Article

The ALMA-PILS survey: first detection of methyl isocyanide (CH 3 NC) in a solar-type protostar

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Abstract

Context. Methyl isocyanide (CH 3 NC) is the isocyanide with the largest number of atoms confirmed in the interstellar medium (ISM), but it is not an abundant molecule, having only been detected towards a handful of objects. Conversely, its isomer, methyl cyanide (CH 3 CN), is one of the most abundant complex organic molecules detected in the ISM, with detections in a variety of low- and high-mass sources. Aims. The aims of this work are to determine the abundances of methyl isocyanide in the solar-type protostellar binary IRAS 16293–2422 and to understand the stark abundance differences observed between methyl isocyanide and methyl cyanide in the ISM. Methods. We use Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) to search for methyl isocyanide and compare its abundance with that of its isomer methyl cyanide. We use a new line catalogue from the Cologne Database for Molecular Spectroscopy (CDMS) to identify methyl isocyanide lines. We also model the chemistry with an updated version of the three-phase chemical kinetics model MAGICKAL, presenting the first chemical modelling of methyl isocyanide to date. Results. We detect methyl isocyanide for the first time in a solar-type protostar, IRAS 16293–2422 B, and present upper limits for its companion protostar, IRAS 16293–2422 A. Methyl isocyanide is found to be at least 20 times more abundant in source B compared to source A, with a CH 3 CN/CH 3 NC abundance ratio of 200 in IRAS 16293–2422 B and >5517 in IRAS 16293–2422 A. We also present the results of a chemical model of methyl isocyanide chemistry in both sources, and discuss the implications for methyl isocyanide formation mechanisms and the relative evolutionary stages of both sources. The chemical modelling is unable to match the observed CH 3 CN/CH 3 NC abundance ratio towards the B source at densities representative of that source. The modelling, however, is consistent with the upper limits for the A source. There are many uncertainties in the formation and destruction pathways of methyl isocyanide, and it is therefore not surprising that the initial modelling attempts do not reproduce observations. In particular, it is clear that some destruction mechanism of methyl isocyanide that does not destroy methyl cyanide is needed. Furthermore, these initial model results suggest that the final density plays a key role in setting the abundance ratio. The next steps are therefore to obtain further detections of methyl isocyanide in more objects, as well as undertaking more detailed physico-chemical modelling of sources such as IRAS16293.

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... Another interesting nitrile molecule to study is the methyl isocyanide (CH 3 NC), the isomer of methyl cyanide (CH 3 CN). First detected toward Sgr B2 (Cernicharo et al. 1988;Remijan et al. 2005), CH 3 NC was also detected toward the Horsehead nebula (Gratier et al. 2013), Orion KL (López et al. 2014), and more recently toward the solar-type binary protostar IRAS16293-2422 (Calcutt et al. 2018). A few theoretical and experimental studies have investigated the isomers' chemistry and their abundance ratio (Huntress & Mitchell 1979;Defrees et al. 1985;Anicich et al. 1995), and converged on the same major gas-phase production pathways for both via the reaction: ...
... While there is good agreement between observations and models in both a classic PDR and a planet-forming disk, it is important to note that the nitrile grain chemistry is still poorly constrained (e.g., Bertin et al. 2017aBertin et al. , 2017bCalcutt et al. 2018;Nguyen et al. 2019), and it may contribute to both kinds of regions. Experiments and theory on nitrile grain-surface chemistry and desorption are needed to make progress here. ...
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... In contrast to the numerous detection of cyanides in astronomical environments, there have been very few confirmed detection of isocyanides, such as methyl isocyanide (CH 3 NC) (Remijan et al. 2005;Gratier et al. 2013). Most recently, the Protostellar Interferometric Line Survey (PILS) observed CH 3 NC in a solar-type star, IRAS 16293-2422, for the first time toward a source of this type (Calcutt et al. 2018). Despite that, there have been no successful detections of CH 2 CHCH 2 NC (Haykal et al. 2013) or CH 3 CH 2 NC (Remijan et al. 2005;Margulès et al. 2018). ...
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... On the other hand, O-COMs toward IRAS 16293-2422 B can be divided in two groups based on their excitation temperature (∼125 K versus ∼300 K; Jørgensen et al. 2018), consistent with different binding energies. N-COMs generally have excitation temperatures of 100-150 K (Calcutt et al. 2018a(Calcutt et al. , 2018b, but a hightemperature component may be hidden inside the unresolved soot line or by the optically thick dust. ...
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... Nonetheless, unpublished and recently published results on column densities of nitriles (−C≡N-bearing organics) are consistent with HCN containing the majority of N in organics in IRAS16293. Using the PILS data, Calcutt et al. (2018aCalcutt et al. ( , 2018b studied complex nitriles in IRAS16293, including CH 3 CN, C 2 H 5 CN, C 2 H 3 CN, HC 3 N, and CH 3 NC. Of these, CH 3 CN is by far the most abundant; it is an order of magnitude more abundant than the next most abundant species, C 2 H 5 CN. ...
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... The COMs emission observed in V883 Ori traces fresh sublimates at the edges of the water sublimation front, where the disk becomes optically thin 5 . COMs abundances in V883 Ori are much more similar to those in comets 6 than to those in the protostellar core IRAS 16293 B [7][8][9] , which represent older and younger evolutionary stages, respectively. The chemical evolution of COMs continues after the volatiles are incorporated to the disk and during the planet formation process. ...
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The interstellar detection of CH 3 CN metastable isomers would suggest that CH 3 NC and H 2 C=C=NH formed in star-forming regions through energetic processing provoked by shocks or others energy sources. In this context, laboratory simulations have been carried out to investigate the chemical transformation of CH 3 CN into CH 3 NC and H 2 C=C=NH induced by UV photolysis and high energy particle irradiation. In the present study, we have carried out the CH 3 CN + N solid state reaction in the 10-40 K temperature range in order to examine the behavior of acetonitrile interacting with nitrogen atoms in icy interstellar grains. We show that CH 3 CN + N is efficient in the solid phase but only in a very specific temperature range, which combines high mobility and relatively long surface residence time of N atoms to allow the CH 3 CN activation. By focusing on the behavior of [CH 3 NC]/[H 2 C=C=NH] abundance ratios versus temperature, we have measured abundance ratios around 10.4 at 10 K, which decreases to 6.8 when the temperature of the reaction increases. These ratios are of the same order of magnitude as those reported from acetonitrile isomers detection toward Sagittarius B2(N). In contrast, previous studies involving energetic processing of solid CH 3 CN, CH 3 NC and CH 2 CNH have found that they have been formed with [CH 3 NC]/[CH 2 CNH] ratios ranging between 0.3 and 1.7. Additionally, the analysis of CH 3 NC and H 2 C=C=NH column densities shows that at low temperatures, the less stable isomer is favored against the most stable one. These results are compared to the puzzling behavior of CN-containing isomers such as HNC, HCN, HCNO and HOCN in molecular clouds.
Article
Hydroxyacetone (CH3COCH2OH) is one of the smallest molecules that contain both hydroxyl and carbonyl group on neighboring carbon atoms. This steric configuration is characteristic of saccharides and determines their biochemical activity. The attempt to search for hydroxyacetone toward the massive star formation region Sagittarius B2(N) was unsuccessful. Here we report the first detection of CH3COCH2OH in the solar-type protostar IRAS 16293–2422B, using the Atacama Large Millimeter Array science verification data at Band 4. In a total of 11 unblended transitions of CH3COCH2OH with upper level energies ranging from 86 to 246 K are identified. From our local thermodynamic equilibrium analysis, we derived that the rotational temperature of CH3COCH2OH is 160±21 K and the column density is (1.2±1.0) ×1016 cm−2, which results in a fractional abundance of 7×10−10 with respect to molecular hydrogen. In this work, we present the identification of CH3COCH2OH in IRAS 16293–2422B and propose a simple formation mechanism. The unambiguous identification of hydroxyacetone may provide the basis for future study of the origin and evolution of saccharides in the interstellar medium.
Article
Star-forming regions show a rich and varied chemistry, including the presence of complex organic molecules—in both the cold gas distributed on large scales and the hot regions close to young stars where protoplanetary disks arise. Recent advances in observational techniques have opened new possibilities for studying this chemistry. In particular, the Atacama Large Millimeter/submillimeter Array has made it possible to study astrochemistry down to Solar System–size scales while also revealing molecules of increasing variety and complexity. In this review, we discuss recent observations of the chemistry of star-forming environments, with a particular focus on complex organic molecules, taking context from the laboratory experiments and chemical models that they have stimulated. The key takeaway points include the following: ▪ The physical evolution of individual sources plays a crucial role in their inferred chemical signatures and remains an important area for observations and models to elucidate. ▪ Comparisons of the abundances measured toward different star-forming environments (high-mass versus low-mass, Galactic Center versus Galactic disk) reveal a remarkable similarity, which is an indication that the underlying chemistry is relatively independent of variations in their physical conditions. ▪ Studies of molecular isotopologues in star-forming regions provide a link with measurements in our own Solar System, and thus may shed light on the chemical similarities and differences expected in other planetary systems. Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 58 is August 18, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Article
Context. Complex organic molecules with three carbon atoms are found in the earliest stages of star formation. In particular, propenal (C 2 H 3 CHO) is a species of interest due to its implication in the formation of more complex species and even biotic molecules. Aims. This study aims to search for the presence of C 2 H 3 CHO and other three-carbon species such as propylene (C 3 H 6 ) in the hot corino region of the low-mass protostellar binary IRAS 16293–2422 to understand their formation pathways. Methods. We use ALMA observations in Band 6 and 7 from various surveys to search for the presence of C 3 H 6 and C 2 H 3 CHO towards the protostar IRAS 16293–2422 B (IRAS 16293B). The identification of the species and the estimates of the column densities and excitation temperatures are carried out by modeling the observed spectrum under the assumption of local thermodynamical equilibrium. Results. We report the detection of both C 3 H 6 and C 2 H 3 CHO towards IRAS 16293B, however, no unblended lines were found towards the other component of the binary system, IRAS 16293A. We derive column density upper limits for C 3 H 8 , HCCCHO, n -C 3 H 7 OH, i -C 3 H 7 OH, C 3 O, and cis-HC(O)CHO towards IRAS 16293B. We then use a three-phase chemical model to simulate the formation of these species in a typical prestellar environment followed by its hydrodynamical collapse until the birth of the central protostar. Different formation paths, such as successive hydrogenation and radical-radical additions on grain surfaces, are tested and compared to the observational results in a number of different simulations, to assess which are the dominant formation mechanisms in the most embedded region of the protostar. Conclusions. The simulations reproduce the abundances within one order of magnitude from those observed towards IRAS 16293B, with the best agreement found for a rate of 10 ⁻¹² cm ³ s ⁻¹ for the gas-phase reaction C 3 + O → C 2 + CO. Successive hydrogenations of C 3 , HC(O)CHO, and CH 3 OCHO on grain surfaces are a major and crucial formation route of complex organics molecules, whereas both successive hydrogenation pathways and radical-radical addition reactions contribute to the formation of C 2 H 5 CHO.
Preprint
Earth's carbon deficit has been an outstanding problem in our understanding of the formation of our Solar System. A possible solution would be the sublimation of carbon grains at the so-called soot line (~300 K) early in the planet-formation process. Here, we argue that the most likely signatures of this process are an excess of hydrocarbons and nitriles inside the soot line, and a higher excitation temperature for these molecules compared to oxygen-bearing complex organics that desorb around the water snowline (~100 K). Such characteristics have been reported in the literature, for example, in Orion KL, although not uniformly, potentially due to differences in observational settings and analysis methods of different studies or related to the episodic nature of protostellar accretion. If this process is active, this would mean that there is a heretofore unknown component to the carbon chemistry during the protostellar phase that is acting from the top down - starting from the destruction of larger species - instead of from the bottom up from atoms. In the presence of such a top-down component, the origin of organic molecules needs to be re-explored.
Article
The photoelectron spectroscopy of CH2NC (isocyanomethyl) radical species is investigated for the first time between 9.3 and 11.2~eV in the vicinity of the first photoionizing transition X⁺¹A1 ← X ²B1. The experiment combines a microwave discharge flow-tube reactor to produce the radicals through the CH3NC + F → CH2NC + HF reaction, a VUV synchrotron radiation excitation, and a double imaging electron/ion coincidence spectrometer which allows the recording of mass-selected threshold photoelectron spectra. Assignment of the observed vibrational structure of CH2NC⁺ cation is guided by ab initio calculations and Franck-Condon simulations. From the experimental spectrum, the first adiabatic ionization energy of the CH2NC radical is measured at 9.439(6) eV. Fundamental wavenumbers are determined for several vibrational modes of the cation: ν1⁺(CH2 symmetric stretch) = 2999(80)~cm⁻¹, ν2⁺(N--C stretch) = 1925(40)~cm⁻¹, ν4⁺(H2C--N stretch) = 1193(40)~cm⁻¹, ν6⁺(CNC out-of-plane bend) = 237(50)~cm⁻¹, and ν8⁺(CH2 rock) = 1185(60)~cm⁻¹.
Article
Context. The Exploring Molecule Complexity with ALMA (EMoCA) survey is an imaging spectral line survey using the Atacama Large Millimeter/submillimeter Array (ALMA) to study the hot-core complex Sagittarius B2(N). Recently, EMoCA revealed the presence of three new hot cores in this complex (N3-N5), in addition to providing detailed spectral data on the previously known hot cores in the complex (N1 and N2). The present study focuses on N2, which is a rich and interesting source for the study of complex molecules whose narrow line widths ameliorate the line confusion problem. Aims. We investigate the column densities and excitation temperatures of cyanide and isocyanide species in Sgr B2(N2). We then use state-of-the-art chemical models to interpret these observed quantities. We also investigate the effect of varying the cosmic-ray ionization rate ( ζ ) on the chemistry of these molecules. Methods. We used the EMoCA survey data to search for isocyanides in Sgr B2(N2) and their corresponding cyanide analogs. We then used the coupled three-phase chemical kinetics code MAGICKAL to simulate their chemistry. Several new species, and over 100 new reactions have been added to the network. In addition, a new single-stage simultaneous collapse/warm-up model has been implemented, thus eliminating the need for the previous two-stage models. A variable, visual extinction-dependent ζ was also incorporated into the model and tested. Results. We report the tentative detection of CH 3 NC and HCCNC in Sgr B2(N2), which represents the first detection of both species in a hot core of Sgr B2. In addition, we calculate new upper limits for C 2 H 5 NC, C 2 H 3 NC, HNC 3 , and HC 3 NH ⁺ . Our updated chemical models can reproduce most observed NC:CN ratios reasonably well depending on the physical parameters chosen. The model that performs best has an extinction-dependent cosmic-ray ionization rate that varies from ~2 × 10 ⁻¹⁵ s ⁻¹ at the edge of the cloud to ~1 × 10 ⁻¹⁶ s ⁻¹ in the center. Models with higher extinction-dependent ζ than this model generally do not agree as well, nor do models with a constant ζ greater than the canonical value of 1.3 × 10 ⁻¹⁷ s ⁻¹ throughout the source. Radiative transfer models are run using results of the best-fit chemical model. Column densities produced by the radiative transfer models are significantly lower than those determined observationally. Inaccuracy in the observationally determined density and temperature profiles is a possible explanation. Excitation temperatures are well reproduced for the true “hot core” molecules, but are more variable for other molecules such as HC 3 N, for which fewer lines exist in ALMA Band 3. Conclusions. The updated chemical models do a very good job of reproducing the observed abundances ratio of CH 3 NC:CH 3 CN towards Sgr B2(N2), while being consistent with upper limits for other isocyanide/cyanide pairs. HCCNC:HC 3 N is poorly reproduced, however. Our results highlight the need for models with AV -depdendent ζ . However, there is still much to be understood about the chemistry of these species, as evidenced by the systematic overproduction of HCCNC. Further study is also needed to understand the complex effect of varying ζ on the chemistry of these species. The new single-stage chemical model should be a powerful tool in analyzing hot-core sources in the future.
Article
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.
Article
Context. Propyne (CH 3 CCH), also known as methyl acetylene, has been detected in a variety of environments, from Galactic star-forming regions to extragalactic sources. These molecules are excellent tracers of the physical conditions in star-forming regions, allowing the temperature and density conditions surrounding a forming star to be determined. Aims. This study explores the emission of CH 3 CCH in the low-mass protostellar binary, IRAS 16293–2422, and examines the spatial scales traced by this molecule, as well as its formation and destruction pathways. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) were used to determine the abundances and excitation temperatures of CH 3 CCH towards both protostars. This data allows us to explore spatial scales from 70 to 2400 au. This data is also compared with the three-phase chemical kinetics model MAGICKAL, to explore the chemical reactions of this molecule. Results. CH 3 CCH is detected towards both IRAS 16293A and IRAS 16293B, and is found the hot corino components, one around each source, in the PILS dataset. Eighteen transitions above 3 σ are detected, enabling robust excitation temperatures and column densities to be determined in each source. In IRAS 16293A, an excitation temperature of 90 K and a column density of 7.8 × 10 ¹⁵ cm ⁻² best fits the spectra. In IRAS 16293B, an excitation temperature of 100 K and 6.8 × 10 ¹⁵ cm ⁻² best fits the spectra. The chemical modelling finds that in order to reproduce the observed abundances, both gas-phase and grain-surface reactions are needed. The gas-phase reactions are particularly sensitive to the temperature at which CH 4 desorbs from the grains. Conclusions. CH 3 CCH is a molecule whose brightness and abundance in many different regions can be utilised to provide a benchmark of molecular variation with the physical properties of star-forming regions. It is essential when making such comparisons, that the abundances are determined with a good understanding of the spatial scale of the emitting region, to ensure that accurate abundances are derived.
Article
Methyl isocyanide, CH3NC, is a key compound in astrochemistry and astrobiology. A combined theoretical and experimental investigation of the single photon ionization of gas phase methyl isocyanide and its fragmentation pathways is presented. Vacuum ultraviolet (VUV) synchrotron radiation based experiments are used to measure the threshold photoelectron photoion coincidence (TPEPICO) spectra between 10.6 and 15.5 eV. This allowed us to experimentally determine the adiabatic ionization energy (AIE) and fragment ion appearance energies (AE) of gas-phase methyl isocyanide. Its AIE has been measured with a precision never achieved before. It is found to be AIEexp = 11.263 ± 0.005 eV. We observe a vibrational progression upon ionization corresponding to the population of vibrational levels of the ground state of the methyl isocyanide cation. In addition, four fragment ion appearance energies (AEs) were measured to be AE (m/z 40) = 12.80 ± 0.05 eV, AE (m/z 39) = 13.70 ± 0.05, AE (m/z 15) = 13.90 ± 0.05 eV, AE (m/z 14) 13.85 ± 0.05 eV, respectively. In order to interpret the experimental data, we performed state-of-the-art computations using the explicitly correlated coupled cluster approach. We also considered the zero-point vibrational energy (ZPVE), core-valence (CV) and scalar relativistic (SR) effects. The results of theoretical calculations of the AIE and AEs are in excellent agreement with the experimental findings allowing for assignment of the fragmentations to the loss of neutral H, H2, CN and HCN upon ionization of CH3NC. The computations show that in addition to the obvious bond breakings, some of the corresponding ionic fragments result from rearrangements - upon photon absorption - either before or after electron ejection.
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Article
Context. Observations with modern instruments such as Herschel reveal that stars form clustered inside filamentary arms of ~1 pc length embedded in giant molecular clouds (GMCs). On smaller scales of ~1000 au, observations of IRAS 16293–2422, for example, show signs of filamentary “bridge” structures connecting young protostars to their birth environment. Aims. We aim to find the origin of bridges associated with deeply embedded protostars by characterizing their connection to the filamentary structure present on GMC scales and to the formation of protostellar multiples. Methods. Using the magnetohydrodynamical code RAMSES , we carried out zoom-in simulations of low-mass star formation starting from GMC scales. We analyzed the morphology and dynamics involved in the formation process of a triple system. Results. Colliding flows of gas in the filamentary arms induce the formation of two protostellar companions at distances of ~1000 au from the primary. After their birth, the stellar companions quickly approach, at Δ t ~ 10 kyr, and orbit the primary on eccentric orbits with separations of ~100 au. The colliding flows induce transient structures lasting for up to a few 10 kyr that connect two forming protostellar objects that are kinematically quiescent along the line-of-sight. Conclusions. Colliding flows compress gas and trigger the formation of stellar companions via turbulent fragmentation. Our results suggest that protostellar companions initially form with a wide separation of ~1000 au. Smaller separations of a ≲ 100 au are a consequence of subsequent migration and capturing. Associated with the formation phase of the companion, the turbulent environment induces the formation of arc- and bridge-like structures. These bridges can become kinematically quiescent when the velocity components of the colliding flows eliminate each other. However, the gas in bridges still contributes to stellar accretion later. Our results demonstrate that bridge-like structures are a transient phenomenon of stellar multiple formation.
Article
An exhaustive chemical characterization of dense cores is mandatory to our understanding of chemical composition changes from a starless to a protostellar stage. However, only a few sources have had their molecular composition characterized in detail. Here we present a λ 3 mm line survey of L483, a dense core around a Class 0 protostar, which was observed with the IRAM 30 m telescope in the 80–116 GHz frequency range. We detected 71 molecules (140 including different isotopologs), most of which are present in the cold and quiescent ambient cloud according to their narrow lines ( FWHM ~ 0.5 km s ⁻¹ ) and low rotational temperatures (≲10 K). Of particular interest among the detected molecules are the cis isomer of HCOOH, the complex organic molecules HCOOCH 3 , CH 3 OCH 3 , and C 2 H 5 OH, a wide variety of carbon chains, nitrogen oxides like N 2 O, and saturated molecules like CH 3 SH, in addition to eight new interstellar molecules (HCCO, HCS, HSC, NCCNH ⁺ , CNCN, NCO, H 2 NCO ⁺ , and NS ⁺ ) whose detection has already been reported. In general, fractional molecular abundances in L483 are systematically lower than in TMC-1 (especially for carbon chains), tend to be higher than in L1544 and B1-b, and are similar to those in L1527. Apart from the overabundance of carbon chains in TMC-1, we find that L483 does not have a marked chemical differentiation with respect to starless/prestellar cores like TMC-1 and L1544, although it does chemically differentiate from Class 0 hot corino sources like IRAS 16293−2422. This fact suggests that the chemical composition of the ambient cloud of some Class 0 sources could be largely inherited from the dark cloud starless/prestellar phase. We explore the use of potential chemical evolutionary indicators, such as the HNCO/C 3 S, SO 2 /C 2 S, and CH 3 SH/C 2 S ratios, to trace the prestellar/protostellar transition. We also derived isotopic ratios for a variety of molecules, many of which show isotopic ratios close to the values for the local interstellar medium (remarkably all those involving ³⁴ S and ³³ S), while there are also several isotopic anomalies like an extreme depletion in ¹³ C for one of the two isotopologs of c -C 3 H 2 , a drastic enrichment in ¹⁸ O for SO and HNCO (SO being also largely enriched in ¹⁷ O), and different abundances for the two ¹³ C substituted species of C 2 H and the two ¹⁵ N substituted species of N 2 H ⁺ . We report the first detection in space of some minor isotopologs like c -C 3 D. The exhaustive chemical characterization of L483 presented here, together with similar studies of other prestellar and protostellar sources, should allow us to identify the main factors that regulate the chemical composition of cores along the process of formation of low-mass protostars.
Article
The interstellar detection of CH3CN metastable isomers would suggest that CH3NC and H2C=C=NH formed in star forming regions through energetic processing provoked by shocks or others energy sources. In this context laboratory simulations have been carried out to investigate the chemical transformation of CH3CN into CH3NC and H2C=C=NH induced by UV photolysis and high energy particle irradiation. In the present study we have carried out the CH3CN + N solid state reaction in the 10-40 K temperature range in order to examine the behavior of acetonitrile interacting with nitrogen atoms in icy interstellar grains. We show that CH3CN + N is efficient in the solid phase but only in a very specific temperature range which combines high mobility and relatively long surface residence time of N atoms to allow the CH3CN activation. By focusing in the behavior of [CH3NC]/[H2C=C=NH] abundance ratios versus temperature, we have measured abundance ratios around 10.4 at 10 K which decreases to 6.8 when the temperature of the reaction increases. These ratios are of the same order of magnitude as those reported from the acetonitrile isomers detection towards Sagittarius B2(N). While in previous studies involving energetic processing of solid CH3CN, CH3NC and CH2CNH have been formed with [CH3NC]/[CH2CNH] ratios ranged between 0.3 and 1.7. Additionally, the analysis of CH3NC and H2C=C=NH column densities shows that at low temperatures the less stable isomer is favored against the most stable one. These results are compared to the puzzling behavior of CN-containing isomers such as HNC, HCN, HCNO and HOCN in molecular clouds.
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Article
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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Preprint
Methyl cyanide (CH3CN) and propyne (CH3CCH) are two molecules commonly used as gas thermometers for interstellar gas. They are detected in several astrophysical environments and in particular towards protostars. Using data of the low-mass protostar IRAS 16293-2422 obtained with the IRAM 30m single-dish telescope, we constrained the origin of these two molecules in the envelope of the source. The line shape comparison and the results of a radiative transfer analysis both indicate that the emission of CH3CN arises from a warmer and inner region of the envelope than the CH3CCH emission. We compare the observational results with the predictions of a gas-grain chemical model. Our model predicts a peak abundance of CH3CCH in the gas-phase in the outer part of the envelope, at around 2000 au from the central star, which is relatively close to the emission size derived from the observations. The predicted CH3CN abundance only rises at the radius where the grain mantle ices evaporate, with an abundance similar to the one derived from the observations.
Article
Studies of deuterated isotopologues of complex organic molecules can provide important constraints on their origin in star formation regions. In particular, the abundances of deuterated species are very sensitive to the physical conditions in the environment where they form. Because the temperatures in star formation regions are low, these isotopologues are enhanced to significant levels, which enables the detection of multiply deuterated species. However, for complex organic species, so far only the multiply deuterated variants of methanol and methyl cyanide have been reported. The aim of this paper is to initiate the characterisation of multiply deuterated variants of complex organic species with the first detection of doubly deuterated methyl formate, CHD 2 OCHO. We use ALMA observations from the Protostellar Interferometric Line Survey (PILS) of the protostellar binary IRAS 16293-2422 in the spectral range of 329.1 GHz to 362.9 GHz. Spectra towards each of the two protostars are extracted and analysed using a local thermal equilibrium model in order to derive the abundances of methyl formate and its deuterated variants. We report the first detection of doubly deuterated methyl formate CHD 2 OCHO in the ISM. The D-to-H ratio (D/H ratio) of CHD 2 OCHO is found to be 2-3 times higher than the D/H ratio of CH 2 DOCHO for both sources, similar to the results for formaldehyde from the same dataset. The observations are compared to a gas-grain chemical network coupled to a dynamical physical model, tracing the evolution of a molecular cloud until the end of the Class 0 protostellar stage. The overall D/H ratio enhancements found in the observations are of about the same magnitude as the predictions from the model for the early stages of Class 0 protostars. However, that the D/H ratio of CHD 2 OCHO is higher than that of CH 2 DOCHO is still not predicted by the model. This suggests that a mechanism enhances the D/H ratio of singly and doubly deuterated methyl formate that is not in the model, for instance, mechanisms for H-D substitutions. This new detection provides an important constraint on the formation routes of methyl formate and outlines a path forward in terms of using these ratios to determine the formation of organic molecules through observations of differently deuterated isotopologues towards embedded protostars.
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Article
We experimentally show that the reaction between ground state nitrogen atoms N(4S) and acetonitrile CH3CN can lead to two distinct chemical pathways that are both thermally activated at very low temperatures. First is CH3CN isomerization which produces CH3NC and H2CCNH. Second is CH3CN decomposition which produces HNC and CH3CNH+CN− fragments, with the possible release of H2. Our results reveal that the mobility of N(4S)-atoms is stimulated in the 3–11 K temperature range, and that its subsequent encounter with one acetonitrile molecule is sufficient for the aforementioned reactions to occur without the need for additional energy to be supplied to the CH3CN + N(4S) system. These findings shed more light on the nitrogen chemistry that can possibly take place in dense molecular clouds, which until now was thought to only involve high-energy processes and therefore be unlikely to occur in such cold and dark interstellar regions. The reaction pathways we propose in the present study have very important astrochemical implications, as it was shown recently that the atomic nitrogen might be more abundant, in many interstellar icy grain mantles, than previously thought. Also, these reaction pathways can now be considered within dense molecular clouds, and possibly affect the branching ratios for N-bearing molecules computed in astrochemical modeling.
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Article
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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Article
Searches for the prebiotically-relevant cyanamide (NH$_2$CN) towards solar-type protostars have not been reported in the literature. We here present the first detection of this species in the warm gas surrounding two solar-type protostars, using data from the Atacama Large Millimeter/Submillimeter Array Protostellar Interferometric Line Survey (PILS) of IRAS 16293-2422 B and observations from the IRAM Plateau de Bure Interferometer of NGC1333 IRAS2A. We furthermore detect the deuterated and $^{13}$C isotopologues of NH$_2$CN towards IRAS 16293-2422 B. This is the first detection of NHDCN in the interstellar medium. Based on a local thermodynamic equilibrium analysis, we find that the deuteration of cyanamide ($\sim$ 1.7%) is similar to that of formamide (NH$_2$CHO), which may suggest that these two molecules share NH$_2$ as a common precursor. The NH$_2$CN/NH$_2$CHO abundance ratio is about 0.2 for IRAS 16293-2422 B and 0.02 for IRAS2A, which is comparable to the range of values found for Sgr B2. We explored the possible formation of NH$_2$CN on grains through the NH$_2$ + CN reaction using the chemical model MAGICKAL. Grain-surface chemistry appears capable of reproducing the gas-phase abundance of NH$_2$CN with the correct choice of physical parameters.
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Article
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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Context. Almost 20% of the ~200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. Aims. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. Methods. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH 3 CN and CH 3 NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption in an ultra-high vacuum between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory to represent the organised solid support. Results. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50% as compared to adsorption on perfect graphene planes. The most stable isomer (CH 3 CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH 3 NC), however.
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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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New laboratory data of CH$_2$CHCN (vinyl cyanide) in its ground and vibrationally excited states at the microwave to THz domain allow searching for these excited state transitions in the Orion-KL line survey. Frequency-modulated spectrometers combined into a single broadband 50-1900 GHz spectrum provided measurements of CH$_2$CHCN covering a spectral range of 18-1893 GHz, whose assignments was confirmed by Stark modulation spectra in the 18-40 GHz region and by ab-initio anharmonic force field calculations. For analyzing the emission lines of CH$_2$CHCN species detected in Orion-KL we used the excitation and radiative transfer code (MADEX) at LTE conditions. The rotational transitions of the ground state of this molecule emerge from four cloud components of hot core nature which trace the physical and chemical conditions of high mass star forming regions in the Orion-KL Nebula. The total column density of CH$_2$CHCN in the ground state is (3.0$\pm$0.9)x10$^{15}$ cm$^{-2}$. We report on the first interstellar detection of transitions in the v10=1/(v11=1,v15=1) dyad in space, and in the v11=2 and v11=3 states in Orion-KL. The lowest energy vibrationally excited states of vinyl cyanide such as v11=1 (at 328.5 K), v15=1 (at 478.6 K), v11=2 (at 657.8 K), the v10=1/(v11=1,v15=1) dyad (at 806.4/809.9 K), and v11=3 (at 987.9 K) are populated under warm and dense conditions, so they probe the hottest parts of the Orion-KL source. Column density and rotational and vibrational temperatures for CH$_2$CHCN in their ground and excited states, as well as for the isotopologues, have been constrained by means of a sample of more than 1000 lines in this survey. Moreover, we present the detection of methyl isocyanide (CH$_3$NC) for the first time in Orion-KL and a tentative detection of vinyl isocyanide (CH$_2$CHNC) and give column density ratios between the cyanide and isocyanide isomers.
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NGC 7129 FIRS 2 (hereafter FIRS 2) is an intermediate-mass (2 to 8 Msun) protostar located at a distance of 1250 pc. High spatial resolution observations are required to resolve the hot core at its center. We present a molecular survey from 218200 MHz to 221800 MHz carried out with the IRAM Plateau de Bure Interferometer. These observations were complemented with a long integration single-dish spectrum taken with the IRAM 30m telescope. We used a Local Thermodynamic Equilibrium (LTE) single temperature code to model the whole dataset. The interferometric spectrum is crowded with a total of ~300 lines from which a few dozens remain unidentified yet. The spectrum has been modeled with a total of 20 species and their isomers, isotopologues and deuterated compounds. Complex molecules like methyl formate (CH3OCHO), ethanol (CH3CH2OH), glycolaldehyde (CH2OHCHO), acetone (CH3COCH3), dimethyl ether (CH3OCH3), ethyl cyanide (CH3CH2CN) and the aGg' conformer of ethylene glycol (aGg'-(CH2OH)_2) are among the detected species. The detection of vibrationally excited lines of CH3CN, CH3OCHO, CH3OH, OCS, HC3N and CH3CHO proves the existence of gas and dust at high temperatures. In fact, the gas kinetic temperature estimated from the vibrational lines of CH3CN, ~405 K, is similar to that measured in massive hot cores. Our data allow an extensive comparison of the chemistry in FIRS~2 and the Orion hot core. We find a quite similar chemistry in FIRS 2 and Orion. Most of the studied fractional molecular abundances agree within a factor of 5. Larger differences are only found for the deuterated compounds D2CO and CH2DOH and a few molecules (CH3CH2CN, SO2, HNCO and CH3CHO). Since the physical conditions are similar in both hot cores, only different initial conditions (warmer pre-collapse phase in the case of Orion) and/or different crossing time of the gas in the hot core can explain this behavior.
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HNC and HCN, typically used as dense gas tracers in molecular clouds, are a pair of isomers that have great potential as a temperature probe because of temperature dependent, isomer-specific formation and destruction pathways. Previous observations of the HNC/HCN abundance ratio show that the ratio decreases with increasing temperature, something that standard astrochemical models cannot reproduce. We have undertaken a detailed parameter study on which environmental characteristics and chemical reactions affect the HNC/HCN ratio and can thus contribute to the observed dependence. Using existing gas and gas-grain models updated with new reactions and reaction barriers, we find that in static models the H + HNC gas-phase reaction regulates the HNC/HCN ratio under all conditions, except for very early times. We quantitatively constrain the combinations of H abundance and H + HNC reaction barrier that can explain the observed HNC/HCN temperature dependence and discuss the implications in light of new quantum chemical calculations. In warm-up models, gas-grain chemistry contributes significantly to the predicted HNC/HCN ratio and understanding the dynamics of star formation is therefore key to model the HNC/HCN system.
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We present CO 3-2, SiO 8-7, C34S 7-6, and 878 mum dust continuum subarcsecond angular resolution observations with the SMA toward IRAS 16293-2422 (I16293). The C34S emission traces well the 878 mum dust continuum, and shows clearly a smooth velocity gradient along the major axis of component I16293A. The CO shows emission at moderate high velocities arising from two bipolar outflows, which appear to be perpendicular with respect to each other. The high sensitivity and higher angular resolution of these observations allows to well pinpoint the origin of these two outflows at the center of component I16293A. Interestingly, the most compact outflow appears to point toward I16293B. Our data show that the previously reported monopolar blueshifted CO outflow associated with component I16293B seems to be part of the compact outflow arising from component I16293A. In addition, the SiO emission is also tracing this compact outflow: on one hand, the SiO emission appears to have a jet-like morphology along the southern redshifted lobe; on the other hand, the SiO emission associated with the blueshifted northern lobe traces a well defined arc on the border of component I16293B facing I16293A. The blueshifted CO lobe of the compact outflow splits in two lobes around the position of this SiO arc. All these leads us to propose that the compact outflow from component I16293A is impacting on the circumstellar gas around component I16293B, possibly being diverged as a consequence of the interaction.
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Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively easily detected in our Galaxy and in other galaxies. We constrain their chemistry through observations of two positions in the Horsehead edge: the photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just behind it. We systematically searched for lines of CH3CN, HC3N, C3N, and some of their isomers in our sensitive unbiased line survey at 3, 2, and 1mm. We derived column densities and abundances through Bayesian analysis using a large velocity gradient radiative transfer model. We report the first clear detection of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and 6 at the dense core position, and we resolved its hyperfine structure for 3 lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR position. We computed new electron collisional rate coefficients for CH3CN, and we found that including electron excitation reduces the derived column density by 40% at the PDR position. While CH3CN is 30 times more abundant in the PDR than in the dense core, HC3N has similar abundance at both positions. The isomeric ratio CH3NC/CH3CN is 0.15+-0.02. In the case of CH3CN, pure gas phase chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of grains are destroyed through photo-desorption or thermal-evaporation in PDRs, and through sputtering in shocks. (abridged)
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We present CO(2-1), 13CO(2-1), CO(6-5), CO(7-6), and SO(65-54) line observations made with the IRAM 30 m and Atacama Pathfinder Experiment (APEX) radiotelescopes and the Submillimeter Array (SMA) toward the highly collimated (11°) and extended (~2') southwest lobe of the bipolar outflow Ori-S6 located in the Orion South region. We report for all these lines, the detection of velocity asymmetries about the flow axis with velocity differences roughly on the order of 1 km s-1 over distances of about 5000 AU, 4 km s-1 over distances of about 2000 AU, and close to the source of between 7 and 11 km s-1 over smaller scales of about 1000 AU. The redshifted gas velocities are located to the southeast of the outflow's axis, the blueshifted ones to the northwest. We interpret these velocity differences as a signature of rotation, but also discuss some alternatives which we recognize as unlikely in view of the asymmetries' large downstream continuation. In particular, any straightforward interpretation by an ambient velocity gradient does not seem viable. This rotation across the Ori-S6 outflow is observed out to (projected) distances beyond 2.5 × 104 AU from the flow's presumed origin. Comparison of our large-scale (single dish) and small-scale (SMA) observations suggests the rotational velocity to decline not faster than 1/R with distance R from the axis; in the innermost few arcsecs an increase of rotational velocity with R is even indicated. The magnetic field lines threading the inner rotating CO shell may well be anchored in a disk of a radius of ~50 AU; the field lines further out need a more extended rotating base. Our high angular resolution SMA observations also suggest this outflow to be energized by the compact millimeter radio source 139-409, a circumbinary flattened ring that is located in a small cluster of very young stars associated with the extended and bright source FIR4.
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Interstellar glycolaldehyde (CH2OHCHO) has been detected with the 100 m Green Bank Telescope (GBT) toward the star-forming region Sagittarius B2(N) by means of the 110-101, 211-2 02, 312-303, and 413-404 rotational transitions at 13.48, 15.18, 17.98, and 22.14 GHz, respectively. An analysis of these four high signal-to-noise ratio rotational transitions yields a glycolaldehyde state temperature of ∼8 K. Previously reported emission-line detections of glycolaldehyde with the NRAO 12 m telescope at millimeter wavelengths (71-103 GHz) are characterized by a state temperature of ∼50 K. By comparison, the GBT detections are surprisingly strong and are seen in emission at 13.48 GHz, emission and absorption at 15.18 GHz, and absorption at 17.98 and 22.14 GHz. We attribute the strong absorption observed by the GBT at the higher frequencies to the correspondingly smaller GET beams coupling better to the continuum source(s) in Sagittarius B2(N). A possible model for the two-temperature regions of glycolaldehyde is discussed.
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We present the results of high angular resolution millimeter observations of gas and dust toward G31.41+0.31 and G24.78+0.08, two high-mass star forming regions where four rotating massive toroids have been previously detected by Beltran et al. (2004). The CH3CN (12-11) emission of the toroids in G31.41+0.31 and core A1 in G24.78+0.08 has been modeled assuming that it arises from a disk-like structure seen edge-on, with a radial velocity field. For G31.41+0.31 the model properly fits the data for a velocity v_rot~1.7 km/s at the outer radius R_out~13400 AU and an inner radius R_inn~1340 AU, while for core A1 in G24.78+0.08 the best fit is obtained for v_rot~2.0 km/s at R_out~7700 AU and R_inn~2300 AU. Unlike the rotating disks detected around less luminous stars, these toroids are not undergoing Keplerian rotation. From the modeling itself, however, it is not possible to distinguish between constant rotation or constant angular velocity, since both velocity fields suitably fit the data. The best fit models have been computed adopting a temperature gradient of the type T proportional R^{-3/4}, with a temperature at the outer radius T_out~100 K for both cores. The M_dyn needed for equilibrium derived from the models is much smaller than the mass of the cores, suggesting that such toroids are unstable and undergoing gravitational collapse. The collapse is also supported by the CH3^{13}CN or CH3CN line width measured in the cores, which increases toward the center of the toroids. The estimates of v_inf and \dot M_acc are ~2 km/s and 3x10^{-2} M_sun/yr for G31.41+0.31, and ~1.2 km/s and ~9x10^{-3} M_sun/yr for G24.78+0.08 A1. Such large accretion rates could weaken the effect of stellar winds and radiation pressure and allow further accretion on the star. Comment: 27 pages, including 25 figures, 12 tables;accepted for publication by A&A
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We present strong detections of methyl cyanide, vinyl cyanide, ethyl cyanide and cyanodiacetylene molecules with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanide isomers were only successful in the case of methyl isocyanide for its J(K)=1(0)-0(0) transition, which is the first interstellar report of this line. To determine the spatial distribution of methyl isocyanide, we used archival Berkeley-Illinois-Maryland Association (BIMA) array data for the J(K)=4(K)-3(K) (K=0-3) transitions but no emission was detected. From ab initio calculations, the bonding energy difference between the cyanide and isocyanide molecules is >8500 cm^-1 (>12,000 K). That we detect methyl isocyanide emission with a single antenna (Gaussian beamsize(Omega_B)=1723 arcsec^2) but not with an interferometer (Omega_B=192 arcsec^2), strongly suggests that methyl isocyanide has a widespread spatial distribution toward the Sgr B2(N) region. Thus, large-scale, non-thermal processes in the surrounding medium may account for the conversion of methyl cyanide to methyl isocyanide while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of methyl isocyanide. Ice analog experiments by other investigators have shown that radiation bombardment of methyl cyanide can produce methyl isocyanide, thus supporting our observations. We conclude that isomers separated by such large bonding energy differences are distributed in different interstellar environments, making the evaluation of column density ratios between such isomers irrelevant unless it can be independently shown that these species are co-spatial. Comment: 17 pages, 2 figures, accepted to the Astrophysical Journal
Article
Context . Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. Their detection is crucial to understanding the chemical evolution of the Universe and exploring the link between the early stages of star formation and the formation of solar system bodies, where complex organic molecules have been found in abundance. In particular, molecules that contain nitrogen are interesting due to the role nitrogen plays in the development of life and the compact scales such molecules have been found to trace around forming protostars. Aims . The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a –C≡N functional group) towards two hot corino sources in the low-mass protostellar binary IRAS 16293–2422. This work explores the abundance differences between the two sources, the isotopic ratios, and the spatial extent derived from molecules containing the nitrile functional group. Methods . Using data from the Protostellar Interferometric Line Survey (PILS) obtained with ALMA, we determine abundances and excitation temperatures for the detected nitriles. We also present a new method for determining the spatial structure of sources with high line density and large velocity gradients – Velocity-corrected INtegrated emission (VINE) maps. Results . We detect methyl cyanide (CH 3 CN) as well as five of its isotopologues, including CHD 2 CN, which is the first detection in the interstellar medium (ISM). We also detect ethyl cyanide (C 2 H 5 CN), vinyl cyanide (C 2 H 3 CN), and cyanoacetylene (HC 3 N). We find that abundances are similar between IRAS 16293A and IRAS 16293B on small scales except for vinyl cyanide which is only detected towards the latter source. This suggests an important difference between the sources either in their evolutionary stage or warm-up timescales. We also detect a spatially double-peaked emission for the first time in molecular emission in the A source, suggesting that this source is showing structure related to a rotating toroid of material. Conclusions . With high-resolution observations, we have been able to show for the first time a number of important similarities and differences in the nitrile chemistry in these objects. These illustrate the utility of nitriles as potential tracers of the physical conditions in star-forming regions.
Article
Rate-equation models are a widely-used and inexpensive tool for the simulation of interstellar chemistry under a range of physical conditions. However, their application to grain-surface chemical systems necessitates a number of simplifying assumptions, due to the requirement to treat only the total population of each species, using averaged rates, rather than treating each surface particle as an independent entity. While the outputs from rate-equation models are strictly limited to such population information, the inputs -- in the form of the averaged rates that control the time-evolution of chemical populations -- can be guided by the results from more exact simulation methods. Here, we examine the effects of back-diffusion, wherein particles diffusing on a surface revisit binding sites on the lattice, slowing the total reaction rate. While this effect has been studied for two-particle systems, its influence at greater surface coverage of reactants has not been explored. Results from two Monte Carlo kinetics models (one a 2-D periodic lattice, the other the surface of a three dimensionally-realized grain) were used to develop a means to incorporate the grain-surface back-diffusion effect into rate-equation methods. The effects of grain size, grain morphology, and surface coverage on the magnitude of the back-diffusion effect were studied for the simple H+H reaction system. The results were fit with expressions that can be easily incorporated into astrochemical rate-equation models to reproduce accurately the effects of back-diffusion on grain-surface reaction rates. Back-diffusion reduces reaction rates by a maximum factor of around 5 for the canonical grain of $\sim$10$^6$ surface sites, but this falls to unity at close to full surface coverage.
Article
Using mm-wavelength data from ALMA, the EMoCA spectral line survey revealed the presence of both the straight-chain and branched forms of propyl cyanide (C$_3$H$_7$CN) toward the Galactic Center star-forming source Sgr B2(N2). This was the first interstellar detection of a branched aliphatic molecule. Through computational methods, we seek to explain the observed $i$:$n$ ratio for propyl cyanide, and to predict the abundances of the four different forms of the homologous nitrile, butyl cyanide (C$_4$H$_9$CN). We also investigate whether other molecules will show a similar degree of branching, by modeling alkanes up to pentane (C$_5$H$_{12}$). We use a coupled three-phase chemical kinetics model to simulate the chemistry of Sgr B2(N2), using an updated chemical network that includes grain-surface/ice-mantle formation routes for branched nitriles and alkanes. We use the EMoCA survey data to search for the straight-chain form of butyl cyanide toward Sgr B2(N2). The observed $i$:$n$ ratio for propyl cyanide is reproduced by the models. Butyl cyanide is predicted to show similar abundances to propyl cyanide, and to exhibit strong branching, with the $sec$ form clearly dominant over all others. The addition of CN to acetylene and ethene is found to be important to the production of vinyl, ethyl, propyl, and butyl cyanide. We report a non-detection of $n$-C$_4$H$_9$CN toward Sgr B2(N2), with an abundance at least 1.7 times lower than that of $n$-C$_3$H$_7$CN. This value is within the range predicted by the chemical models. The models indicate that the degree of branching rises with increasing molecular size. The efficiency of CN addition to unsaturated hydrocarbons boosts the abundances of nitriles in the model, and enhances the ratio of straight-to-branched molecule production. The predicted abundance of $s$-C$_4$H$_9$CN makes it a good candidate for future detection toward Sgr B2(N2).
Article
N-methylformamide, CH3NHCHO, may be an important molecule for interstellar pre-biotic chemistry because it contains a peptide bond. The rotational spectrum of the most stable trans conformer of CH3NHCHO is complicated by strong torsion-rotation interaction due to the low barrier of the methyl torsion. We use two absorption spectrometers in Kharkiv and Lille to measure the rotational spectra over 45--630 GHz. The analysis is carried out using the Rho-axis method and the RAM36 code. We search for N-methylformamide toward the hot molecular core Sgr B2(N2) using a spectral line survey carried out with ALMA. The astronomical results are put into a broader astrochemical context with the help of a gas-grain chemical kinetics model. The laboratory data set for the trans conformer of CH3NHCHO consists of 9469 line frequencies with J <= 62, including the first assignment of the rotational spectra of the first and second excited torsional states. All these lines are fitted within experimental accuracy. We report the tentative detection of CH3NHCHO towards Sgr B2(N2). We find CH3NHCHO to be more than one order of magnitude less abundant than NH2CHO, a factor of two less abundant than CH3NCO, but only slightly less abundant than CH3CONH2. The chemical models indicate that the efficient formation of HNCO via NH + CO on grains is a necessary step in the achievement of the observed gas-phase abundance of CH3NCO. Production of CH3NHCHO may plausibly occur on grains either through the direct addition of functional-group radicals or through the hydrogenation of CH3NCO. Provided the detection of CH3NHCHO is confirmed, the only slight underabundance of this molecule compared to its more stable structural isomer acetamide and the sensitivity of the model abundances to the chemical kinetics parameters suggest that the formation of these two molecules is controlled by kinetics rather than thermal equilibrium.
Article
One of the open questions in astrochemistry is how complex organic and prebiotic molecules are formed. Aims. Our aim is to start the process of compiling an inventory of oxygen-bearing complex organic molecules toward the solar-type Class 0 protostellar binary IRAS16293-2422 from an unbiased spectral survey with ALMA (PILS). Here we focus on the new detections of ethylene oxide (c-C$_2$H$_4$O), acetone (CH$_3$COCH$_3$), and propanal (C$_2$H$_5$CHO). Methods. With ALMA, we surveyed the spectral range from 329 to 363 GHz at 0.5$"$ (60 AU diameter) resolution. Using a simple model for the molecular emission in LTE, the excitation temperatures and column densities of each species were constrained. Results. We successfully detect propanal (44 lines), ethylene oxide (20 lines) and acetone (186 lines) toward one component of the protostellar binary, IRAS16293B. The high resolution maps demonstrate that the emission for all investigated species originates from the compact central region close to the protostar. This, along with a derived common excitation temperature of $\sim$ 125 K, is consistent with a coexistence of these molecules in the same gas. Conclusions. The observations mark the first detections of acetone, propanal and ethylene oxide toward a low-mass protostar. The relative abundance ratios of the two sets of isomers (CH$_3$COCH$_3$/C$_2$H$_5$CHO $\sim$ 8 and CH$_3$CHO/c-C$_2$H$_4$O $\sim$ 12) are comparable to previous observations toward high-mass protostars. The majority of observed abundance ratios from these results as well as those measured toward high-mass protostars are up to an order of magnitude above the predictions from chemical models. This may reflect either missing reactions or uncertain rates in the chemical networks. The physical conditions, such as temperatures or densities, used in the models, may not be applicable to solar-type protostars either.
Article
Context. Almost 20% of the ~200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Among these 37 molecules, 30 are nitrile R-CN compounds, the remaining seven belonging to the isonitrile R-NC family. How these species behave in presence of the grain surfaces is still an open question. Aims. In this contribution we investigate whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of different nature and morphologies. Methods. The question was addressed by means of a concerted experimental and theoretical study of the adsorption energies of CH 3 CN and CH 3 NC on the surface water ice and silica. The experimental determination of the molecule – surface interaction energies was carried out using temperature programmed desorption (TPD) under an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question was addressed using first principle periodic density functional theory (DFT) to represent the organized solid support. Results. The most stable isomer (CH 3 CN) interacts more efficiently with the solid support than the higher energy isomer (CH 3 NC) for water ice and silica. Comparing with the HCN and HNC pair of isomers, the simulations show an opposite behaviour, in which isonitrile HNC are more strongly adsorbed than nitrile HCN provided that hydrogen bonds are compatible with the nature of the model surface. Conclusions. The present study confirms that the strength of the molecule surface interaction between isomers is not related to their intrinsic stability but instead to their respective ability to generate different types of hydrogen bonds. Coupling TPD to first principle simulations is a powerful method for investigating the possible role of interstellar surfaces in the release of organic species from grains, depending on the environment.
Article
The inner regions of the envelopes surrounding young protostars are characterised by a complex chemistry, with prebiotic molecules present on the scales where protoplanetary disks eventually may form. This paper introduces a systematic survey, "Protostellar Interferometric Line Survey (PILS)" of the Class 0 protostellar binary IRAS 16293-2422 using the Atacama Large Millimeter/submillimeter Array (ALMA). The survey covers the full frequency range from 329 to 363 GHz (0.8 mm) with additional targeted observations at 3.0 and 1.3 mm. More than 10,000 features are detected toward one component in the protostellar binary. Glycolaldehyde, its isomers, methyl formate and acetic acid, and its reduced alcohol, ethylene glycol, are clearly detected. For ethylene glycol both lowest state conformers, aGg' and gGg', are detected, the latter for the first time in the ISM. The abundance of glycolaldehyde is comparable to or slightly larger than that of ethylene glycol. In comparison to the Galactic Center, these two species are over-abundant relative to methanol, possibly an indication of formation at low temperatures in CO-rich ices. Both 13C and deuterated isotopologues of glycolaldehyde are detected, also for the first time ever in the ISM. For the deuterated species, a D/H ratio of approximately 5% is found with no differences between the deuteration in the different functional groups of glycolaldehyde. Measurements of the 13C-species lead to a 12C:13C ratio of approximately 30, lower than the typical ISM value. This low ratio may reflect an enhancement of 13CO in the ice due to either ion-molecule reactions in the gas before freeze-out or differences in the temperatures where 12CO and 13CO ices sublimate. The results reinforce the importance of low-temperature grain surface chemistry for the formation of prebiotic molecules seen here in the gas after sublimation of the entire ice mantle.
Article
The Cologne Database for Molecular Spectroscopy, CDMS, was founded 1998 to provide in its catalog section line lists of mostly molecular species which are or may be observed in various astronomical sources by means of (usually) radio astronomical means. The line lists contain transition frequencies with qualified accuracies, intensities, quantum numbers, as well as further auxilary information. They have been generated from critically evaluated experimental line lists, mostly from laboratory experiments, employing established Hamiltonian models. Seperate entries exist for different isotopic species and usually also for different vibrational states. As of December 2015, the number of entries is 792. They are available online as ascii tables with additional files documenting information on the entries.
Article
Observations of higher-excited transitions of abundant molecules such as CO are important for determining where energy in the form of shocks is fed back into the parental envelope of forming stars. The nearby prototypical and protobinary low-mass hot core, IRAS 16293-2422 (I16293) is ideal for such a study. The source was targeted with ALMA for science verification purposes in band 9, which includes CO J = 6-5 (Eup/kB ~ 116 K), at an unprecedented spatial resolution (~0.''2, 25 AU). I16293 itself is composed of two sources, A and B, with a projected distance of 5''. CO J = 6-5 emission is detected throughout the region, particularly in small, arcsecond-sized hotspots, where the outflow interacts with the envelope. The observations only recover a fraction of the emission in the line wings when compared to data from single-dish telescopes, with a higher fraction of emission recovered at higher velocities. The very high angular resolution of these new data reveal that a bow shock from source A coincides, in the plane of the sky, with the position of source B. Source B, on the other hand, does not show current outflow activity. In this region, outflow entrainment takes place over large spatial scales, ≳100 AU, and in small discrete knots. This unique dataset shows that the combination of a high-temperature tracer (e.g., CO J = 6-5) and very high angular resolution observations is crucial for interpreting the structure of the warm inner environment of low-mass protostars. Appendices are available in electronic form at http://www.aanda.org
Article
This paper describes a computer-accessible catalog of submillimeter, millimeter, and microwave spectral lines in the frequency range between 0 and 10 000 GHz (i.e. wavelengths longer than 30 μm). The catalog can be used as a planning guide or as an aid in the identification and analysis of observed spectral lines in the interstellar medium, the Earth’s atmosphere, and the atmospheres of other planets. The information listed for each spectral line includes the frequency and its estimated error, the intensity, the lower state energy, and the quantum number assignment. The catalog is continuously updated and at present has information on 331 atomic and molecular species and includes a total of 1 845 866 lines. The catalog has been constructed by using theoretical least-squares fits of published spectral lines to accepted molecular models. The associated predictions and their estimated errors are based upon the resultant fitted parameters and their covariance. Future versions of this catalog will add more atoms and molecules and update the present listings as new data appear. The catalog is available on-line via anonymous FTP at spec.jpl.nasa.gov and on the world wide web at http: //spec.jpl.nasa.gov.
Article
The weak perpendicular bands of the low-frequency fundamental bands ν8 and ν7 of CH3NC were measured in a long path cell with a high-resolution Fourier transform spectrometer. Analysis of the ν8 band shows its origin to be at ν0 = 267.3173 cm−1, some 4.3 cm−1 higher than previous estimates that were based on an unresolved peak of the bunched-up Q branches seen in a low-resolution scan. The ν7 band, at ν0 = 1130.6588 cm−1, is slightly affected by Fermi resonance with the combination ν4 + ν8, whose band is observed just above ν7. On combining the observed ΔK = +1 transitions of ν−7 + ν−8 we reported recently, with the ΔK = −1 transitions of ν−8 and of the hot band ν−7 + ν−8 − ν−8 accompanying ν7, we have been able to obtain accurate values for the ground state rotational constant A0 = 5.247234(77) cm−1 and for the centrifugal distortion constant D0K = 8.5626(24) × 10−5 cm−1, which had never been experimentally determined. Accurate spectroscopic constants are also reported for the ν8, ν7, and ν4 + ν8 states.
Article
A new chemical model is presented that simulates fully-coupled gas-phase, grain-surface and bulk-ice chemistry in hot cores. Glycine (NH2CH2COOH), the simplest amino acid, and related molecules such as glycinal, propionic acid and propanal, are included in the chemical network. Glycine is found to form in moderate abundance within and upon dust-grain ices via three radical-addition mechanisms, with no single mechanism strongly dominant. Glycine production in the ice occurs over temperatures ~40-120 K. Peak gas-phase glycine fractional abundances lie in the range 8 x 10^{-11} - 8 x 10^{-9}, occuring at ~200 K, the evaporation temperature of glycine. A gas-phase mechanism for glycine production is tested and found insignificant, even under optimal conditions. A new spectroscopic radiative-transfer model is used, allowing the translation and comparison of the chemical-model results with observations of specific sources. Comparison with the nearby hot-core source NGC 6334 IRS1 shows excellent agreement with integrated line intensities of observed species, including methyl formate. The results for glycine are consistent with the current lack of a detection of this molecule toward other sources; the high evaporation temperature of glycine renders the emission region extremely compact. Glycine detection with ALMA is predicted to be highly plausible, for bright, nearby sources with narrow emission lines. Photodissociation of water and subsequent hydrogen-abstraction from organic molecules by OH, and NH2, are crucial to the build-up of complex organic species in the ice. The inclusion of alternative branches within the network of radical-addition reactions appears important to the abundances of hot-core molecules; less favorable branching ratios may remedy the anomalously high abundance of glycolaldehyde predicted by this and previous models.
Article
Motivated by detections of nitriles in Titan's atmosphere, cometary comae, and the interstellar medium, we report laboratory investiga-tions of the low-temperature chemistry of acetonitrile, propionitrile, acrylonitrile, cyanoacetylene, and cyanogen (CH 3 CN, CH 3 CH 2 CN, CH 2 CHCN, HCCCN, and NCCN, respectively). A few experiments were also done on isobutyronitrile and trimethylacetonitrile ((CH 3) 2 CHCN and (CH 3) 3 CCN, respectively). Trends were sought, and found, in the photo-and radiation chemical products of these molecules at 12–25 K. In the absence of water, all of these molecules isomerized to isonitriles, and CH 3 CN, CH 3 CH 2 CN, and (CH 3) 2 CHCN also formed ketenimines. In the presence of H 2 O, no isonitriles were detected but rather the cyanate ion (OCN −) was seen in all cases. Although isonitriles, ketenimines, and OCN − were the main focus of our work, we also describe cases of hydrogen loss, to make smaller nitriles, and hydrogen addition (reduction), to make larger nitriles. HCN formation also was seen in most experiments. The results are pre-sented in terms of nitrile ice chemistry on Titan, in cometary ice, and in the interstellar medium. Possible connections to prebiotic chemistry are briefly discussed.
Article
A spectral survey of the W51 e1/e2 star-forming region at 84–115 GHz has yielded detections of 105 molecules and their isotopic species, from simple diatomic or triatomic molecules, such as CO, CS, HCN, up to complex organic compounds, such as CH3OCH3, CH3COCH3, and C2H5OOCH. Ninety-three lines that are absent from the Lovas list of molecular lines observed from space were detected, and approximately half of these were identified. A significant number of the detectedmolecules are typical for hot cores. These include the neutral molecules CH3OCHO, C2H5OH, CH3COCH3 etc., which are currently believed to exist in the gas phase only in hot cores and shock-heated gas. In addition, vibrationally excited SiO, C4H, HCN, l-C3H, HCCCN, CH3CN, CH3OH, H2O, and SO2 lines with upper-level temperatures of several hundred Kelvin were found. Such lines can arise only in hot gas with temperatures of the order of 100 K or higher. Apart from neutral molecules, various molecular ions were also detected. Some of these (N2H+, HCO+, HCS+) usually exist in molecular clouds with high visual extinctions A V . At the same time, the CF+ ion should be observed in photon-dominated regions with A V values of about unity or lower. An interesting result is the tentative detection of two molecules that have thus far been observed only in the atmospheres of late-type giant stars—MgCN and NaCN. This suggests that the conditions in the hottest W51 regions (probably, in the vicinities of protostars) are close to those in the atmospheres of giant stars. It would be desirable to search for other lines of these molecules to verify these tentative detections. Analysis of the radial velocities of the detected molecules suggests that the contribution from the e2 core dominates the emission of some O-bearing molecules (CH3OCHO, CH3CH2OH), while the contribution of the e1 core dominates the emission of some N-bearing molecules (e.g., CH3CH2CN). Thus, the molecular composition of the e2 core may be closer to the composition of the “Compact Ridge” in OMC-1, while the composition of the e1 core is closer to that for the “Hot Core” in the same cloud.
Article
Aims.The production of saturated organic molecules in hot cores and corinos is not well understood. The standard approach is to assume that, as temperatures heat up during star formation, methanol and other species evaporate from grain surfaces and undergo a warm gas-phase chemistry at 100 K or greater to produce species such as methyl formate, dimethyl ether, and others. But a series of laboratory results shows that protonated ions, typical precursors to final products in ion-molecule schemes, tend to fragment upon dissociative recombination with electrons rather than just ejecting a hydrogen atom. Moreover, the specific proposed reaction to produce protonated methyl formate is now known not to occur at all. Methods: .We utilize a gas-grain chemical network to probe the chemistry of the relatively ignored stage of hot core evolution during which the protostar switches on and the temperature of the surrounding gas and dust rises from 10 K to over 100 K. During this stage, surface chemistry involving heavy radicals becomes more important as surface hydrogen atoms tend to evaporate rather than react. Results: .Our results show that complex species such as methyl formate, formic acid, and dimethyl ether can be produced in large abundance during the protostellar switch-on phase, but that both grain-surface and gas-phase processes help to produce most species. The longer the timescale for protostellar switch-on, the more important the surface processes.
Article
The parallel vibration-rotation band ν(4) of methyl isocyanide (CH(3)NC), with a band center at 944.9 cm(-1), was studied by FTIR spectroscopy between 890 and 980 cm(-1) in order to improve the ground-state rotational constants. Such improvement is essential for the scheduled studies of excited vibrational levels and their mutual anharmonic resonances occurring at higher values of the K rotational number. Ground-state combination differences generated from this band, spanning values of J/K from 0 to 85/13, were combined with rotational data from the literature and newly measured rotational transitions, extending the J/K range from 3/0 up to 31/14, and fitted simultaneously with a fully quantitative reproduction of the data. The infrared data of the ν(4) band were analyzed together with rotational data of the ν(4) = 1 level, spanning values of J/K from 4/0 to 14/12. The fit in the approximation of an isolated vibrational state, with the transitions perturbed by weak local resonances excluded, yields reproduction of the data within experimental uncertainties.
Article
8 pages, 2 tables, 2 figures.-- Accepted for publication in ApJ Letters. Glycolaldehyde is the simplest of the monosaccharide sugars and is directly linked to the origin of life. We report on the detection of glycolaldehyde (CH2OHCHO) towards the hot molecular core G31.41+0.31 through IRAM PdBI observations at 1.4, 2.1, and 2.9 mm. The CH2OHCHO emission comes from the hottest (>= 300 K) and densest (>= 2E8 cm^-3) region closest (<= 10^4 AU) to the (proto)stars. The comparison of data with gas-grain chemical models of hot cores suggests for G31.41+0.31 an age of a few 10^5 yr. We also show that only small amounts of CO need to be processed on grains in order for existing hot core gas-grain chemical models to reproduce the observed column densities of glycolaldehyde, making surface reactions the most feasible route to its formation. SV acknowledges financial support from an individual PPARC Advanced Fellowship. Peer reviewed
Article
We have observed emission from HCN, H13CN, HC15N, HN13C, H15NC, HC3N, CH3CN, and possibly CH3NC, and determined an upper limit for NH2CN, toward the cold, dark cloud TMC-1. The abundance ratio [HNC]/[HCN] = 1.55 +/- 0.16 is at least a factor approximately 4 and approximately 100 greater than that observed toward the giant molecular clouds DR 21(OH) and Orion KL, respectively. In contrast, for the corresponding methylated isomers we obtain [CH3NC]/CH3CN] < or approximately 0.1. We also find [NH2CN]/[CH3CN] < or approximately 0.1 and [HC3N]/[CH3CN] = 30 +/- 10. We find no evidence for anomalous hyperfine ratios for H13CN, indicating that the ratios for HCN (cf. recent work of Walmsley et al.) are the result of self-absorption by cold foreground gas.
Article
Calculations have been performed to determine the abundance ratio of the metastable isomer CH3NC to the stable isomer CH3CN in dense interstellar clouds. According to gas phase, ion-molecule treatments, these molecules are both synthesized via protonated ion precursors. We have calculated the ratio of the formation rates of the protonated precursor ions-- CH3NCH+ and CH3CNH+ --synthesized via the radiative association reaction between CH3+ and HCN, which is thought to the dominant formation process of the two isomeric ions. Our calculations, which involve both ab initio quantum chemistry and equilibrium determinations, lead to a predicted CH3NCH+/CH3CNH+ formation rate ratio between 0.1 and 0.4. If this ratio is maintained in the neutral species formed from the precursor ions, theory predicts a sizable abundance for methyl isocyanide (CH3NC) and lends credence to its tentative observation.
Article
Aims: To investigate the chemical relations between complex organics based on their spatial distributions and excitation conditions in the low-mass young stellar objects IRAS 16293-2422 A and B. Methods: Interferometric observations with the Submillimeter Array have been performed at 5''x3'' resolution revealing emission lines of HNCO, CH3CN, CH2CO, CH3CHO and C2H5OH. Rotational temperatures are determined from rotational diagrams when a sufficient number of lines are detected. Results: Compact emission is detected for all species studied here. For HNCO and CH3CN it mostly arises from source A, CH2CO and C2H5OH have comparable strength for both sources and CH3CHO arises exclusively from source B. HNCO, CH3CN and CH3CHO have rotational temperatures >200 K. The (u,v)-visibility data reveal that HNCO also has extended cold emission. Conclusions: The abundances of the molecules studied here are very similar within factors of a few to those found in high-mass YSOs. Thus the chemistry between high- and low-mass objects appears to be independent of luminosity and cloud mass. Bigger abundance differences are seen between the A and B source. The HNCO abundance relative to CH3OH is ~4 times higher toward A, which may be due to a higher initial OCN- ice abundances in source A compared to B. Furthermore, not all oxygen-bearing species are co-existent. The different spatial behavior of CH2CO and C2H5OH compared with CH3CHO suggests that hydrogenation reactions on grain-surfaces are not sufficient to explain the observed gas phase abundances. Selective destruction of CH3CHO may result in the anti-coincidence of these species in source A. These results illustrate the power of interferometric compared with single dish data in terms of testing chemical models. Comment: 11 pages, 15 figures, accepeted by A&A
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