Article

The ALMA-PILS survey: Stringent limits on small amines and nitrogen-oxides towards IRAS 16293-2422B

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Abstract

Context. Hydroxylamine (NH 2 OH) and methylamine (CH 3 NH 2 ) have both been suggested as precursors to the formation of amino acids and are therefore, of interest to prebiotic chemistry. Their presence in interstellar space and formation mechanisms, however, are not well established. Aims. We aim to detect both amines and their potential precursor molecules NO, N 2 O, and CH 2 NH towards the low-mass protostellar binary IRAS 16293–2422, in order to investigate their presence and constrain their interstellar formation mechanisms around a young Sun-like protostar. Methods. ALMA observations from the unbiased, high-angular resolution and sensitivity Protostellar Interferometric Line Survey (PILS) are used. Spectral transitions of the molecules under investigation are searched for with the CASSIS line analysis software. Results. CH 2 NH and N 2 O are detected for the first time, towards a low-mass source, the latter molecule through confirmation with the single-dish TIMASSS survey. NO is also detected. CH 3 NH 2 and NH 2 OH are not detected and stringent upper limit column densities are determined. Conclusions. The non-detection of CH 3 NH 2 and NH 2 OH limits the importance of formation routes to amino acids involving these species. The detection of CH 2 NH makes amino acid formation routes starting from this molecule plausible. The low abundances of CH 2 NH and CH 3 NH 2 compared to Sgr B2 indicate that different physical conditions influence their formation in low- and high-mass sources.

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... Ratios of methylamine with CH 3 OH and CH 3 CN, on the other hand, are somewhat high, perhaps by as much as an order of magnitude or so, dependent on the model and the observational datapoint. Ligterink et al. (2018) recently searched unsuccessfully for methylamine toward IRAS 16293B with ALMA. Their upper limit for the CH 3 NH 2 /NH 2 CHO ratio was 0.053, which is at least two orders of magnitude lower than that of our models, and a factor of nearly 400 lower than our fast model (which we crudely identify as the closest general match for IRAS 16293B abundances). ...
... Hydroxylamine (NH 2 OH) was recently detected for the first time by Rivilla et al. (2020), toward a quiescent molecular cloud. However, this molecule has yet to be detected toward a star-forming source (Pulliam et al. 2012;Ligterink et al. 2018). Experimental work by Congiu et al. (2012) suggests that this molecule may form on grain surfaces through the repetitive addition of H to NO (see also the discussion of Rivilla et al. 2020). ...
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A new, more comprehensive model of gas-grain chemistry in hot molecular cores is presented, in which nondiffusive reaction processes on dust-grain surfaces and in ice mantles are implemented alongside traditional diffusive surface/bulk-ice chemistry. We build on our nondiffusive treatments used for chemistry in cold sources, adopting a standard collapse/warm-up physical model for hot cores. A number of other new chemical model inputs and treatments are also explored in depth, culminating in a final model that demonstrates excellent agreement with gas-phase observational abundances for many molecules, including some (e.g.~methoxymethanol) that could not be reproduced by conventional diffusive mechanisms. Observed ratios of structural isomers methyl formate, glycolaldehyde and acetic acid are well reproduced by the models. The main temperature regimes are identified in which various complex organic molecules (COMs) are formed. Nondiffusive chemistry advances the production of many COMs to much earlier times and lower temperatures than in previous model implementations. Those species may form either as by-products of simple-ice production, or via early photochemistry within the ices while external UV photons can still penetrate. Cosmic ray-induced photochemistry is less important than in past models, although it affects some species strongly over long timescales. Another production regime occurs during the high-temperature desorption of solid water, whereby radicals trapped in the ice are released onto the grain/ice surface, where they rapidly react. Several recently-proposed gas-phase COM-production mechanisms are also introduced, but they rarely dominate. New surface/ice reactions involving CH and CH$_2$ are found to contribute substantially to the formation of certain COMs.
... As recently shown by Becker et al. (2019), NH 2 OH is a key precursor in the unified synthesis of both pyrimidine and purine ribonucleotides. However, observational searches of NH 2 OH in the ISM have so far been unsuccessful (Pulliam et al. 2012;McGuire et al. 2015;Ligterink et al. 2018). This is a puzzling result since NH 2 OH is expected to form efficiently on dust grains according to chemical models and laboratory experiments (Garrod et al. 2008;Zheng & Kaiser 2010;Fedoseev et al. 2012;Garrod 2013;He et al. 2015). ...
... The derived physical parameters for all these species are reported in Table 2. Table 3 compares the abundance of NH 2 OH in G+0.693 with the upper limits found in other regions where it has been previously searched for. We find that the abundance of NH 2 OH in G+0.693 is about a factor of 6 and 25 higher than the upper limits measured in the hot corino IRAS 16293−2422 B and the hot core Sgr B2(N), respectively (Pulliam et al. 2012;Ligterink et al. 2018). Unfortunately, the upper limit toward the protostellar shock L1157-B1 (McGuire et al. 2015) is too high for a meaningful comparison. ...
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One of the proposed scenarios for the origin of life is the primordial RNA world, which considers that RNA molecules were likely responsible for the storage of genetic information and the catalysis of biochemical reactions in primitive cells, before the advent of proteins and DNA. In the last decade, experiments in the field of prebiotic chemistry have shown that RNA nucleotides can be synthesized from relatively simple molecular precursors, most of which have been found in space. An important exception is hydroxylamine, NH2OH, which, despite several observational attempts, it has not been detected in space yet. Here we present the first detection of NH2OH in the interstellar medium toward the quiescent molecular cloud G+0.693-0.027 located in the Galactic Center. We have targeted the three groups of transitions from the J = 2-1, 3-2, and 4-3 rotational lines, detecting five transitions that are unblended or only slightly blended. The derived molecular abundance of NH2OH is (2.1 ± 0.9) × 10-10. From the comparison of the derived abundance of NH2OH and chemically related species, with those predicted by chemical models and measured in laboratory experiments, we favor the formation of NH2OH in the interstellar medium via hydrogenation of NO on dust grain surfaces, with possibly a contribution of ice-mantle NH3 oxidation processes. Further laboratory studies and quantum chemical calculations are needed to completely rule out the formation of NH2OH in the gas phase. © 2020. The American Astronomical Society. All rights reserved.
... As recently shown by Becker et al. (2019), NH 2 OH is a key precursor in the unified synthesis of both pyrimidine and purine ribonucleotides. However, observational searches of NH 2 OH in the ISM have so far been unsuccessful (Pulliam et al. 2012;McGuire et al. 2015;Ligterink et al. 2018). This is a puzzling result since NH 2 OH is expected to form efficiently on dust grains according to chemical models and laboratory experiments (Garrod et al. 2008;Garrod 2013;Zheng & Kaiser 2010;Fedoseev et al. 2012;He et al. 2015). ...
... The derived physical parameters for all these species are reported in Table 2. Table 3 compares the abundance of NH 2 OH in G+0.693 with the upper limits found in other regions where it has been previously searched for. We find that the abundance of NH 2 OH in G+0.693 is about a factor of 6 and 25 higher than the upper limits measured in the hot corino IRAS 16293−2422 B and the hot core Sgr B2(N), respectively (Ligterink et al. 2018;Pulliam et al. 2012). Unfortunately, the upper limit toward the protostellar shock L1157-B1 (McGuire et al. 2015) is too high for a meaningful comparison. ...
Preprint
One of the proposed scenarios for the origin of life is the primordial RNA world, which considers that RNA molecules were likely responsible for the storage of genetic information and the catalysis of biochemical reactions in primitive cells, before the advent of proteins and DNA. In the last decade, experiments in the field of prebiotic chemistry have shown that RNA nucleotides can be synthesized from relatively simple molecular precursors, most of which have been found in space. An important exception is hydroxylamine, NH$_2$OH, which, despite several observational attempts, it has not been detected in space yet. Here we present the first detection of NH$_2$OH in the interstellar medium towards the quiescent molecular cloud G+0.693-0.027 located in the Galactic Center. We have targeted the three groups of transitions from the $J$=2$-$1, 3$-$2, and 4$-$3 rotational lines, detecting 5 transitions that are unblended or only slightly blended. The derived molecular abundance of NH$_2$OH is (2.1$\pm$0.9)$\times$10$^{-10}$. From the comparison of the derived abundance of NH$_2$OH and chemically related species, with those predicted by chemical models and measured in laboratory experiments, we favor the formation of NH$_2$OH in the interstellar medium via hydrogenation of NO on dust grain surfaces, with possibly a contribution of ice mantle NH$_3$ oxidation processes. Further laboratory studies and quantum chemical calculations are needed to completely rule out the formation of NH$_2$OH in the gas phase.
... Although the H 2 CNH +• and HCNH 2 +• ions have not yet been identified in the ISM, neutral methanimine (i.e. methylene imine, H 2 CNH) is ubiquitous, having been detected in several objects including giant molecular clouds as well as both high mass and solar-type protostellar systems Dickens et al. (1997), Suzuki et al. (2016), Widicus Weaver et al. (2017), Ligterink et al. (2018), Bogelund et al. (2019). Methanimine, which could form either on the surface of dust grains Bernstein et al. (1995), Theule et al. (2011) or via gas phase reactions Suzuki et al. (2016), could serve as the basis for synthetic routes to form more complex nitrogencontaining molecules, including biomolecule precursors such as glycine and its corresponding α-aminonitrile Basiuk and Bogillo (2002), Aponte et al. (2017). ...
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Experimental and theoretical studies are presented on the reactivity of the radical cation isomers methanimine and aminomethylene with ethyne. Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors and via direct photoionization of methanimine. Reactive cross sections and product branching ratios are measured as a function of photon and collision energies. Results are discussed in light of ab initio calculations of reaction mechanisms. The major channels, for both isomers, are due to H atom elimination from covalently bound adducts to give [C3NH4]+. Theoretical calculations show that while for the reaction of aminomethylene with acetylene any of the three lowest energy [C3NH4]+ isomers can form via barrierless and exothermic pathways, for the methanimine reagent the only barrierless pathway is the one leading to the production of protonated vinyl cyanide (CH2CHCNH+), a prototypical branched nitrile species that has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan. The astrochemical implications of the results are briefly addressed.
... Although the H 2 CNH +• and HCNH 2 +• ions have not yet been identified in the ISM, neutral methanimine (i.e. methylene imine, H 2 CNH) is ubiquitous, having been detected in several objects including giant molecular clouds as well as both high mass and solar-type protostellar systems Dickens et al. (1997), Suzuki et al. (2016), Widicus Weaver et al. (2017), Ligterink et al. (2018), Bogelund et al. (2019). Methanimine, which could form either on the surface of dust grains Bernstein et al. (1995), Theule et al. (2011) or via gas phase reactions Suzuki et al. (2016), could serve as the basis for synthetic routes to form more complex nitrogencontaining molecules, including biomolecule precursors such as glycine and its corresponding α-aminonitrile Basiuk and Bogillo (2002), Aponte et al. (2017). ...
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Experimental and theoretical studies are presented on the reactivity of the radical cation isomers H 2 CNH +• (methanimine) and HCNH 2 +• (aminomethylene) with ethyne (C 2 H 2 ). Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors as well as via direct photoionization of methanimine. Reactive cross sections (in absolute scales) and product branching ratios are measured as a function of photon and collision energies. Differences between isomers’ reactivity are discussed in light of ab-initio calculations of reaction mechanisms. The major channels, for both isomers, are due to H atom elimination from covalently bound adducts to give [C 3 NH 4 ] ⁺ . Theoretical calculations show that while for the reaction of HCNH 2 +• with acetylene any of the three lowest energy [C 3 NH 4 ] ⁺ isomers can form via barrierless and exothermic pathways, for the H 2 CNH +• reagent the only barrierless pathway is the one leading to the production of protonated vinyl cyanide (CH 2 CHCNH ⁺ ), a prototypical branched nitrile species that has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan. The astrochemical implications of the results are briefly addressed.
... More recently, N 2 O (nitrous oxide) was detected, for the first time, towards the low-mass protostar IRAS 16293−2422B, through confirmation with the single-dish TIMASSS survey (e.g. Ligterink et al. 2018). Observations suggest that presence of nitrous oxide is related to NO (e.g. ...
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... As a matter of fact, current "grain-surface-formation" models are, in some cases, not capable to reproduce recent observations. This is, for instance, the case of the methanimine (CH 2 NH) and methoxymethanol (CH 3 OCH 2 OH) abundances, 6,26 in which significant discrepancies were reported. Without doubts, laboratory experiments are very useful in telling us the nature of the products formed. ...
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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 (COMs) are thought to form on icy dust grains in the earliest phase of star formation. The evolution of these COMs from the youngest Class 0/I protostellar phases toward the more evolved Class II phase is still not fully understood. Since planet formation seems to start early, and mature disks are too cold for characteristic COM emission lines, studying the inventory of COMs on Solar- System scales in the Class 0/I stage is relevant. Aims. Our aim is to determine the abundance ratios of oxygen-bearing COMs in Class 0 protostellar systems on scales of ~100 AU radius. We aim to compare these abundances with one another, and to the abundances of other low-mass protostars such as IRAS 16293-2422B and HH 212. Additionally, using both cold and hot COM lines, the gas-phase abundances can be tracked from a cold to a hot component, and ultimately be compared with those in ices to be measured with the James Webb Space Telescope (JWST). The abundance of deuterated methanol allows us to probe the ambient temperature during the formation of this species. Methods. ALMA Band 3 (3 mm) and Band 6 (1 mm) observations are obtained for seven Class 0 protostars in the Perseus and Serpens star-forming regions. By modeling the inner protostellar region using local thermodynamic equilibrium models, the excitation temperature and column densities are determined for several O-bearing COMs including methanol (CH 3 OH), acetaldehyde (CH 3 CHO), methyl formate (CH 3 OCHO), and dimethyl ether (CH 3 OCH 3 ). Abundance ratios are taken with respect to CH 3 OH. Results. Three out of the seven of the observed sources, B1-c, B1-bS (both Perseus), and Serpens S68N (Serpens), show COM emission. No clear correlation seems to exist between the occurrence of COMs and source luminosity. The abundances of several COMs such as CH 3 OCHO, CH 3 OCH 3 , acetone (CH 3 COCH 3 ), and ethylene glycol ((CH 2 OH) 2 ) are remarkably similar for the three COM-rich sources; this similarity also extends to IRAS 16293-2422B and HH 212, even though collectively these sources originate from four different star-forming regions (i.e., Perseus, Serpens, Ophiuchus, and Orion). For other COMs like CH 3 CHO, ethanol (CH 3 CH 2 OH), and glycolaldehyde (CH 2 OHCHO), the abundances differ by up to an order of magnitude, indicating that local source conditions become important. B1-c hosts a cold ( T ex ≈ 60 K), more extended component of COM emission with a column density of typically a few percent of the warm/hot ( T ex ~ 200 K) central component. A D/H ratio of 1–3% is derived for B1-c, S68N, and B1-bS based on the CH 2 DOH/CH 3 OH ratio (taking into account statistical weighting) suggesting a temperature of ~15 K during the formation of methanol. This ratio is consistent with other low-mass protostars, but is lower than for high-mass star-forming regions. Conclusions. The abundance ratios of most O-bearing COMs are roughly fixed between different star-forming regions, and are presumably set at an earlier cold prestellar phase. For several COMs, local source properties become important. Future mid-infrared facilities such as JWST/MIRI will be essential for the direct observation of COM ices. Combining this with a larger sample of COM-rich sources with ALMA will allow ice and gas-phase abundances to be directly linked in order to constrain the routes that produce and maintain chemical complexity during the star formation process.
Article
Applying various action spectroscopic techniques in a 4 K cryogenic ion trap instrument, protonated methanimine, CH2NH2⁺, has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 39 rovibrational transitions within the fundamental band of the ν2 symmetric C-H stretch were measured, which were used to predict pure rotational transition frequencies of CH2NH2⁺ in the ground vibrational state. Based on these predictions, 9 rotational transitions were observed between 109 and 283 GHz using a novel double resonance method, which significantly improved the sensitivity of the rotational measurements. The double resonance method consists of rotational excitation followed by vibrational excitation, which is finally detected as a dip in the number of CH2NH2⁺-He complexes formed in the 4 K He bath of the trap. The new measurements and the derived predictions of pure rotational transitions will enable the first radio-astronomical search for CH2NH2⁺.
Article
This paper is a review of molecules with internal rotors detected or tentatively detected with upper limits in the interstellar medium. Internal rotor molecules containing one or two methyl groups are present in any of the different phases leading to stellar and planetary formation from the molecular clouds to the star and its planets. They are also found in comets and asteroids. They represent a ubiquitous hydrogen-rich substituent in many complex organic molecules. The paper shows how high resolution spectra (mainly in the microwave, millimeter and sub-millimeter wave range) are analyzed in the laboratory and what theoretical methods can be used to properly reproduce the spectra. The paper also presents a range of examples of important methyl rotors studied and how they can be used as tools to study the chemistry of interstellar medium.
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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.
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 chemical influence of luminosity outbursts on the environments of young solar-type stars is explored. Species are categorized into several types according to their response to the outburst. The first and second types imply chemical changes only during the outburst (with slightly different behaviours). These response types are mostly observed close to the star and are caused by icy mantle evaporation. However, mantles recover after the outburst almost immediately. A notable exception is benzene ice, which is accumulated on dust surfaces during and after the outburst, so that its abundance exceeds the pre-outburst level by orders of magnitude. The third type of response is mostly seen at the disc periphery and implies alteration of abundances during the outburst and preservation of these ‘abnormal’ abundances for centuries. This behaviour is typical of organic compounds, like HCOOCH 3 , CH 3 CN, and CH 2 CO. Their presence in the dark disc regions can be a manifestation of the past outburst. CO and CO 2 only trace past outbursts at the remote disc regions. The outburst changes the C/O ratio, but it quickly returns to the pre-outburst value almost everywhere in the disc. An important factor determining the sensitivity of molecular composition to the outburst is the dust size distribution. The duration of the pre-outburst stage and of the outburst itself influence the chemical effects, if the burst duration is shorter than 50 yr and the duration of the quiescent phase between the bursts is shorter than 100 kyr.
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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.
Article
Context. In the search for the building blocks of life, nitrogen-bearing molecules are of particular interest since nitrogen-containing bonds are essential for the linking of amino acids and ultimately the formation of larger biological structures. The elusive molecule methylamine (CH 3 NH 2 ) is thought to be a key pre-biotic species but has so far only been securely detected in the giant molecular cloud Sagittarius B2. Aims. We identify CH 3 NH 2 and other simple nitrogen-bearing species involved in the synthesis of biologically relevant molecules towards three hot cores associated with the high-mass star-forming region NGC 6334I, located at a distance of 1.3 kpc. Column density ratios are derived in order to investigate the relevance of the individual species as precursors of biotic molecules. Methods. High sensitivity, high angular and spectral resolution observations obtained with the Atacama Large Millimeter/ submillimeter Array were used to study transitions of CH 3 NH 2 , CH 2 NH, NH 2 CHO, and the ¹³ C- and ¹⁵ N-methyl cyanide (CH 3 CN) isotopologues, detected towards NGC 6334I. Column densities are derived for each species assuming local thermodynamic equilibrium and excitation temperatures in the range 220–340 K for CH 3 NH 2 , 70–110 K for the CH 3 CN isotopologues and 120–215 K for NH 2 CHO and CH 2 NH. Results. We report the first detections of CH 3 NH 2 towards NGC 6334I with column density ratios with respect to CH 3 OH of 5.9 × 10 ⁻³ , 1.5 × 10 ⁻³ and 5.4 × 10 ⁻⁴ for the three hot cores MM1, MM2, and MM3, respectively. These values are slightly lower than the values derived for Sagittarius B2 but higher by more than an order of magnitude as compared with the values derived for the low-mass protostar IRAS 16293–2422B. The column density ratios of NH 2 CHO, ¹³ CH 3 CN, and CH 3 C ¹⁵ N with respect to CH 3 OH are (1.5 – 1.9) × 10 ⁻⁴ , (1.0 – 4.6) × 10 ⁻³ and (1.7 – 3.0) × 10 ⁻³ respectively. Lower limits of 5.2, 1.2, and 3.0 are reported for the CH 3 NH 2 to CH 2 NH column density ratio for MM1, MM2, and MM3 respectively. These limits are largely consistent with the values derived for Sagittarius B2 and higher than those for IRAS 16293–2422B. Conclusions. The detections of CH 3 NH 2 in the hot cores of NGC 6334I hint that CH 3 NH 2 is generally common in the interstellar medium, albeit that high-sensitivity observations are essential forthe detection of the species. The good agreement between model predictions of CH 3 NH 2 ratios and the observations towards NGC 6334I indicate a main formation pathway via radical recombination on grain surfaces. This process may be stimulated further by high grain temperatures allowing a lager degree of radical mobility. Further observations with ALMA will help evaluate the degree to which CH 3 NH 2 chemistry depends on the temperature of the grains in high- and low-mass star-forming regions respectively.
Article
Context. One of the important questions of astrochemistry is how complex organic molecules, including potential prebiotic species, are formed in the envelopes around embedded protostars. The abundances of minor isotopologues of a molecule, in particular the D- and ¹³ C-bearing variants, are sensitive to the densities, temperatures and timescales characteristic of the environment in which they form, and can therefore provide important constraints on the formation routes and conditions of individual species. Aims. The aim of this paper is to systematically survey the deuteration and the ¹³ C content of a variety of oxygen-bearing complex organic molecules on solar system scales toward the “B component” of the protostellar binary IRAS16293–2422. Methods. We have used the data from an unbiased molecular line survey of the protostellar binary IRAS16293−2422 between 329 and 363 GHz from the Atacama Large Millimeter/submillimeter Array (ALMA). The data probe scales of 60 AU (diameter) where most of the organic molecules are expected to have sublimated off dust grains and be present in the gas phase. The deuterated and ¹³ C isotopic species of ketene, acetaldehyde and formic acid, as well as deuterated ethanol, are detected unambiguously for the first time in the interstellar medium. These species are analysed together with the ¹³ C isotopic species of ethanol, dimethyl ether and methyl formate along with mono-deuterated methanol, dimethyl ether and methyl formate. Results. The complex organic molecules can be divided into two groups with one group, the simpler species, showing a D/H ratio of ≈2% and the other, the more complex species, D/H ratios of 4–8%. This division may reflect the formation time of each species in the ices before or during warm-up/infall of material through the protostellar envelope. No significant differences are seen in the deuteration of different functional groups for individual species, possibly a result of the short timescale for infall through the innermost warm regions where exchange reactions between different species may be taking place. The species show differences in excitation temperatures between 125 and 300 K. This likely reflects the binding energies of the individual species, in good agreement with what has previously been found for high-mass sources. For dimethyl ether, the ¹² C/ ¹³ C ratio is found to be lower by up to a factor of 2 compared to typical ISM values similar to what has previously been inferred for glycolaldehyde. Tentative identifications suggest that the same may apply for ¹³ C isotopologues of methyl formate and ethanol. If confirmed, this may be a clue to their formation at the late prestellar or early protostellar phases with an enhancement of the available ¹³ C relative to ¹² C related to small differences in binding energies for CO isotopologues or the impact of FUV irradiation by the central protostar. Conclusions. The results point to the importance of ice surface chemistry for the formation of these complex organic molecules at different stages in the evolution of embedded protostars and demonstrate the use of accurate isotope measurements for understanding the history of individual species.
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The high-sensitivity of the IRAM 30-m ASAI unbiased spectral survey in the mm-window allows us to detect NO emission towards both the Class I object SVS13-A and the protostellar outflow shock L1157-B1. We detect the hyperfine components of the $^2\Pi_{\rm 1/2}$ $J$ = 3/2 $\to$ 1/2 (at 151 GHz) and the $^2\Pi_{\rm 1/2}$ $J$ = 5/2 $\to$ 3/2 (250 GHz) spectral pattern. The two objects show different NO profiles: (i) SVS13-A emits through narrow (1.5 km s$^{-1}$) lines at the systemic velocity, while (ii) L1157-B1 shows broad ($\sim$ 5 km s$^{-1}$) blue-shifted emission. For SVS13-A the analysis leads to $T_{\rm ex}$ $\geq$ 4 K, $N(\rm NO)$ $\leq$ 3 $\times$ 10$^{15}$ cm$^{-2}$, and indicates the association of NO with the protostellar envelope. In L1157-B1, NO is tracing the extended outflow cavity: $T_{\rm ex}$ $\simeq$ 4--5 K, and $N(\rm NO)$ = 5.5$\pm$1.5 $\times$ 10$^{15}$ cm$^{-2}$. Using C$^{18}$O, $^{13}$C$^{18}$O, C$^{17}$O, and $^{13}$C$^{17}$O ASAI observations we derive an NO fractional abundance less than $\sim$ 10$^{-7}$ for the SVS13-A envelope, in agreement with previous measurements towards extended PDRs and prestellar objects. Conversely, a definite $X(NO)$ enhancement is measured towards L1157-B1, $\sim$ 6 $\times$ 10$^{-6}$, showing that the NO production increases in shocks. The public code UCLCHEM was used to interpret the NO observations, confirming that the abundance observed in SVS13-A can be attained in an envelope with a gas density of 10$^5$ cm$^{-3}$ and a kinetic temperature of 40 K. The NO abundance in L1157-B1 is reproduced with pre-shock densities of 10$^5$ cm$^{-3}$ subjected to a $\sim$ 45 km s$^{-1}$ shock.
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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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The laboratory work presented here simulates the chemistry on icy dust grains as typical for the ‘CO freeze-out stage’ in dark molecular clouds. It differs from previous studies in that solid-state hydrogenation and vacuum UV photoprocessing are applied simultaneously to co-depositing molecules. In parallel, the reactions at play are described for fully characterized laboratory conditions. The focus is on the formation of molecules containing both carbon and nitrogen atoms, starting with NO in CO-, H2CO-, and CH3OH-rich ices at 13 K. The experiments yield three important conclusions. (1) Without UV processing hydroxylamine (NH2OH) is formed, as reported previously. (2) With UV processing (energetic) NH2 is formed through photodissociation of NH2OH. This radical is key in the formation of species with an N–C bond. (3) The formation of three N–C bearing species, HNCO, OCN−, and NH2CHO, is observed. The experiments put a clear chemical link between these species; OCN− is found to be a direct derivative of HNCO and the latter is shown to have the same precursor as formamide (NH2CHO). Moreover, the addition of VUV competing channels decreases the amount of NO molecules converted into NH2OH by at least one order of magnitude. Consequently, this decrease in NH2OH formation yield directly influences the amount of NO molecules that can be converted into HNCO, OCN−, and NH2CHO.
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The importance of comets for the origin of life on Earth has been advocated for many decades. Amino acids are key ingredients in chemistry, leading to life as we know it. Many primitive meteorites contain amino acids, and it is generally believed that these are formed by aqueous alterations. In the collector aerogel and foil samples of the Stardust mission after the flyby at comet Wild 2, the simplest form of amino acids, glycine, has been found together with precursor molecules methylamine and ethylamine. Because of contamination issues of the samples, a cometary origin was deduced from the (13)C isotopic signature. We report the presence of volatile glycine accompanied by methylamine and ethylamine in the coma of 67P/Churyumov-Gerasimenko measured by the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) mass spectrometer, confirming the Stardust results. Together with the detection of phosphorus and a multitude of organic molecules, this result demonstrates that comets could have played a crucial role in the emergence of life on Earth.
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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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A deep search for the potential glycine precursor hydroxylamine (NH$_2$OH) using the Caltech Submillimeter Observatory (CSO) at $\lambda = 1.3$ mm and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at $\lambda = 3$ mm is presented toward the molecular outflow L1157, targeting the B1 and B2 shocked regions. We report non-detections of NH$_2$OH in both sources. We a perform non-LTE analysis of CH$_3$OH observed in our CSO spectra to derive kinetic temperatures and densities in the shocked regions. Using these parameters, we derive upper limit column densities of NH$_2$OH of $\leq1.4 \times 10^{13}$~cm$^{-2}$ and $\leq1.5 \times 10^{13}$~cm$^{-2}$ toward the B1 and B2 shocks, respectively, and upper limit relative abundances of $N_{NH_2OH}/N_{H_2} \leq1.4 \times 10^{-8}$ and $\leq1.5 \times 10^{-8}$, respectively.
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Comets harbor the most pristine material in our solar system in the form of ice, dust, silicates, and refractory organic material with some interstellar heritage. The evolved gas analyzer Cometary Sampling and Composition (COSAC) experiment aboard Rosetta's Philae lander was designed for in situ analysis of organic molecules on comet 67P/Churyumov-Gerasimenko. Twenty-five minutes after Philae's initial comet touchdown, the COSAC mass spectrometer took a spectrum in sniffing mode, which displayed a suite of 16 organic compounds, including many nitrogen-bearing species but no sulfur-bearing species, and four compounds-methyl isocyanate, acetone, propionaldehyde, and acetamide-that had not previously been reported in comets. Copyright © 2015, American Association for the Advancement of Science.
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The quest to detect prebiotic molecules in space, notably amino acids, requires an understanding of the chemistry involving nitrogen atoms. Hydroxylamine (NH$_2$OH) is considered a precursor to the amino acid glycine. Although not yet detected, NH$_2$OH is considered a likely target of detection with ALMA. We report on an experimental investigation of the formation of hydroxylamine on an amorphous silicate surface via the oxidation of ammonia. The experimental data are then fed into a simulation of the formation of NH$_2$OH in dense cloud conditions. On ices at 14 K and with a modest activation energy barrier, NH$_2$OH is found to be formed with an abundance that never falls below a factor 10 with respect to NH$_3$. Suggestions of conditions for future observations are provided.
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Frequency measurements are given for the 1000←0000 band of N2O near 4.5 μm and for pure rotational transitions beyond 151.5 μm. The infrared measurements utilize a periodically poled lithium niobate (PPLN) based difference frequency generation (DFG) source locked to the saturated absorption center of an N2O absorption line. The DFG frequency is calibrated by an optical frequency comb and an iodine hyperfine transition. We report 44 transitions ranging from J=1−100 for both the P and R branches and the accuracy is better than 10 kHz for most transitions. In addition, 175 pure rotational transitions have been measured including 33 measurements with sub-Doppler precision (≤3 kHz), and 142 Doppler limited measurements. These are combined with other precision rotational and vibrational measurements to provide improved quantum mechanical parameters, as well as frequency calibration tables for the N2O bands near 4.5 μm.
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We present a comprehensive analysis of a broadband spectral line survey of the Orion Kleinmann-Low nebula (Orion KL), one of the most chemically rich regions in the Galaxy, using the HIFI instrument on board the Herschel Space Observatory. This survey spans a frequency range from 480 to 1907 GHz at a resolution of 1.1 MHz. These observations thus encompass the largest spectral coverage ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and include frequencies >1 THz, where the Earth's atmosphere prevents observations from the ground. In all, we detect emission from 39 molecules (79 isotopologues). Combining this data set with ground-based millimeter spectroscopy obtained with the IRAM 30 m telescope, we model the molecular emission from the millimeter to the far-IR using the XCLASS program, which assumes local thermodynamic equilibrium (LTE). Several molecules are also modeled with the MADEX non-LTE code. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. A reduced χ2 analysis indicates that models for most species reproduce the observed emission well. In particular, most complex organics are well fit by LTE implying gas densities are high (>106 cm–3) and excitation temperatures and column densities are well constrained. Molecular abundances are computed using H2 column densities also derived from the HIFI survey. The distribution of rotation temperatures, T rot, for molecules detected toward the hot core is significantly wider than the compact ridge, plateau, and extended ridge T rot distributions, indicating the hot core has the most complex thermal structure.
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A sensitive broadband molecular line survey of the Sagittarius B2(N) star-forming region has been obtained with the HIFI instrument on the Herschel Space Observatory, offering the first high-spectral resolution look at this well-studied source in a wavelength region largely inaccessible from the ground (625-157 um). From the roughly 8,000 spectral features in the survey, a total of 72 isotopologues arising from 44 different molecules have been identified, ranging from light hydrides to complex organics, and arising from a variety of environments from cold and diffuse to hot and dense gas. We present an LTE model to the spectral signatures of each molecule, constraining the source sizes for hot core species with complementary SMA interferometric observations, and assuming that molecules with related functional group composition are cospatial. For each molecule, a single model is given to fit all of the emission and absorption features of that species across the entire 480-1910 GHz spectral range, accounting for multiple temperature and velocity components when needed to describe the spectrum. As with other HIFI surveys toward massive star forming regions, methanol is found to contribute more integrated line intensity to the spectrum than any other species. We discuss the molecular abundances derived for the hot core, where the local thermodynamic equilibrium approximation is generally found to describe the spectrum well, in comparison to abundances derived for the same molecules in the Orion KL region from a similar HIFI survey.
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We have investigated the synthesis of the simplest amino acid, glycine, by Galactic cosmic-ray particles in extraterrestrial ices. Laboratory experiments combined with electronic structure calculations showed that a methylamine molecule [CH3NH2(X 1A')] can be dissociated through interaction with energetic electrons in the track of a cosmic-ray particle to form atomic hydrogen and the radicals CH2NH2(X 2A') and CH3NH(X 2A'). Hydrogen atoms with sufficient kinetic energy could overcome the entrance barrier to add to a carbon dioxide molecule [CO2(X 1Σ)], yielding a trans-hydroxycarbonyl radical, HOCO(X 2A'). Neighboring radicals with the correct geometric orientation then recombine to form glycine, NH2CH2COOH(X 1A), and also its isomer, CH3NHCOOH(X 1A). These findings expose for the first time detailed reaction mechanisms of how the simplest amino acid glycine and its isomer can be synthesized via nonequilibrium chemistry in interstellar and cometary ices. Our results offer an important alternative to aqueous and photon-induced formation of amino acids in comets and in molecular clouds. These results also predict the existence of a hitherto undetected isomer of glycine in the interstellar medium, suggest that glycine should be observable on Saturn's moon Titan, and help to account for the synthesis of more complex amino acids in the Murchison and Orgueil meteorites.
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Context. According to traditional gas-phase chemical models, O_2 should be abundant in molecular clouds, but until recently, attempts to detect interstellar O_2 line emission with ground- and space-based observatories have failed. Aims. Following the multi-line detections of O_2 with low abundances in the Orion and ρ Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O_2 abundance near a solar-mass protostar. Methods. Observations of molecular oxygen, O_2, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333-IRAS 4A, are presented, using the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models. Results. The deep HIFI spectrum fails to show O_2 at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O_2/H_2 at ≤6 × 10^(-9) (3σ). The O_2/CO abundance ratio is less than 0.005. However, a tentative (4.5σ) detection of O_2 is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km s^(-1) relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~0.7–1 × 10^6 years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H_2O, to avoid overproduction of O_2 in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule if hydrogenation of NO on the grains to more complex molecules such as NH_2OH, found in recent laboratory experiments, is included. The tentative detection of O_2 in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates. Conclusions. The low O_2 abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O_2.
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The available data on the microwave spectrum of methylenimine are critically reviewed for information applicable to radio astronomy. Molecular data such as rotational constants, centrifugal distortion parameters, hyperfine coupling constants, and dipole moments are tabulated. A detailed centrifugal distortion calculation has been carried out for the most abundant isotopic form of this molecule, H2 12C 14NH. Transitions have been predicted and tabulated for the frequency range 100 MHz to 300 GHz. All predicted transitions include 95 percent confidence limits; error limits have been reported for all measured transitions.
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Context. Hydrogenation reactions dominate grain surface chemistry in dense molecular clouds and lead to the formation of complex saturated molecules in the interstellar medium. Aims. We investigate in the laboratory the hydrogenation reaction network of hydrogen cyanide HCN. Methods. Pure hydrogen cyanide HCN and methanimine CH2NH ices are bombarded at room temperature by H-atoms in an ultra-high vacuum experiment. Warm H-atoms are generated in an H-2 plasma source. The ices are monitored with Fourier-transform infrared spectroscopy in reflection absorption mode. The hydrogenation products are detected in the gas phase by mass spectroscopy during temperature-programmed desorption experiments. Results. HCN hydrogenation leads to the formation of methylamine CH3NH2, and CH2NH hydrogenation leads to the formation of methylamine CH3NH2, suggesting that CH2NH can be a hydrogenation- intermediate species between HCN and CH3NH2. Conclusions. In cold environments the HCN hydrogenation reaction can produce CH3NH2, which is known to be a glycine precursor, and to destroy solid-state HCN, preventing its observation in molecular clouds ices.
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Context. Studing chemical reactivity in astrophysical environments is an important means for improving our understanding of the origin of the organic matter in molecular clouds, in protoplanetary disks, and possibly, as a final destination, in our solar system. Laboratory simulations of the reactivity of ice analogs provide important insight into the reactivity in these environments. Here, we use these experimental simulations to investigate the Strecker synthesis leading to the formation of aminoacetonitrile in astrophysical-like conditions. The aminoacetonitrile is an interesting compound because it was detected in SgrB2, hence could be a precursor of the smallest amino acid molecule, glycine, in astrophysical environments. Aims. We present the first experimental investigation of the formation of aminoacetonitrile NH2CH2CN from the thermal processing of ices including methanimine (CH2NH), ammonia (NH3), and hydrogen cyanide (HCN) in interstellar-like conditions without VUV photons or particules. Methods. We use Fourier Transform InfraRed (FTIR) spectroscopy to monitor the ice evolution during its warming. Infrared spectroscopy and mass spectroscopy are then used to identify the aminoacetonitrile formation. Results. We demonstrate that methanimine can react with - CN during the warming of ice analogs containing at 20 K methanimine, ammonia, and [NH4+ -CN] salt. During the ice warming, this reaction leads to the formation of poly(methylene-imine) polymers. The polymer length depend on the initial ratio of mass contained in methanimine to that in the [NH4+ -CN] salt. In a methanimine excess, long polymers are formed. As the methanimine is progressively diluted in the [NH4+ -CN] salt, the polymer length decreases until the aminoacetonitrile formation at 135 K. Therefore, these results demonstrate that aminoacetonitrile can be formed through the second step of the Strecker synthesis in astrophysical-like conditions.
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A computational study of the reactions of hydroxylamine and its ionized and protonated derivatives with acetic acid is provided. The reaction of neutral hydroxylamine with acetic acid, despite being clearly exothermic, involves a very large energy barrier. The reaction of ionized hydroxylamine with acetic acid is also clearly exothermic, but again a significant energy barrier is found (around 24 kcal mol–1 at the CCSD(T) level). The reaction of the most stable protonated isomer of hydroxylamine, NH3OH+, with acetic acid also involves a high barrier (more than 27 kcal mol–1 at the CCSD(T) level). Only the higher energy isomer, NH2OH+ 2, leads to a sensibly lower energy barrier (about 2.3 kcal mol–1 at the CCSD(T) level). Nevertheless, an estimate of the reaction coefficient at low temperatures such as those reigning in the interstellar medium gives very low values. Therefore, it seems that precursors of interstellar glycine could not be efficiently produced from the reactions of hydroxylamine-derived ions with acetic acid.
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New measurements of 12C/13C ratios in Galactic molecular clouds have been conducted using the N = 1 → 0 transition of the CN radical. This species is unique in that it has extensive hyperfine structure that can be accurately used to correct for line saturation effects. Combined with the past observations of Savage and coworkers, the ratios derived from CN are the most extensive data set to date for molecular clouds, and they include sources that lie in the range of 0.09-16.41 kpc in distance from the Galactic center (DGC). The ratios derived from CN indicate a gradient with Galactic distance of 12C/13C = 6.01DGC + 12.28. This gradient agrees rather closely with those derived from measurements of CO and H2CO. The least-squares fit to all data points for the three molecules is 12C/13C = 6.21DGC + 18.71. CO, CN, and H2CO are synthesized from quite varied reactions, and any 13C fractionation must follow different pathways for these three species. The relatively good agreement between the 12C/13C ratios of the three molecules, as well as their lack of correlation with gas kinetic temperature, suggests that chemical fractionation and isotope-selective photodissociation both do not play a substantial role in influencing such ratios. Therefore, the 12C/13C gradient found in the Galaxy is a true indicator of Galactic chemical evolution. The apparent discrepancy between the solar system (12C/13C = 89) and local interstellar medium values (12C/13C ≈ 68) of this ratio may be a result of 13C enrichment since the formation of the solar system, as predicted by recent models.
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A recent experimental study reported that glycine and other amino acids were formed when cryogenic H2O ice containing small amounts of CH3OH, NH3, and HCN was subjected to ultraviolet (UV) irradiation. Quantum chemical calculations were employed to evaluate the viability of various pathways to the formation of glycine, alanine, and serine in dilute H2O ice containing CH3OH and HCN. Under the experimental processing conditions of deposition and UV irradiation at 15 K followed by heating to room temperature, amino acids can form by recombining radicals produced by dehydrogenating H2O and CH3OH and subsequently hydrogenating HCN. The study indicates that isotopic substitution experiments would identify CH3OH as the source of the C atom in the COOH carboxylic acid group of the amino acids observed in the irradiation experiments, with the CO+OH reaction playing an important role. The remaining C and N atoms in glycine are predicted to originate from HCN via sequential hydrogenation to yield CH2NH2. Formation pathways for alanine and serine are also discussed.
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Hydroxylamine (NH(2)OH) is one of the potential precursors of complex pre-biotic species in space. Here, we present a detailed experimental study of hydroxylamine formation through nitric oxide (NO) surface hydrogenation for astronomically relevant conditions. The aim of this work is to investigate hydroxylamine formation efficiencies in polar (water-rich) and non-polar (carbon monoxide-rich) interstellar ice analogues. A complex reaction network involving both final (N(2)O, NH(2)OH) and intermediate (HNO, NH(2)O·, etc.) products is discussed. The main conclusion is that hydroxyl-amine formation takes place via a fast and barrierless mechanism and it is found to be even more abundantly formed in a water-rich environment at lower temperatures. In parallel, we experimentally verify the non-formation of hydroxylamine upon UV photolysis of NO ice at cryogenic temperatures as well as the non-detection of NC- and NCO-bond bearing species after UV processing of NO in carbon monoxide-rich ices. Our results are implemented into an astrochemical reaction model, which shows that NH(2)OH is abundant in the solid phase under dark molecular cloud conditions. Once NH(2)OH desorbs from the ice grains, it becomes available to form more complex species (e.g., glycine and β-alanine) in gas phase reaction schemes.
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Our previous analysis of cometary samples returned to Earth by NASA's Stardust spacecraft showed several amines and amino acids, but the origin of these compounds could not be firmly established. Here, we present the stable carbon isotopic ratios of glycine and ε-amino-n-caproic acid (EACA), the two most abundant amino acids identified in Stardust-returned foil samples measured by gas chromatography–mass spectrometry coupled with isotope ratio mass spectrometry. The δ 13 C value for glycine of +29 ± 6‰ strongly suggests an extraterrestrial origin for glycine, while the δ 13 C value for EACA of −25 ± 2‰ indicates terrestrial contamination by Nylon-6 during curation. This represents the first detection of a cometary amino acid.
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Icy dust grains in space act as catalytic surfaces onto which complex molecules form. These molecules are synthesized through exothermic reactions from precursor radicals and, mostly, hydrogen atom additions. Among the resulting products are species of biological relevance, such as hydroxylamine—NH2OH—a precursor molecule in the formation of amino acids. In this Letter, laboratory experiments are described that demonstrate NH2OH formation in interstellar ice analogs for astronomically relevant temperatures via successive hydrogenation reactions of solid nitric oxide (NO). Inclusion of the experimental results in an astrochemical gas–grain model proves the importance of a solid-state NO + H reaction channel as a starting point for prebiotic species in dark interstellar clouds and adds a new perspective to the way molecules of biological importance may form in space.
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Observations of 14 rotational transitions of hydroxylamine (NH_2OH) using the NRAO 12 m telescope on Kitt Peak are reported toward IRC+10216, Orion KL, Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and W51M. Although recent models suggest the presence of NH_2OH in high abundance, these observations resulted in non-detection. Upper limits are calculated to be as much as six orders of magnitude lower than those predicted by models. Possible explanations for the lower-than-expected abundance are explored.
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Context: The chemical composition of a molecular cloud changes dramatically as it collapses to form a low-mass protostar and circumstellar disk. Two-dimensional (2D) chemodynamical models are required to properly study this process. Aims: The goal of this work is to follow, for the first time, the chemical evolution in two dimensions all the way from a pre-stellar core into a circumstellar disk. Of special interest is the question whether the chemical composition of the disk is a result of chemical processing during the collapse phase, or whether it is determined by in situ processing after the disk has formed. Methods: Our model combines a semi-analytical method to get 2D axisymmetric density and velocity structures with detailed radiative transfer calculations to get temperature profiles and UV fluxes. Material is followed in from the core to the disk and a full gas-phase chemistry network -- including freeze-out onto and evaporation from cold dust grains -- is evolved along these trajectories. The abundances thus obtained are compared to the results from a static disk model and to observations of comets. Results: The chemistry during the collapse phase is dominated by a few key processes, such as the evaporation of CO or the photodissociation of H2O. At the end of the collapse phase, the disk can be divided into zones with different chemical histories. The disk is not in chemical equilibrium at the end of the collapse, so care must be taken when choosing the initial abundances for stand-alone disk chemistry models. Our model results imply that comets must be formed from material with different chemical histories: some of it is strongly processed, some of it remains pristine. Variations between individual comets are possible if they formed at different positions or different times in the solar nebula.
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While unbiased surveys observable from ground-based telescopes have previously been obtained towards several high mass protostars, very little exists on low mass protostars. To fill up this gap, we carried out a complete spectral survey of the bands at 3, 2, 1 and 0.8 mm towards the solar type protostar IRAS16293-2422. The observations covered about 200\,GHz and were obtained with the IRAM-30m and JCMT-15m telescopes. Particular attention was devoted to the inter-calibration of the obtained spectra with previous observations. All the lines detected with more than 3 sigma and free from obvious blending effects were fitted with Gaussians to estimate their basic kinematic properties. More than 4000 lines were detected (with sigma \geq 3) and identified, yielding a line density of approximatively 20 lines per GHz, comparable to previous surveys in massive hot cores. The vast majority (~2/3) of the lines are weak and due to complex organic molecules. The analysis of the profiles of more than 1000 lines belonging 70 species firmly establishes the presence of two distinct velocity components, associated with the two objects, A and B, forming the IRAS16293-2422 binary system. In the source A, the line widths of several species increase with the upper level energy of the transition, a behavior compatible with gas infalling towards a ~1 Mo object. The source B, which does not show this effect, might have a much lower central mass of ~0.1 Mo. The difference in the rest velocities of both objects is consistent with the hypothesis that the source B rotates around the source A. This spectral survey, although obtained with single-dish telescope with a low spatial resolution, allows to separate the emission from 2 different components, thanks to the large number of lines detected. The data of the survey are public and can be retrieved on the web site http://www-laog.obs.ujf-grenoble.fr/heberges/timasss.
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Context. Analyses of dust cometary grains collected by the Stardust spacecraft have shown the presence of amines and amino acids molecules, and among them glycine (NH$_{2}$CH$_{2}$COOH). We show how the glycine molecule could be produced in the protostellar environments before its introduction into comets.Aims. We study the evolution of the interstellar ice analogues affected by both thermal heating and vacuum ultraviolet (VUV) photons, in addition to the nature of the formed molecules and the confrontation of our experimental results with astronomical observations.Methods. Infrared spectroscopy and mass spectrometry are used to monitor the evolution of the H$_{2}$O$:$CO$_{2}$$:$CH$_{3}$NH$_{2}$ and CO$_{2}$$:$CH$_{3}$NH$_{2}$ ice mixtures during both warming processes and VUV photolysis.Results. We first show how carbon dioxide (CO$_{2}$) and methylamine (CH$_{3}$NH$_{2}$) thermally react in water-dominated ice to form methylammonium methylcarbamate [ CH$_{3}$NH$_{3}^{+}$] [ CH$_{3}$NHCOO$^{-}$] noted C. We then determine the reaction rate and activation energy. We show that C thermal formation can occurs in the 50–70 K temperature range of a protostellar environment. Secondly, we report that a VUV photolysis of a pure C sample produces a glycine salt, methylammonium glycinate [ CH$_{3}$NH$_{3}^{+}$] [ NH$_{2}$CH$_{2}$COO$^{-}$] noted G. We propose a scenario explaining how C and subsequently G can be synthesized in interstellar ices and precometary grains.Conclusions. [ CH$_{3}$NH$_{3}^{+}$] [ CH$_{3}$NHCOO$^{-}$] could be readily formed and would act as a glycine salt precursor in protostellar environments dominated by thermal and UV processing. We propose a new pathway leading to a glycine salt, which is consistent with the detection of glycine and methylamine within the returned samples of comet 81P/Wild 2 from the Stardust mission.
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A new interstellar molecule, N2O, known as nitrous oxide or 'laughing gas,' has been detected using the NRAO 12 m telescope. The J = 3 - 2, 4 - 3, 5 - 4, and 6 - 5 rotational transitions of this species at 75, 100, 125, and 150 GHz, respectively, were observed toward Sgr B2(M). The column density derived for N2O in this source is N(sub tot) approx. 10(exp 15)/sq. cm, which corresponds to a fractional abundance of approx. 10(exp -9), relative to H2. This value implies abundance ratios of N2O/NO approx. 0.1 and N2O/HNO approx. 3 in the Galactic center. Such ratios are in excellent agreement with predictions of ion-molecule models of interstellar chemistry using early-time calculations and primarily neutral-neutral reactions. N2O is the third interstellar molecule detected thus far containing an N-O bond. Such bonds cannot be so rare as previously thought.
Article
The –CH–NH 2 moiety represents the fundamental building block of all proteinogenic amino acids, with the cyclic amino acid proline being a special case (–CH–NH– in proline). Exploiting a chemical retrosynthesis, we reveal that methylamine (CH 3 NH 2) and/or ethylamine (CH 3 CH 2 NH 2) are essential precursors in the formation of each proteinogenic amino acid. In the present study we elucidate the abiotic formation of methylamine and ethylamine from ammonia (NH 3) and methane (CH 4) ices exposed to secondary electrons generated by energetic cosmic radiation in cometary and interstellar model ices. Our experiments show that methylamine and ethylamine are crucial reaction products in irradiated ices composed of ammonia and methane. Using isotopic substitution studies we further obtain valuable information on the specific reaction pathways toward methylamine. The very recent identification of methylamine and ethylamine together with glycine in the coma of 67P/Churyumov–Gerasimenko underlines their potential to the extraterrestrial formation of amino acids.
Article
Context. We wish to improve our understanding of the Orion central star formation region (Orion-KL) and disentangle its complexity. Aims. We collected data with ALMA during cycle 2 in 16 GHz of total bandwidth spread between 215.1 and 252.0 GHz with a typical sensitivity of 5 mJy/beam (2.3 mJy/beam from 233.4 to 234.4 GHz) and a typical beam size of 1.̋7 × 1.̋0 (average position angle of 89°). We produced a continuum map and studied the emission lines in nine remarkable infrared spots in the region including the hot core and the compact ridge, plus the recently discovered ethylene glycol peak. Methods. We present the data, and report the detection of several species not previously seen in Orion, including n- and i-propyl cyanide (C 3 H 7 CN), and the tentative detection of a number of other species including glycolaldehyde (CH 2 (OH)CHO). The first detections of gGg′ ethylene glycol (gGg′ (CH 2 OH) 2 ) and of acetic acid (CH 3 COOH) in Orion are presented in a companion paper. We also report the possible detection of several vibrationally excited states of cyanoacetylene (HC 3 N), and of its ¹³ C isotopologues. We were not able to detect the ¹⁶ O ¹⁸ O line predicted by our detection of O 2 with Herschel, due to blending with a nearby line of vibrationally excited ethyl cyanide. We do not confirm the tentative detection of hexatriynyl (C 6 H) and cyanohexatriyne (HC 7 N) reported previously, or of hydrogen peroxide (H 2 O 2 ) emission. Results. We report a complex velocity structure only partially revealed before. Components as extreme as −7 and +19 km s ⁻¹ are detected inside the hot region. Thanks to different opacities of various velocity components, in some cases we can position these components along the line of sight. We propose that the systematically redshifted and blueshifted wings of several species observed in the northern part of the region are linked to the explosion that occurred ~500 yr ago. The compact ridge, noticeably farther south displays extremely narrow lines (~1 km s ⁻¹ ) revealing a quiescent region that has not been affected by this explosion. This probably indicates that the compact ridge is either over 10 000 au in front of or behind the rest of the region. Conclusions. Many lines remain unidentified, and only a detailed modeling of all known species, including vibrational states of isotopologues combined with the detailed spatial analysis offered by ALMA enriched with zero-spacing data, will allow new species to be detected.
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
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
We conducted survey observations of a glycine precursor, methanimine or methylenimine (CH$_2$NH), with the NRO 45 m telescope and the SMT telescope towards 12 high-mass and two low-mass star-forming regions in order to increase number of CH$_2$NH sources and to better understand the characteristics of CH2NH sources. As a result of our survey, CH$_2$NH was detected in eight sources, including four new sources. The estimated fractional abundances were ~10$^8$ in Orion KL and G10.47+0.03, while they were ~10$^9$ towards the other sources. Our hydrogen recombination line and past studies suggest that CH$_2$NH-rich sources have less evolved HII regions. The less destruction rates by UV flux from the central star would be contributed to the high CH$_2$NH abundances towards CH$_2$NH-rich sources. Our gas-grain chemical simulations suggest that CH$_2$NH is mostly formed in the gas-phase by neutral-neutral reactions rather than the product of thermal evaporation from the dust surfaces.
Article
The presence of NH2OH, one of the main precursors in the formation of amino-acids, on dust grain mantles, may be the most obvious elucidation for the creation of large prebiotic molecules in the interstellar medium. However, while many laboratory experimental studies, to simulate the icy grain chemistry in space, found that NH2OH molecules may be easily formed in solid phase with high abundances and then they should desorb, through a temperature induced desorption into the gas phase, with the same high abundances; all the spatial observations conclude that NH2OH is not detected in gas phase within any of the explored astronomical sources. Such inconsistencies between laboratory experiment simulations and spatial observations lead our investigations towards this experimental study to see if there is any chemical transformation of NH2OH, occurring in the solid phase before the desorption processes of NH2OH from the mantle of interstellar icy grains. Our experimental results show that the heating of NH2OH-H2O ices lead to a decomposition of NH2OH into HNO, NH3 and O2, even before reaching its desorption temperature. We show through this work that the NH2OH non-detection from previous examined astronomical sources could mainly due to its high reactivity in solid phase on the icy interstellar grains.
Article
Nitric oxide (/sup 14/N/sup 16/O) has been detected in the source Sgr B2 by means of its two /sup 2/Pi/sub 1/2/, J=3/2..-->..1/2, rotational transitions at 150.2 and 150.5 GHz. The inferred column density and related abundance are N/sub NO/approx.2 x 10/sup 16/ cm/sup -2/ and X (NO) approx.1 x 10/sup -8/, respectively.
Article
We aim to detect methylamine, CH$_{3}$NH$_{2}$, in a variety of hot cores and use it as a test for the importance of photon-induced chemistry in ice mantles and mobility of radicals. Specifically, CH$_3$NH$_2$ cannot be formed from atom addition to CO whereas other NH$_2$-containing molecules such as formamide, NH$_2$CHO, can. Submillimeter spectra of several massive hot core regions were taken with the James Clerk Maxwell Telescope. Abundances are determined with the rotational diagram method where possible. Methylamine is not detected, giving upper limit column densities between 1.9 $-$ 6.4 $\times$ 10$^{16}$ cm$^{-2}$ for source sizes corresponding to the 100 K envelope radius. Combined with previously obtained JCMT data analyzed in the same way, abundance ratios of CH$_{3}$NH$_{2}$, NH$_{2}$CHO and CH$_{3}$CN with respect to each other and to CH$_{3}$OH are determined. These ratios are compared with Sagittarius B2 observations, where all species are detected, and to hot core models. The observed ratios suggest that both methylamine and formamide are overproduced by up to an order of magnitude in hot core models. Acetonitrile is however underproduced. The proposed chemical schemes leading to these molecules are discussed and reactions that need further laboratory studies are identified. The upper limits obtained in this paper can be used to guide future observations, especially with ALMA.
Article
Multiple observations of methanimine (CH2NH) and methyl amine (CH3NH2) have been performed toward Sgr B2(N) at 1, 2, and 3 mm using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. In the frequency range 68-280 GHz, 23 transitions of CH2NH and 170 lines of CH3NH2 have been observed as individual, distinguishable features, although some are partially blended with other lines. For CH2NH, the line profiles indicate V LSR = 64.2 ± 1.4 km s–1 and ΔV 1/2 = 13.8 ± 2.8 km s–1, while V LSR = 63.7 ± 1.6 km s–1 and ΔV 1/2 = 15.1 ± 3.0 km s–1 for CH3NH2, parameters that are very similar to those of other organic species in Sgr B2(N). From these data, rotational diagrams were constructed for both species. In the case of CH2NH, a rotational temperature of T rot = 44 ± 13 K and a column density of N tot = (9.1 ± 4.4) × 1014 cm–2 were determined from the analysis. For CH3NH2, T rot = 159 ± 30 K and N tot = (5.0 ± 0.9) × 1015 cm–2, indicating that this species is present in much warmer gas than CH2NH. The fractional abundances for CH2NH and CH3NH2 were established to be f (H2) 3.0 × 10–10 and f (H2) 1.7 × 10–9, respectively. It has been proposed that CH2NH is formed on grains via hydrogenation of HCN; further hydrogenation of CH2NH on surfaces leads to CH3NH2. However, given the dissimilarity between the rotational temperatures and distributions of CH2NH and CH3NH2 in Sgr B2, it is improbable that these species are closely related synthetically, at least in this source. Both CH2NH and CH3NH2 are more likely created by neutral-neutral processes in the gas phase.
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 discovery of amino acids in meteorites and the detection of glycine in samples returned from a comet to Earth suggest that the interstellar chemistry is capable of producing such complex organic molecules. Our goal is to investigate the degree of chemical complexity that can be reached in the ISM. We performed an unbiased, spectral line survey toward Sgr B2(N) and (M) with the IRAM 30m telescope in the 3mm window. The spectra were analyzed with a simple radiative transfer model that assumes LTE but takes optical depth effects into account. About 3675 and 945 spectral lines with a peak signal-to-noise ratio higher than 4 are detected toward N and M, i.e. about 102 and 26 lines per GHz, respectively. This represents an increase by about a factor of 2 over previous surveys of Sgr B2. About 70% and 47% of the lines detected toward N and M are identified and assigned to 56 and 46 distinct molecules as well as to 66 and 54 less abundant isotopologues of these molecules, respectively. We also report the detection of transitions from 59 and 24 catalog entries corresponding to vibrationally or torsionally excited states of some of these molecules, respectively. Excitation temperatures and column densities were derived for each species but should be used with caution. Among the detected molecules, aminoacetonitrile, n-propyl cyanide, and ethyl formate were reported for the first time in space based on this survey, as were 5 rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We also report the detection of transitions from within 12 new vib. or tors. excited states of known molecules. Although the large number of unidentified lines may still allow future identification of new molecules, we expect most of these lines to belong to vib. or tors. excited states or to rare isotopologues of known molecules for which spectroscopic predictions are currently missing. (abridged)
Article
Recent progress is reported in measuring, assigning, and fitting the rotational spectrum of the ground vibrational state of methylamine, CH3NH2, a spectrum complicated both by internal rotation of the methyl top and by inversion of the amino group. New measurements of 513 rotational transitions with J up to 30 and K up to 9 were carried out between 49 and 326 GHz using the millimeter-wave spectrometer in Kharkov. After removing the observed quadrupole hyperfine splittings, these new data along with previously published measurements were fitted to a group-theoretical high-barrier tunneling Hamiltonian from the literature, using 53 parameters to give an overall weighted standard deviation of 0.80 for 850 far-infrared and 673 microwave transitions in the ground state. The root-mean-square deviation of 0.018 MHz obtained for 346 millimeter-wave transitions measured with 0.020 MHz uncertainty represents an approximately 30-fold improvement in fitting accuracy over past attempts.
Article
Over 800 lines in OMC-1 and over 700 in Sgr B2 were observed in the NRAO 3-mm survey. Detailed LTE analysis of 33 species has been performed using numbers of transitions in the range 4-98. Limitations of the LTE optically thin approach are emphasized, in particular its failure to account for the anomalously strong emission from intrisically weak transitions in all the most complex species including CH3OHCO, (CH3)2O, CH3CHO, EtOH, NH2CHO, CH2CHCN, and EtCN. A detailed radiative-transfer analysis of all species with a sufficient number of transitions was also performed. The intensity and abundance ratios of the existing 3-mm surveys of Sgr B2 (NRAO, BTL) and OMC-1 (NRAO, Onsala) are compared to deduce the relative contributions to the emission of each molecular species from compact hot sources and from extended cool regions. It is concluded that the chemistries of the two regions are similar, which suggests a similar cosmic-ray flux, evolutionary stage, and elemental abundance distribution.
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
Mapping observations of the J = 6 → 5 transition of N2O and the Π, J = 3/2 → 1/2 line of NO in the 2 mm band toward the core region of the Sagittarius B2 complex have been carried out using the Kitt Peak 12 m telescope. Emission from NO was found to be extended over a region 2' × 5' in size that includes the Sgr B2 (N), Sgr B2 (M), and Sgr B2 (OH) positions, very similar to the distribution found for HNO. In contrast, N2O emission was confined to a source approximately 1' in extent, slightly elongated in the north-south direction and centered on the Sgr B2 (N) core. A virtually identical distribution was found for the JKτ = 140 → 14-1 E transition of methanol, which lies 255 K above ground state and samples very hot gas. Excitation conditions are favorable for the J = 6 → 5 line of N2O over the entire NO region; hence, the confined nature of this species is a result of chemistry. The J = 3 → 2 and J = 9 → 8 lines of N2O at 75 and 226 GHz, respectively, were also detected at Sgr B2 (N). Combined with the J = 6 → 5 data, these transitions indicate a column density for this molecule of Ntot ~ 1.5 × 1015 cm-2 at this position and an abundance of f(N2O/H2) ~ 1.5 × 10-9. This fractional abundance is almost 2 orders of magnitude higher than predicted by low-temperature chemical models. The N2O observations suggest that this molecule is preferentially formed in high-temperature gas; a likely mechanism is the neutral-neutral reaction NO + NH → N2O + H, which has an appreciable rate only at T > 125 K. The column density of NO found over the Sgr B2 cloud was Ntot ~ (0.8-1.5) × 1016 cm-2, corresponding to a fractional abundance of f(NO/H2) ~ (0.8-1.5) × 10-8, which is about 1 order of magnitude less than model predictions. The similar distributions of NO and HNO suggest a chemical connection. It is likely that the major route to HNO is from NO via the ion-molecule process NO + HNO+ → NO+ + HNO, which occurs readily at low temperatures. The NO molecule thus appears to be the main precursor species in the N/O chemical network.
Article
The present laboratory study simulated cosmic-ray-induced grain chemistry of nitrogen-bearing organic molecules in interstellar and cometary ices. Model ices of ammonia (NH3)-methane (CH4) were prepared and irradiated at 10 K under contamination-free, ultrahigh vacuum conditions with energetic electrons generated in the track of galactic cosmic-ray particles. The radiolysis-induced processing of nitrogen-bearing molecules was then monitored on line and in situ by a Fourier transform infrared spectrometer and a quadrupole mass spectrometer during the irradiation phase and subsequent warm-up phases. The analogous processing was also achieved in ammonia (NH3) and six hydrocarbon (C nH2n+2; n = 1-6) ices. The formation of cyanide anion (CN–) was commonly observed in both ices at 10 K, the temporal column density fit of which traced back the involvement of methylamine (CH3NH2)-based intermediates. Traces of CH3NH2 were evident at about 110 K through thin ammonia matrices in sublimation. From the point of radiative transfer, we further constrain the formation mechanism of aminoacetonitrile (NH2CH2CN) on icy grains of Sgr B2(N) under a cosmic-ray-induced photon field.
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Measurements of the present-day abundances of elements and isotopes, combined with model calculations, allow us to trace the history of nucleosynthesis in the universe. Throughout this review, emphasis will be placed on descriptions of the measurement processes and the interpretations needed to obtain actual isotope and element abundances from measurements. Comparisons of the abundances of isotopomers of a given element are less affected by systematic effects than are comparisons of the abundances of different elements. Thus ratios of isotopomers should be given a greater weight when data and models are compared. As is generally accepted, the universe began with an explosive event, the Big Bang. The nucleosynthesis associated with this event produced `primordial' abundances of the `light elements', deuterium, , , and . Subsequent stellar processing of the light elements has altered the relative abundances, and also produced heavier elements such as carbon, nitrogen and oxygen. Stellar nucleosynthesis products from solar and larger mass stars are expelled into the interstellar medium (ISM). The goal of studies of the abundances of the light elements is to estimate the primordial abundances, that is, the abundances produced in the Big Bang. It is believed that D is always net destroyed in stars; and may be net produced, is certainly net produced. In the Solar System itself, results are obtained from in situ measurements with space probes to Jupiter, measurements of solar wind