scholarly journals Gas-grain model of carbon fractionation in dense molecular clouds

2020 ◽  
Vol 498 (4) ◽  
pp. 4663-4679
Author(s):  
Jean-Christophe Loison ◽  
Valentine Wakelam ◽  
Pierre Gratier ◽  
Kevin M Hickson
Keyword(s):  
Hydrogen Atom ◽  
Solar System ◽  
Gas Phase ◽  
Molecular Clouds ◽  
Isotopic Fractionation ◽  
Chemical Model ◽  
Nominal Model ◽  
Multiple Observations ◽  
Grain Model ◽  
Global Agreement

ABSTRACT Carbon containing molecules in cold molecular clouds show various levels of isotopic fractionation through multiple observations. To understand such effects, we have developed a new gas-grain chemical model with updated 13C fractionation reactions (also including the corresponding reactions for 15 N, 18O, and 34S). For chemical ages typical of dense clouds, our nominal model leads to two 13C reservoirs: CO and the species that derive from CO, mainly s-CO and s-CH3OH, as well as C3 in the gas phase. The nominal model leads to strong enrichment in C3, c-C3H2, and C2H in contradiction with observations. When C3 reacts with oxygen atoms, the global agreement between the various observations and the simulations is rather good showing variable 13C fractionation levels that are specific to each species. Alternatively, hydrogen atom reactions lead to notable relative 13C fractionation effects for the two non-equivalent isotopologues of C2H, c-C3H2, and C2S. As there are several important fractionation reactions, some carbon bearing species are enriched in 13C, particularly CO, depleting atomic 13C in the gas phase. This induces a 13C depletion in CH4 formed on grain surfaces, an effect that is not observed in the CH4 in the Solar system, in particular on Titan. This seems to indicate a transformation of matter between the collapse of the molecular clouds, leading to the formation of the protostellar disc, and the formation of the planets. Or it means that the atomic carbon sticking to the grains reacts with the species already on the grains giving very little CH4.

scholarly journals The D/H Ratio in Molecular Clouds

1992 ◽  
Vol 150 ◽  
pp. 91-95
Author(s):  
A.G.G.M. Tielens
Keyword(s):  
Solar System ◽  
Gas Phase ◽  
Molecular Clouds ◽  
Large Fraction ◽  
Total Density ◽  
Dense Cores

This paper summarizes our understanding of the dominant D-reservoirs in molecular clouds and suggests possible direct determinations of the D-abundance. It is concluded that rotational HD lines from shock regions provide the best way to determine the gas phase D-abundance. In cold dense cores, the dominant gas phase D-reservoir is likely to be atomic D, because of expected inefficient HD formation on grain surfaces. The gas phase D-abundance derived from observations of dense cores is ≍5 × 10-6/n4 (with n4 the total density in units of 104 cm−3). A large fraction of the D (≍50%) may be locked up in deuterated molecules in grain mantles. A small fraction (≍2%) may be locked up in the photolyzed residues of such grain mantles. PAHs will also be deuterated (PADs), containing ≍1% of the D. Finally, it is likely that all of these processes have contributed to the D-enrichment observed in solar system materials.

2-Deoxyribose Radicals in the Gas Phase and Aqueous Solution. Transient Intermediates of Hydrogen Atom Abstraction from 2-Deoxyribofuranose

2005 ◽  
Vol 70 (11) ◽  
pp. 1769-1786 ◽  
Author(s):  
Luc A. Vannier ◽  
Chunxiang Yao ◽  
František Tureček
Keyword(s):  
Aqueous Solution ◽  
Hydrogen Atom ◽  
Gas Phase ◽  
Computational Study ◽  
Hydrogen Atom Abstraction ◽  
Oh Radical ◽  
Free Energies ◽  
Intramolecular Hydrogen ◽  
Solvation Free Energies ◽  
Transient Intermediates

A computational study at correlated levels of theory is reported to address the structures and energetics of transient radicals produced by hydrogen atom abstraction from C-1, C-2, C-3, C-4, C-5, O-1, O-3, and O-5 positions in 2-deoxyribofuranose in the gas phase and in aqueous solution. In general, the carbon-centered radicals are found to be thermodynamically and kinetically more stable than the oxygen-centered ones. The most stable gas-phase radical, 2-deoxyribofuranos-5-yl (5), is produced by H-atom abstraction from C-5 and stabilized by an intramolecular hydrogen bond between the O-5 hydroxy group and O-1. The order of radical stabilities is altered in aqueous solution due to different solvation free energies. These prefer conformers that lack intramolecular hydrogen bonds and expose O-H bonds to the solvent. Carbon-centered deoxyribose radicals can undergo competitive dissociations by loss of H atoms, OH radical, or by ring cleavages that all require threshold dissociation or transition state energies >100 kJ mol-1. This points to largely non-specific dissociations of 2-deoxyribose radicals when produced by exothermic hydrogen atom abstraction from the saccharide molecule. Oxygen-centered 2-deoxyribose radicals show only marginal thermodynamic and kinetic stability and are expected to readily fragment upon formation.

scholarly journals Nautilus multi-grain model: Importance of cosmic-ray-induced desorption in determining the chemical abundances in the ISM

2018 ◽  
Vol 615 ◽  
pp. A20 ◽  
Author(s):  
Wasim Iqbal ◽  
Valentine Wakelam
Keyword(s):  
Grain Size ◽  
Surface Temperature ◽  
Gas Phase ◽  
Numerical Models ◽  
Cosmic Ray ◽  
Grain Sizes ◽  
Small Grains ◽  
Grain Model ◽  
Grain Surface ◽  
The Impact

Context. Species abundances in the interstellar medium (ISM) strongly depend on the chemistry occurring at the surfaces of the dust grains. To describe the complexity of the chemistry, various numerical models have been constructed. In most of these models, the grains are described by a single size of 0.1 μm. Aims. We study the impact on the abundances of many species observed in the cold cores by considering several grain sizes in the Nautilus multi-grain model. Methods. We used grain sizes with radii in the range of 0.005 μm to 0.25 μm. We sampled this range in many bins. We used the previously published, MRN and WD grain size distributions to calculate the number density of grains in each bin. Other parameters such as the grain surface temperature or the cosmic-ray-induced desorption rates also vary with grain sizes. Results. We present the abundances of various molecules in the gas phase and also on the dust surface at different time intervals during the simulation. We present a comparative study of results obtained using the single grain and the multi-grain models. We also compare our results with the observed abundances in TMC-1 and L134N clouds. Conclusions. We show that the grain size, the grain size dependent surface temperature and the peak surface temperature induced by cosmic ray collisions, play key roles in determining the ice and the gas phase abundances of various molecules. We also show that the differences between the MRN and the WD models are crucial for better fitting the observed abundances in different regions in the ISM. We show that the small grains play a very important role in the enrichment of the gas phase with the species which are mainly formed on the grain surface, as non-thermal desorption induced by collisions of cosmic ray particles is very efficient on the small grains.

Size-Dependent Hydrogen Atom Attachment to Gas-Phase Hydrogen-Deficient Polypeptide Radical Cations

2018 ◽  
Vol 140 (2) ◽  
pp. 531-533 ◽  
Author(s):  
Luciano H. Di Stefano ◽  
Dimitris Papanastasiou ◽  
Roman A. Zubarev
Keyword(s):  
Hydrogen Atom ◽  
Gas Phase ◽  
Radical Cations ◽  
Size Dependent

The chemistry in clumpy AGB outflows

2018 ◽  
Vol 14 (S343) ◽  
pp. 531-532
Author(s):  
M. Van de Sande ◽  
J. O. Sundqvist ◽  
T. J. Millar ◽  
D. Keller ◽  
L. Decin
Keyword(s):  
Density Distribution ◽  
Gas Phase ◽  
Thermodynamic Equilibrium ◽  
Angular Resolution ◽  
Abundance Ratio ◽  
Chemical Model ◽  
High Angular Resolution ◽  
Gas Phase Chemistry ◽  
Phase Chemistry ◽  
Smooth Density

AbstractThe chemistry within the outflow of an AGB star is determined by its elemental C/O abundance ratio. Thanks to the advent of high angular resolution observations, it is clear that most outflows do not have a smooth density distribution, but are inhomogeneous or “clumpy”. We have developed a chemical model that takes into account the effect of a clumpy outflow on its gas-phase chemistry by using a theoretical porosity formalism. The clumpiness of the model increases the inner wind abundances of all so-called unexpected species, i.e. species that are not predicted to be present assuming an initial thermodynamic equilibrium chemistry. By applying the model to the distribution of cyanopolyynes and hydrocarbon radicals within the outflow of IRC+10216, we find that the chemistry traces the underlying density distribution.

scholarly journals Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM III. Elemental depletion and shortcomings of the current physico-chemical models

2020 ◽  
Vol 497 (2) ◽  
pp. 2309-2319
Author(s):  
V Wakelam ◽  
W Iqbal ◽  
J-P Melisse ◽  
P Gratier ◽  
M Ruaud ◽  
...  
Keyword(s):  
Gas Phase ◽  
Molecular Form ◽  
Dense Medium ◽  
Chemical Model ◽  
Physical Conditions ◽  
Dense Cores ◽  
Chemical Models ◽  
Physico Chemical ◽  
Galactic Model ◽  
Elemental Depletion

ABSTRACT We present a study of the elemental depletion in the interstellar medium. We combined the results of a Galactic model describing the gas physical conditions during the formation of dense cores with a full-gas-grain chemical model. During the transition between diffuse and dense medium, the reservoirs of elements, initially atomic in the gas, are gradually depleted on dust grains (with a phase of neutralization for those which are ions). This process becomes efficient when the density is larger than 100 cm−3. If the dense material goes back into diffuse conditions, these elements are brought back in the gas phase because of photo-dissociations of the molecules on the ices, followed by thermal desorption from the grains. Nothing remains on the grains for densities below 10 cm−3 or in the gas phase in a molecular form. One exception is chlorine, which is efficiently converted at low density. Our current gas–grain chemical model is not able to reproduce the depletion of atoms observed in the diffuse medium except for Cl, which gas abundance follows the observed one in medium with densities smaller than 10 cm−3. This is an indication that crucial processes (involving maybe chemisorption and/or ice irradiation profoundly modifying the nature of the ices) are missing.

scholarly journals Enhanced UV radiation and dense clumps in the molecular outflow of Mrk 231

2020 ◽  
Vol 633 ◽  
pp. A163 ◽  
Author(s):  
Claudia Cicone ◽  
Roberto Maiolino ◽  
Susanne Aalto ◽  
Sebastien Muller ◽  
Chiara Feruglio
Keyword(s):  
Gas Phase ◽  
Uv Radiation ◽  
Molecular Clouds ◽  
Line Emission ◽  
Molecular Gas ◽  
Spectral Features ◽  
Giant Molecular Clouds ◽  
Dense Phase ◽  
Molecular Outflow ◽  
First Time

We present interferometric observations of the CN(1–0) line emission in Mrk 231 and combine them with previous observations of CO and other H2 gas tracers to study the physical properties of the massive molecular outflow. We find a strong boost of the CN/CO(1–0) line luminosity ratio in the outflow of Mrk 231, which is unprecedented compared to any other known Galactic or extragalactic astronomical source. For the dense gas phase in the outflow traced by the HCN and CN emissions, we infer XCN ≡ [CN]/[H2]> XHCN by at least a factor of three, with H2 gas densities of nH2 ∼ 105−6 cm−3. In addition, we resolve for the first time narrow spectral features in the HCN(1–0) and HCO+(1–0) high-velocity line wings tracing the dense phase of the outflow. The velocity dispersions of these spectral features, σv ∼ 7−20 km s−1, are consistent with those of massive extragalactic giant molecular clouds detected in nearby starburst nuclei. The H2 gas masses inferred from the HCN data are quite high, Mmol ∼ 0.3−5 × 108 M⊙. Our results suggest that massive complexes of denser molecular gas survive embedded into the more diffuse H2 phase of the outflow, and that the chemistry of these outflowing dense clouds is strongly affected by UV radiation.

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