15 epochs of VLBA observations of the 22.2 GHz water maser line. By modeling the displacement on the sky of the H$_2$O maser spots, we derived a trigonometric parallax of $7.1\pm1.3$ mas, corresponding to a distance of $141_{-21}^{+30}$ pc. This new distance is in good agreement with recent values obtained for other magnetically active young stars in the L1689 cloud. We relate the kinematics of these masers with the outflows and the recent ejections powered by source A in the system.
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The evolutionary past of our Solar System can be pieced together by comparing analogous low-mass protostars with remnants of our Protosolar Nebula - comets. Sulphur-bearing molecules may be unique tracers of the joint evolution of the volatile and refractory components. ALMA Band 7 data from the large unbiased Protostellar Interferometric Line Survey (PILS) are used to search for S-bearing molecules in the outer disc-like structure, 60 au from IRAS 16293-2422 B, and are compared with data on 67P/C-G stemming from the ROSINA instrument aboard Rosetta. Species such as SO$_{2}$, SO, OCS, CS, H$_{2}$CS, H$_{2}$S and CH$_{3}$SH are detected via at least one of their isotopologues towards IRAS 16293-2422 B. The search reveals a first-time detection of OC$^{33}$S towards this source and a tentative first-time detection of C$^{36}$S towards a low-mass protostar. The data show that IRAS 16293-2422 B contains much more OCS than H$_{2}$S in comparison to 67P/C-G; meanwhile, the SO/SO$_{2}$ ratio is in close agreement between the two targets. IRAS 16293-2422 B has a CH$_{3}$SH/H$_{2}$CS ratio in range of that of our Solar System (differences by a factor of 0.7-5.3). It is suggested that the levels of UV radiation during the initial collapse of the systems may have varied and have potentially been higher for IRAS 16293-2422 B due to its binary nature; thereby, converting more H$_{2}$S into OCS. It remains to be conclusively tested if this also promotes the formation of S-bearing complex organics. Elevated UV levels of IRAS 16293-2422 B and a warmer birth cloud of our Solar System may jointly explain the variations between the two low-mass systems.
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The astronomical gas-phase detection of simple species and small organic molecules in cold pre-stellar cores, with abundances as high as ∼10⁻⁸-10⁻⁹ n H, contradicts the generally accepted idea that at 10 K, such species should be fully frozen out on grain surfaces. A physical or chemical mechanism that results in a net transfer from solid-state species into the gas phase offers a possible explanation. Reactive desorption, i.e., desorption following the exothermic formation of a species, is one of the options that has been proposed. In astronomical models, the fraction of molecules desorbed through this process is handled as a free parameter, as experimental studies quantifying the impact of exothermicity on desorption efficiencies are largely lacking. In this work, we present a detailed laboratory study with the goal of deriving an upper limit for the reactive desorption efficiency of species involved in the CO-H2CO-CH3OH solid-state hydrogenation reaction chain. The limit for the overall reactive desorption fraction is derived by precisely investigating the solid-state elemental carbon budget, using reflection absorption infrared spectroscopy and the calibrated solid-state band-strength values for CO, H2CO and CH3OH. We find that for temperatures in the range of 10 to 14 K, an upper limit of 0.24 ±0.02 for the overall elemental carbon loss upon CO conversion into CH3OH. This corresponds with an effective reaction desorption fraction of ≤0.07 per hydrogenation step, or ≤0.02 per H-atom induced reaction, assuming that H-atom addition and abstraction reactions equally contribute to the overall reactive desorption fraction along the hydrogenation sequence. The astronomical relevance of this finding is discussed. © 2018. The American Astronomical Society. All rights reserved.
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Glycolaldehyde (HOCH$_2$CHO) and ethylene glycol ((CH$_2$OH)$_2$) are two complex organic molecules detected in the hot cores and hot corinos of several star-forming regions. The ethylene glycol/glycolaldehyde abundance ratio seems to show an increase with the source luminosity. In the literature, several surface-chemistry formation mechanisms have been proposed for these two species. With the UCLCHEM chemical code, we explored the different scenarios and compared the predictions for a range of sources of different luminosities with the observations. None of the scenarios reproduce perfectly the trend. A better agreement is, however, found for a formation through recombination of two HCO radicals followed by successive hydrogenations. The reaction between HCO and CH$_2$OH could also contribute to the formation of glycolaldehyde in addition to the hydrogenation pathway. The predictions are improved when a trend of decreasing H$_2$ density within the core region with T $\geq$ 100 K as a function of luminosity, is included in the model. Destruction reactions of complex organic molecules in the gas phase would also need to be investigated, since they can affect the abundance ratios once the species have desorbed in the warm inner regions of the star-forming regions.
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Despite the harsh conditions of the interstellar medium, chemistry thrives in it, especially in star forming regions where several interstellar complex organic molecules (iCOMs) have been detected. Yet, how these species are synthesised is a mystery. The majority of current models claim that this happens on interstellar grain surfaces. Nevertheless, evidence is mounting that neutral gas-phase chemistry plays an important role. In this article, we propose a new scheme for the gas-phase synthesis of glycolaldehyde, a species with a prebiotic potential and for which no gas-phase formation route was previously known. In the proposed scheme, the ancestor is ethanol and the glycolaldehyde sister species are acetic acid (another iCOM with unknown gas-phase formation routes) and formic acid. For the reactions of the new scheme with no available data, we have performed electronic structure and kinetics calculations deriving rate coefficients and branching ratios. Furthermore, after a careful review of the chemistry literature, we revised the available chemical networks, adding and correcting several reactions related to glycolaldehyde, acetic acid and formic acid. The new chemical network has been used in an astrochemical model to predict the abundance of glycolaldehyde, acetic acid and formic acid. The predicted abundance of glycolaldehyde depends on the ethanol abundance in the gas phase and is in excellent agreement with the measured one in hot corinos and shock sites. Our new model overpredicts the abundance of acetic acid and formic acid by about a factor of ten, which might imply a yet incomplete reaction network.
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Context. The Class 0 protostellar binary IRAS 16293–2422 is an interesting target for (sub)millimeter observations due to, both, the rich chemistry toward the two main components of the binary and its complex morphology. Its proximity to Earth allows the study of its physical and chemical structure on solar system scales using high angular resolution observations. Such data reveal a complex morphology that cannot be accounted for in traditional, spherical 1D models of the envelope. Aims. The purpose of this paper is to study the environment of the two components of the binary through 3D radiative transfer modeling and to compare with data from the Atacama Large Millimeter/submillimeter Array. Such comparisons can be used to constrain the protoplanetary disk structures, the luminosities of the two components of the binary and the chemistry of simple species. Methods. We present ¹³ CO, C ¹⁷ O and C ¹⁸ O J = 3 – 2 observations from the ALMA Protostellar Interferometric Line Survey (PILS), together with a qualitative study of the dust and gas density distribution of IRAS 16293–2422. A 3D dust and gas model including disks and a dust filament between the two protostars is constructed which qualitatively reproduces the dust continuum and gas line emission. Results. Radiative transfer modeling in our sampled parameter space suggests that, while the disk around source A could not be constrained, the disk around source B has to be vertically extended. This puffed-up structure can be obtained with both a protoplanetary disk model with an unexpectedly high scale-height and with the density solution from an infalling, rotating collapse. Combined constraints on our 3D model, from observed dust continuum and CO isotopologue emission between the sources, corroborate that source A should be at least six times more luminous than source B. We also demonstrate that the volume of high-temperature regions where complex organic molecules arise is sensitive to whether or not the total luminosity is in a single radiation source or distributed into two sources, affecting the interpretation of earlier chemical modeling efforts of the IRAS 16293–2422 hot corino which used a single-source approximation. Conclusions. Radiative transfer modeling of source A and B, with the density solution of an infalling, rotating collapse or a protoplanetary disk model, can match the constraints for the disk-like emission around source A and B from the observed dust continuum and CO isotopologue gas emission. If a protoplanetary disk model is used around source B, it has to have an unusually high scale-height in order to reach the dust continuum peak emission value, while fulfilling the other observational constraints. Our 3D model requires source A to be much more luminous than source B; L A ~ 18 L ⊙ and L B ~ 3 L ⊙ .
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We report the detection of interstellar methoxymethanol (CH$_3$OCH$_2$OH) in ALMA Bands 6 and 7 toward the MM1 core in the high-mass star-forming region NGC 6334I at ~0.1" - 1" spatial resolution. A column density of 4(2) x $10^{18}$ cm$^{-2}$ at $T_{ex}$ = 200 K is derived toward MM1, ~34 times less abundant than methanol (CH$_3$OH), and significantly higher than predicted by astrochemical models. Probable formation and destruction pathways are discussed, primarily through the reaction of the CH$_3$OH photodissociation products, the methoxy (CH$_3$O) and hydroxymethyl (CH$_2$OH) radicals. Finally, we comment on the implications of these mechanisms on gas-phase vs grain-surface routes operative in the region, and the possibility of electron-induced dissociation of CH$_3$OH rather than photodissociation.
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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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Complex organic molecules have been observed for decades in the interstellar medium. Some of them might be considered as small bricks of the macromolecules at the base of terrestrial life. It is hence particularly important to understand organic chemistry in Solar-like star forming regions. In this article, we present a new observational project: SOLIS (Seeds Of Life In Space). This is a Large Project at the IRAM-NOEMA interferometer, and its scope is to image the emission of several crucial organic molecules in a sample of Solar-like star forming regions in different evolutionary stage and environments. Here, we report the first SOLIS results, obtained from analysing the spectra of different regions of the Class 0 source NGC1333-IRAS4A, the protocluster OMC-2 FIR4, and the shock site L1157-B1. The different regions were identified based on the images of formamide (NH2CHO) and cyanodiacetylene (HC5N) lines. We discuss the observed large diversity in the molecular and organic content, both on large (3000-10000 au) and relatively small (300-1000 au) scales. Finally, we derive upper limits to the methoxy fractional abundance in the three observed regions of the same order of magnitude of that measured in few cold prestellar objects, namely ~10^-12-10^-11 with respect to H2 molecules.
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Organohalogens, a class of molecules that contain at least one halogen atom bonded to carbon, are abundant on the Earth where they are mainly produced through industrial and biological processes ¹. Consequently, they have been proposed as biomarkers in the search for life on exoplanets ². Simple halogen hydrides have been detected in interstellar sources and in comets, but the presence and possible incorporation of more complex halogen-containing molecules such as organohalogens into planet-forming regions is uncertain 3,4. Here we report the interstellar detection of two isotopologues of the organohalogen CH3Cl and put some constraints on CH3F in the gas surrounding the low-mass protostar IRAS 16293-2422, using the Atacama Large Millimeter/submillimeter Array (ALMA). We also find CH3Cl in the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) by using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument. The detections reveal an efficient pre-planetary formation pathway of organohalogens. Cometary impacts may deliver these species to young planets and should thus be included as a potential abiotical production source when interpreting future organohalogen detections in atmospheres of rocky planets.
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We report the detection of widespread CH$_2$OHCHO and HOCH$_2$CH$_2$OH emission in Galactic center giant molecular cloud Sagittarius B2 using the Shanghai Tianma 65m Radio Telescope. Our observations show for the first time that the spatial distribution of these two important prebiotic molecules extends over 15 arc-minutes, corresponding to a linear size of approximately 36 pc. These two molecules are not just distributed in or near the hot cores. The abundance of these two molecules seems to decrease from the cold outer region to the central region associated with star-formation activity. Results present here suggest that these two molecules are likely to form through a low temperature process. Recent theoretical and experimental studies demonstrated that prebiotic molecules can be efficiently formed in icy grain mantles through several pathways. However, these complex ice features cannot be directly observed, and most constraints on the ice compositions come from millimeter observations of desorbed ice chemistry products. These results, combined with laboratory studies, strongly support the existence of abundant prebiotic molecules in ices.
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Observational studies reveal that complex organic molecules (COMs) can be found in various objects associated with different star formation stages. The identification of COMs in prestellar cores, i.e., cold environments in which thermally induced chemistry can be excluded and radiolysis is limited by cosmic rays and cosmic ray induced UV-photons, is particularly important as this stage sets up the initial chemical composition from which ultimately stars and planets evolve. Recent laboratory results demonstrate that molecules as complex as glycolaldehyde and ethylene glycol are efficiently formed on icy dust grains via non-energetic atom addition reactions between accreting H atoms and CO molecules, a process that dominates surface chemistry during the 'CO-freeze out stage' in dense cores. In the present study we demonstrate that a similar mechanism results in the formation of the biologically relevant molecule glycerol - HOCH2CH(OH)CH2OH - a three-carbon bearing sugar alcohol necessary for the formation of membranes of modern living cells and organelles. Our experimental results are fully consistent with a suggested reaction scheme in which glycerol is formed along a chain of radical-radical and radical-molecule interactions between various reactive intermediates produced upon hydrogenation of CO ice or its hydrogenation products. The tentative identification of the chemically related simple sugar glyceraldehyde - HOCH2CH(OH)CHO - is discussed as well. These new laboratory findings indicate that the proposed reaction mechanism holds much potential to form even more complex sugar alcohols and simple sugars.
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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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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.
Article
Our modern day Solar System has 4.6 × 109 yr of evolution behind it with just a few relics of its birth conditions remaining. Comets are thought to be some of the most pristine tracers of the initial ingredients that were combined to produce the Earth and the other planets. Other low-mass protostars may be analogous to our proto-Sun and hence, could be used to study the building blocks necessary to form Solar-like systems. This study tests this idea on the basis of new high sensitivity, high spatial resolution ALMA data on the protoplanetary disc-scales (∼70 au) of IRAS 16293-2422 and the bulk composition of comet 67P/Churyumov-Gerasimenko, as determined for the first time with the unique in situ monitoring carried out by Rosetta. The comparative analysis of the observations from the Protostellar Interferometric Line Survey (PILS) and the measurements made with Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) shows that the relative abundances of CHO-, N-, and S-bearing molecules correlate, with some scatter, between protostellar and cometary data. A tentative correlation is seen for the first time for P- and Cl-bearing compounds. The results imply that the volatile composition of cometesimals and planetesimals is partially inherited from the pre- and protostellar phases of evolution.
Article
Context. Spectral lines of minor isotopic species of molecules that are abundant in space may also be detectable. Their respective isotopic ratios may provide clues about the formation of these molecules. Emission lines of acetone in the hot molecular core Sagittarius B2(N2) are strong enough to warrant a search for its singly substituted ¹³ C isotopologs. Aims. We want to study the rotational spectra of CH 3¹³ C(O)CH 3 and ¹³ CH 3 C(O)CH 3 and search for them in Sagittarius B2(N2). Methods. We investigated the laboratory rotational spectrum of isotopically enriched CH 3¹³ C(O)CH 3 between 40 GHz and 910 GHz and of acetone between 36 GHz and 910 GHz in order to study ¹³ CH 3 C(O)CH 3 in natural isotopic composition. In addition, we searched for emission lines produced by these species in a molecular line survey of Sagittarius B2(N) carried out with the Atacama Large Millimeter/submillimeter Array (ALMA). Discrepancies between predictions of the main isotopic species and the ALMA spectrum prompted us to revisit the rotational spectrum of this isotopolog. Results. We assigned 9711 new transitions of CH 3¹³ C(O)CH 3 and 63 new transitions of ¹³ CH 3 C(O)CH 3 in the laboratory spectra. More than 1000 additional transitions were assigned for the main isotopic species. We modeled the ground state data of all three isotopologs satisfactorily with the ERHAM program. We find that models of the torsionally excited states v12 = 1 and v17 = 1 of CH 3 C(O)CH 3 improve only marginally. No transitrrrion of CH 3¹³ C(O)CH 3 is clearly detected toward the hot molecular core Sgr B2(N2). However, we report a tentative detection of ¹³ CH 3 C(O)CH 3 with a ¹² C/ ¹³ C isotopic ratio of 27 that is consistent with the ratio previously measured for alcohols in this source. Several dozens of transitions of both torsional states of the main isotopolog are detected as well. Conclusion. Our predictions of CH 3¹³ C(O)CH 3 and CH 3 C(O)CH 3 are reliable into the terahertz region. The spectrum of ¹³ CH 3 C(O)CH 3 should be revisited in the laboratory with an enriched sample. The torsionally excited states v12 = 1 and v17 = 1 of CH 3 C(O)CH 3 were not reproduced satisfactorily in our models. Nevertheless, transitions pertaining to both states could be identified unambiguously in Sagittarius B2(N2).
Article
Context. The chemical composition of high-mass protostars reflects the physical evolution associated with different stages of star formation. In addition, the spatial distribution and velocity structure of different molecular species provide valuable information on the physical structure of these embedded objects. Despite an increasing number of interferometric studies, there is still a high demand for high angular resolution data to study chemical compositions and velocity structures for these objects. Aims. The molecular inventory of the forming high-mass star AFGL 4176, located at a distance of ~3.7 kpc, is studied in detail at a high angular resolution of ~0.35′′, equivalent to ~1285 au at the distance of AFGL 4176. This high resolution makes it possible to separate the emission associated with the inner hot envelope and disc around the forming star from that of its cool outer envelope. The composition of AFGL 4176 is compared with other high- and low-mass sources, and placed in the broader context of star formation. Methods. Using the Atacama Large Millimeter/submillimeter Array (ALMA) the chemical inventory of AFGL 4176 has been characterised. The high sensitivity of ALMA made it possible to identify weak and optically thin lines and allowed for many isotopologues to be detected, providing a more complete and accurate inventory of the source. For the detected species, excitation temperatures in the range 120–320 K were determined and column densities were derived assuming local thermodynamic equilibrium and using optically thin lines. The spatial distribution of a number of species was studied. Results. A total of 23 different molecular species and their isotopologues are detected in the spectrum towards AFGL 4176. The most abundant species is methanol (CH 3 OH) with a column density of 5.5 × 10 ¹⁸ cm ⁻² in a beam of ~0.3′′, derived from its ¹³ C-isotopologue. The remaining species are present at levels between 0.003 and 15% with respect to methanol. Hints that N-bearing species peak slightly closer to the location of the peak continuum emission than the O-bearing species are seen. A single species, propyne (CH 3 C 2 H), displays a double-peaked distribution. Conclusions. AFGL 4176 comprises a rich chemical inventory including many complex species present on disc scales. On average, the derived column density ratios, with respect to methanol, of O-bearing species are higher than those derived for N-bearing species by a factor of three. This may indicate that AFGL 4176 is a relatively young source since nitrogen chemistry generally takes longer to evolve in the gas phase. Taking methanol as a reference, the composition of AFGL 4176 more closely resembles that of the low-mass protostar IRAS 16293–2422B than that of high-mass, star-forming regions located near the Galactic centre. This similarity hints that the chemical composition of complex species is already set in the cold cloud stage and implies that AFGL 4176 is a young source whose chemical composition has not yet been strongly processed by the central protostar.
Article
Context. The majority of stars form in binary or higher order systems. The evolution of each protostar in a multiple system may start at different times and may progress differently. The Class 0 protostellar system IRAS 16293–2422 contains two protostars, “A” and “B”, separated by ~600 au and embedded in a single, 10 ⁴ au scale envelope. Their relative evolutionary stages have been debated. Aims. We aim to study the relation and interplay between the two protostars A and B at spatial scales of 60 au up to ~10 ³ au. Methods. We selected molecular gas line transitions of the species CO, H 2 CO, HCN, CS, SiO, and C 2 H from the ALMA-PILS spectral imaging survey (329–363 GHz) and used them as tracers of kinematics, density, and temperature in the IRAS 16293–2422 system. The angular resolution of the PILS data set allows us to study these quantities at a resolution of 0.5′′ (60 au at the distance of the source). Results. Line-of-sight velocity maps of both optically thick and optically thin molecular lines reveal: (i) new manifestations of previously known outflows emanating from protostar A; (ii) a kinematically quiescent bridge of dust and gas spanning between the two protostars, with an inferred density between 4 × 10 ⁴ cm ⁻³ and ~3 × 10 ⁷ cm ⁻³ ; and (iii) a separate, straight filament seemingly connected to protostar B seen only in C 2 H, with a flat kinematic signature. Signs of various outflows, all emanating from source A, are evidence of high-density and warmer gas; none of them coincide spatially and kinematically with the bridge. Conclusions. We hypothesize that the bridge arc is a remnant of filamentary substructure in the protostellar envelope material from which protostellar sources A and B have formed. One particular morphological structure appears to be due to outflowing gas impacting the quiescent bridge material. The continuing lack of clear outflow signatures unambiguously associated to protostar B and the vertically extended shape derived for its disk-like structure lead us to conclude that source B may be in an earlier evolutionary stage than source A.
Article
Dimethyl ether is one of the most abundant interstellar complex organic molecules. Yet its formation route remains elusive. In this work, we have performed electronic structure and kinetics calculations to derive the rate coefficients for two ion-molecule reactions recently proposed as a gas-phase formation route of dimethyl ether in interstellar objects, namely CH 3 OH+CH 3 OH ⁺2 →(CH 3 ) 2 OH+ +H 2 Ofollowed by (CH 3 ) 2 OH+ +NH 3 →CH 3 OCH 3 + NH ⁺4 . A comparison with previous experimental rate coefficients for the reaction CH 3 OH + CH 3 OH ⁺2 sustains the accuracy of the present calculations and allows a more reliable extrapolation at the low temperatures of interest in interstellar objects (10-100 K). The rate coefficient for the reaction (CH 3 ) 2 OH+ + NH 3 is, instead, provided for the first time ever. The rate coefficients derived in this work essentially confirm the prediction by Taquet,Wirström & Charnley concerning dimethyl ether formation in hot cores/corinos. Nevertheless, this formation route cannot be efficient in cold objects (like pre-stellar cores) where dimethyl ether is also detected, because ammonia has a very low abundance in those environments.
Article
Context . Detection of deuterated species may provide information on the evolving chemistry in the earliest phases of star-forming regions. For molecules with two isomeric forms of the same isotopic variant, gas-phase and solid-state formation pathways can be differentiated using their abundance ratio. Aims . Spectroscopic databases for astrophysical purposes are built for the two mono deuterated isomeric species CH 2 DCOH and CH 3 COD of the complex organic molecule acetaldehyde. These databases can be used to search and detect these two species in astrophysical surveys, retrieving their column density and therefore abundances. Methods . Submillimeter wave and terahertz transitions were measured for mono deuterated acetaldehyde CH 2 DCOH which is a non-rigid species displaying internal rotation of its asymmetrical CH 2 D methyl group. An analysis of a dataset consisting of previously measured microwave data and the newly measured transition was carried out with a model accounting for the large amplitude torsion. Results . The frequencies of 2556 transitions are reproduced with a unitless standard deviation of 2.3 yielding various spectroscopic constants. Spectroscopic databases for astrophysical purposes were built for CH 2 DCOH using the results of the present analysis and for CH 3 COD using the results of a previous spectroscopic investigation. These two species were both searched for and are detected toward a low-mass star-forming region. Conclusions . We report the first detection of CH 2 DCOH (93 transitions) and the detection of CH 3 COD (43 transitions) species in source B of the IRAS 16293−2422 young stellar binary system located in the ρ Ophiuchus cloud region, using the publicly available ALMA Protostellar Interferometric Line Survey.