Oxygen depletion in dense molecular clouds: a clue to a low O<sub>2</sub>abundance?

Hincelin, Ugo; Wakelam, Valentine; Hersant, F.; Guilloteau, S.; Loison, Jean‐Christophe; Honvault, Pascal; Troe, J.

doi:10.1051/0004-6361/201016328

Cited by 141 publications

(167 citation statements)

References 71 publications

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“…The model runs were initially performed with a depleted oxygen abundance compared to that of carbon as suggested by Jenkins (20) and Whittet (21), yielding an elemental C/O ratio of 1.2 (SI Text). Depleted elemental oxygen was required to reproduce the observed low gas-phase abundance of O 2 in dense clouds in a recent modeling study (19). However, given the probable model dependence on initial elementary abundances that are themselves poorly constrained, secondary model runs were also undertaken in which the elemental C/O ratio was fixed to 0.7 using a larger oxygen abundance of 2.4 × 10 −4 with respect to total hydrogen [n H ¼ nðHÞ þ 2nðH 2 Þ] to test the effect of excess elemental oxygen.…”

Section: Resultsmentioning

confidence: 99%

“…To test their importance to N 2 formation, we used the chemical model Nautilus (19), which solves the kinetic equations for both gas-phase and grain surface chemistry. This model incorporates gas-phase reactions, the sticking of gas-phase species onto interstellar grains, the evaporation of surface species into the gas-phase, and chemical reactions occurring at the surface of the grains.…”

Section: Resultsmentioning

confidence: 99%

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Elemental nitrogen partitioning in dense interstellar clouds

Daranlot

Hincelin

Bergeat

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

100

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Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N 2 , with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N 2 is difficult to detect spectroscopically through infrared or millimeterwavelength transitions. Therefore, its abundance is often inferred indirectly through its reaction product N 2 H þ . Two main formation mechanisms, each involving two radical-radical reactions, are the source of N 2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N þ CN reaction, down to 56 K. The measured rate constants for this reaction, and those recently determined for two other reactions implicated in N 2 formation, are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N 2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N 2 formation are inefficient, we argue that N 2 does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH 3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.astrochemistry | chemical kinetics E lemental nitrogen is present in the atmospheres of telluric planets predominantly in the form of N 2 . As a stable molecule with a high bond energy, it is hard to break N out of N 2 , so the amount of nitrogen available for the formation of complex prebiotic molecules in such environments depends on the presence of nitrogen in more labile forms such as NH 3 . Despite the high abundance of N 2 in the solar system, observations of N 2 beyond our own planetary system are scarce, as N 2 possesses no allowed rotational or vibrational transitions. Only one direct measurement (1) of N 2 in the far ultraviolet wavelength range (thereby accessing its electronic transitions) has been reported in a diffuse interstellar cloud, where low column densities allow light to pass and a star along the line of sight was used to probe the species within. In dense molecular clouds where temperatures as low as 10 K prevail, UV starlight is absorbed long before it can penetrate to the cloud core where N 2 is predicted to form efficiently (2, 3). Instead, gas-phase N 2 densities are inferred through observations of N 2 H þ , a product of the N 2 þ H 3 þ reaction (4-6). The major pathways for N 2 formation in dense clouds rely on reactions involving neutral radical species:Mechanism (II) Recent experimental and theoretical studies of reactions 1 (7) and 2 (8-10) indicate that these reactions are slower than previously thought, suggesting that current astrochemical models overestimate molecular nitrogen abundances produced by Mechanism (I) both at steady state and at specific times. The influence of Mechani...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Elemental nitrogen partitioning in dense interstellar clouds

Daranlot

Hincelin

Bergeat

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

100

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“…All elements are assumed to be initially in atomic form, except for hydrogen which is entirely molecular, with abundances listed in Table 1 of (Hincelin et al 2011), the C/O elemental ratio being equal to 0.7 in this study.…”

Section: The Chemical Modelmentioning

confidence: 99%

The interstellar gas-phase chemistry of HCN and HNC

Loison¹,

Wakelam²,

Hickson³

2014

Monthly Notices of the Royal Astronomical Society

104

109

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+ CH 2 reaction and two new reactions: H + CCN and C + HNC. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio significantly above one as long as the carbon atom abundance remains high. However, the reaction of HCN with H 3 + followed by DR of HCNH + acts to isomerize HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio close to or slightly greater than 1. This agrees well with observations in TMC-1 and L134N taking into consideration the overestimation of HNC abundances through the use of the same rotational excitation rate constants for HNC as for HCN in many radiative transfer models.

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“…The instantaneous jump from a 0D to a 1D structure is a crude approximation which future work should address in more detail. In addition to the lm abundance set, we considered several initial molecular abundance sets with non-solar C/O ratios, which were proposed to explain observed peculiarities of the sulfur and oxygen chemistries in high-mass star-forming regions and protoplanetary disks (Hincelin et al 2011;Dutrey et al 2011), called d11c and h11 (see Table 2). Both these elemental abundance sets have C/O > 1, an abundance of nitrogen 3 times higher than in our standard lm set, and most of the atoms in the ionized state.…”

Section: Initial Abundancesmentioning

confidence: 99%

“…Similarly, the model of initial elemental abundances of Hincelin et al (2011) adopted to calculate pre-AFGL 2591 the initial abundances is called h11 (see Table 3). …”

Section: Initial Abundancesmentioning

confidence: 99%

The HIFI spectral survey of AFGL 2591 (CHESS)

Kaźmierczak-Barthel

Semenov

Tak

et al. 2015

A&A

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Aims. The aim of this work is to understand the richness of chemical species observed in the isolated high-mass envelope of AFGL 2591, a prototypical object for studying massive star formation. Methods. Based on HIFI and JCMT data, the molecular abundances of species found in the protostellar envelope of AFGL 2591 were derived with a Monte Carlo radiative transfer code (Ratran), assuming a mixture of constant and 1D stepwise radial profiles for abundance distributions. The reconstructed 1D abundances were compared with the results of the time-dependent gas-grain chemical modeling, using the best-fit 1D power-law density structure. The chemical simulations were performed considering ages of 1−5 × 10 4 years, cosmic ray ionization rates of 5−500 × 10 −17 s −1 , uniformly-sized 0.1−1 μm dust grains, a dust/gas ratio of 1%, and several sets of initial molecular abundances with C/O < 1 and >1. The most important model parameters varied one by one in the simulations are age, cosmic ray ionization rate, external UV intensity, and grain size. Results. Constant abundance models give good fits to the data for CO, CN, CS, HCO + , H 2 CO, N 2 H + , CCH, NO, OCS, OH, H 2 CS, O, C, C + , and CH. Models with an abundance jump at 100 K give good fits to the data for NH 3 , SO, SO 2 , H 2 S, H 2 O, HCl, and CH 3 OH. For HCN and HNC, the best models have an abundance jump at 230 K. The time-dependent chemical model can accurately explain abundance profiles of 15 out of these 24 species. The jump-like radial profiles for key species like HCO + , NH 3 , and H 2 O are consistent with the outcome of the time-dependent chemical modeling. The best-fit model has a chemical age of ∼10−50 kyr, a solar C/O ratio of 0.44, and a cosmic-ray ionization rate of ∼5 × 10 −17 s −1 . The grain properties and the intensity of the external UV field do not strongly affect the chemical structure of the AFGL 2591 envelope, whereas its chemical age, the cosmic-ray ionization rate, and the initial abundances play an important role. Conclusions. We demonstrate that simple constant or jump-like abundance profiles constrained with 1D Ratran line radiative transfer simulations are in agreement with time-dependent chemical modeling for most key C-, O-, N-, and S-bearing molecules. The main exceptions are species with very few observed transitions (C, O, C + , and CH) or with a poorly established chemical network (HCl, H 2 S) or whose chemistry is strongly affected by surface processes (CH 3 OH).

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Oxygen depletion in dense molecular clouds: a clue to a low O₂abundance?

Cited by 141 publications

References 71 publications

Elemental nitrogen partitioning in dense interstellar clouds

Elemental nitrogen partitioning in dense interstellar clouds

The interstellar gas-phase chemistry of HCN and HNC

The HIFI spectral survey of AFGL 2591 (CHESS)

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Oxygen depletion in dense molecular clouds: a clue to a low O2abundance?

Cited by 141 publications

References 71 publications

Elemental nitrogen partitioning in dense interstellar clouds

Elemental nitrogen partitioning in dense interstellar clouds

The interstellar gas-phase chemistry of HCN and HNC

The HIFI spectral survey of AFGL 2591 (CHESS)

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Oxygen depletion in dense molecular clouds: a clue to a low O₂abundance?