Modeling sulfur depletion in interstellar clouds

Context. The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value toward lines of sight containing higher-density environments. Aims. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas-grain astrochemical code and an enhanced sulfur reaction network. Methods. A common gas-grain astrochemical reaction network has been systematically updated and greatly extended based on previous literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Results. Our new gas-grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 106 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochemical models. Conclusions. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the “missing” sulfur is in the form of organo-sulfur species that are trapped on grains.

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Astrochemistry of Interstellar Clouds: II. Molecular Formation in a Contracting Cloud

Symposium - International Astronomical Union ◽

10.1017/s0074180900154142 ◽

1987 ◽

Vol 120 ◽

pp. 273-274

Author(s):

M.A. El Shalaby ◽

A. Aiad

Keyword(s):

Gas Phase ◽

Optical Depth ◽

Chemical Reactions ◽

Particle Density ◽

Interstellar Cloud ◽

Gas Phase Reactions ◽

Interstellar Clouds ◽

Continous Function ◽

Molecular Formation ◽

Self Gravity

The chemistry of an 667 Mo interstellar cloud was studied using 142 reactions for 40 species during the contraction under self gravity in two steps. At first the contraction is allowed without gas phase reactions untill certain optical depth is reached. Secondly, at this optical depth the chemical reactions are started for sufficient cycles in a time dependant scheme till only very small additionally changes in the abundances occur. The so obtained, relative abundances and coulmn densities for different species represent a continous function of the optical depths. The values arround τ=6.3 represent the observations for H2, H2+, H3+, OH, OH+, CH, CH+, CH2, CH2+, CH3+, H2O and H3O+. The region of τ between 1 and 5 i.e. of particle density between 4 102–6 103 is the preferable formation place for the majority of molecules.

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Molecular evolution in star-forming cores: From prestellar cores to protostellar cores

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308021364 ◽

2008 ◽

Vol 4 (S251) ◽

pp. 129-136 ◽

Cited By ~ 1

Author(s):

Yuri Aikawa ◽

Valentine Wakelam ◽

Nami Sakai ◽

R. T. Garrod ◽

E. Herbst ◽

...

Keyword(s):

Gas Phase ◽

Surface Reactions ◽

Reaction Network ◽

Central Star ◽

Carbon Chain ◽

Complex Species ◽

Star Forming ◽

Organic Species ◽

Molecular Abundances ◽

Grain Surface

AbstractWe investigate the molecular abundances in protostellar cores by solving the gas-grain chemical reaction network. As a physical model of the core, we adopt a result of one-dimensional radiation-hydrodynamics calculation, which follows the contraction of an initially hydrostatic prestellar core to form a protostellar core. Temporal variation of molecular abundances is solved in multiple infalling shells, which enable us to investigate the spatial distribution of molecules in the evolving core. The shells pass through the warm region of T ~ 20–100 K in several 104 yr and falls onto the central star in ~100 yr after they enter the region of T > 100 K. We found that the complex organic species such as HCOOCH3 are formed mainly via grain-surface reactions at T ~ 20–40 K, and then sublimated to the gas phase when the shell temperature reaches their sublimation temperatures (T ≥ 100 K). Carbon-chain species can be re-generated from sublimated CH4 via gas-phase and grain-surface reactions. HCO2+, which is recently detected towards L1527, are abundant at r = 100–2,000 AU, and its column density reaches ~1011 cm−2 in our model. If a core is isolated and irradiated directly by interstellar UV radiation, photo-dissociation of water ice produces OH, which reacts with CO to form CO2 efficiently. Complex species then become less abundant compared with the case of embedded core in ambient clouds. Although a circumstellar (protoplanetary) disk is not included in our core model, we can expect similar chemical reactions (i.e., production of large organic species, carbon-chains and HCO2+) to proceed in disk regions with T ~ 20–100 K.

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The Effect of an Inert Solid Reservoir on Molecular Abundances in Dense Interstellar Clouds

Open Astronomy ◽

10.1515/astro-2017-0402 ◽

2012 ◽

Vol 21 (4) ◽

Author(s):

Juris Kalvāns ◽

Ivar Shmeld

Keyword(s):

Gas Phase ◽

Reference Model ◽

Solid Phase ◽

Chemical Species ◽

Interstellar Gas ◽

Interstellar Clouds ◽

Major Chemical ◽

Molecular Abundances ◽

Phase Chemistry ◽

Grain Surface

AbstractThe question, what is the role of freeze-out of chemical species in determining the molecular abundances in the interstellar gas is a matter of debate. We investigate a theoretical case of a dense interstellar molecular cloud core by time-dependent modeling of chemical kinetics, where grain surface reactions deliberately are not included. That means, the gas-phase and solid-phase abundances are influenced only by gas reactions, accretion on grains and desorption. We compare the results to a reference model where no accretion occurs, and only gas-phase reactions are included. We can trace that the purely physical processes of molecule accretion and desorption have major chemical consequences on the gas-phase chemistry. The main effect of introduction of the gas-grain interaction is long-term molecule abundance changes that come nowhere near an equilibrium during the typical lifetime of a prestellar core.

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Molecule Formation in External Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900089828 ◽

1992 ◽

Vol 150 ◽

pp. 121-126

Author(s):

T J Millar ◽

E. Herbst

Keyword(s):

Starburst Galaxies ◽

Molecular Gas ◽

Dark Cloud ◽

Elemental Abundances ◽

Molecule Formation ◽

Cloud Models ◽

External Galaxies ◽

Molecular Abundances

We discuss the parameters needed to model chemistry in extragalactic clouds. While density and temperature can be constrained by multiline observations, molecular abundances may be severely affected by the adopted elemental abundances. While the observations of the Magelllanic Clouds can be reasonably interpreted in terms of dark cloud models, molecular gas in starburst galaxies could well be dominated by photoeffects. The detection of deuterium in extragalactic molecules would provide a valuable diagnostic.

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Chemical Signatures of the Evolutionary State of Cores

Symposium - International Astronomical Union ◽

10.1017/s0074180900241466 ◽

2004 ◽

Vol 221 ◽

pp. 67-74 ◽

Cited By ~ 1

Author(s):

Yuri Aikawa

Keyword(s):

Gas Phase ◽

Angular Resolution ◽

Reaction Network ◽

High Angular Resolution ◽

Gas Phase Reactions ◽

Parent Molecule ◽

The Core ◽

Chemical Signatures ◽

Prestellar Cores ◽

Molecular Abundances

Recent observations with high angular resolution revealed chemical differentiation in several prestellar cores; while N2H+ emission peaks at the core center, CO, CS and CCS emission peaks are offset from the center. Molecular abundances also vary among cores; some cores have high CCS abundance and low N2H+ abundance, but others have less CCS and more N2H+. Numerical calculations of a chemical-reaction network in contracting cores show that these differentiations and variations can be diagnostics of physical evolution of cores, because molecular abundances and distributions are determined by the balance between the dynamical and chemical time scales. In prestellar cores, low temperatures and high densities cause adsorption of molecules onto grains. Depletion time scale varies among species; early-phase species deplete first because of destruction via gas-phase reactions in addition to the adsorption. N2H+ is the last to deplete because of the low adsorption energy of its parent molecule N2 and depletion of main reactants such as CO. Molecular D/H ratio is another probe of core evolution, since it increases as the adsorption proceeds.

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The formation of glycine and other complex organic molecules in exploding ice mantles

Faraday Discussions ◽

10.1039/c3fd00155e ◽

2014 ◽

Vol 168 ◽

pp. 369-388 ◽

Cited By ~ 6

Author(s):

J. M. C. Rawlings ◽

D. A. Williams ◽

S. Viti ◽

C. Cecchi-Pestellini ◽

W. W. Duley

Keyword(s):

Gas Phase ◽

Molecular Clouds ◽

Organic Molecules ◽

Alternative Mechanism ◽

Hydrogen Atoms ◽

Interstellar Gas ◽

Dark Cloud ◽

Short Period ◽

Molecular Abundances ◽

Complex Organic

Complex Organic Molecules (COMs), such as propylene (CH3CHCH2) and the isomers of C2H4O2 are detected in cold molecular clouds (such as TMC-1) with high fractional abundances (Marcelino et al., Astrophys. J., 2007, 665, L127). The formation mechanism for these species is the subject of intense speculation, as is the possibility of the formation of simple amino acids such as glycine (NH2CH2COOH). At typical dark cloud densities, normal interstellar gas-phase chemistries are inefficient, whilst surface chemistry is at best ill defined and does not easily reproduce the abundance ratios observed in the gas phase. Whatever mechanism(s) is/are operating, it/they must be both efficient at converting a significant fraction of the available carbon budget into COMs, and capable of efficiently returning the COMs to the gas phase. In our previous studies we proposed a complementary, alternative mechanism, in which medium- and large-sized molecules are formed by three-body gas kinetic reactions in the warm high density gas phase. This environment exists, for a very short period of time, after the total sublimation of grain ice mantles in transient co-desorption events. In order to drive the process, rapid and efficient mantle sublimation is required and we have proposed that ice mantle ‘explosions’ can be driven by the catastrophic recombination of trapped hydrogen atoms, and other radicals, in the ice. Repeated cycles of freeze-out and explosion can thus lead to a cumulative molecular enrichment of the interstellar medium. Using existing studies we based our chemical network on simple radical addition, subject to enthalpy and valency restrictions. In this work we have extended the chemistry to include the formation pathways of glycine and other large molecular species that are detected in molecular clouds. We find that the mechanism is capable of explaining the observed molecular abundances and complexity in these sources. We find that the proposed mechanism is easily capable of explaining the large abundances of all three isomers of C2H4O2 that are observationally inferred for star-forming regions. However, the model currently does not provide an obvious explanation for the predominance of methyl formate, suggesting that some refinement to our (very simplistic) chemistry is necessary. The model also predicts the production of glycine at a (lower) abundance level, that is consistent with its marginal detection in astrophysical sources.

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Theoretical Studies of Dense Cloud Chemistry

Symposium - International Astronomical Union ◽

10.1017/s0074180900154087 ◽

1987 ◽

Vol 120 ◽

pp. 235-244 ◽

Cited By ~ 2

Author(s):

Eric Herbst

Keyword(s):

Gas Phase ◽

Low Temperatures ◽

Rate Coefficients ◽

Theoretical Studies ◽

Cloud Chemistry ◽

Physical Processes ◽

Gas Phase Reactions ◽

Interstellar Clouds ◽

Molecular Abundances ◽

Quantitative Calculations

Based on analyses by a variety of investigators, it has become understood that gas phase reactions can account for much of the chemistry observed in dense interstellar clouds. However, quantitative calculations of molecular abundances utilizing gas phase reactions are beset with difficulties. These difficulties include uncertainties in needed rate coefficients at the low temperatures of interstellar clouds, uncertainties in the dynamics of physical processes such as cloud collapse and clumping, and uncertainties in our understanding of gasgrain interactions. New work in some of these areas and its impact on modelling is emphasized.

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High-Resolution Optical Observations of Interstellar Absorption Lines

Highlights of Astronomy ◽

10.1017/s1539299600017196 ◽

2005 ◽

Vol 13 ◽

pp. 808-810

Author(s):

Daniel E. Welty

Keyword(s):

High Resolution ◽

Isotopic Ratio ◽

Optical Observations ◽

Small Scale ◽

Molecular Gas ◽

Interstellar Clouds ◽

Physical Conditions ◽

Molecular Abundances

AbstractWe briefly note several current topics concerning the properties of interstellar clouds for which high-resolution optical spectra play a significant role: (1) the recognition and characterization of small-scale (sub-pc) structure in both atomic and molecular gas; (2) the discovery of variations in the 7Li/6Li isotopic ratio in the nearby Galactic ISM; (3) the determination of atomic and molecular abundances and physical conditions for heavily reddened (“translucent”) Galactic sightlines; and (4) studies of interstellar clouds in the LMC and SMC.

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