Monte Carlo Simulations of the Formation and Morphology of Interstellar Ices

Cazaux, S.; Bossa, Jean-Baptiste; Martín-Doménech, R.; Muñoz, G. M.; Chen, Yu-Jung; Linnartz, H.; Tielens, A. G. G. M.

doi:10.1007/978-3-319-90020-9_7

Cited by 3 publications

(3 citation statements)

References 55 publications

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“…The different mechanisms used in this model are accretion, diffusion, sublimation, chemical reaction, and photodissociation. These different mechanisms (accretion, diffusion, and sublimation) occur at rates that have been described in a previous work (Cazaux et al 2018). The accretion rate given in number of molecules per second is…”

Section: Monte Carlo Simulationsmentioning

confidence: 85%

Photoprocessing of H₂S on dust grains

Cazaux

Carrascosa

Muñoz

et al. 2022

A&A

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Context. Sulfur is a biogenic element used as a tracer of the evolution of interstellar clouds to stellar systems. However, most of the expected sulfur in molecular clouds remains undetected. Sulfur disappears from the gas phase in two steps. The first depletion occurs during the translucent phase, reducing the gas-phase sulfur by 7–40 times, while the following freeze-out step occurs in molecular clouds, reducing it by another order of magnitude. This long-standing question awaits an explanation. Aims. The aim of this study is to understand under what form the missing sulfur is hiding in molecular clouds. The possibility that sulfur is depleted onto dust grains is considered. Methods. Experimental simulations mimicking H2S ice UV photoprocessing in molecular clouds were conducted at 8 K under ultra-high vacuum. The ice was subsequently warmed up to room temperature. The ice was monitored using infrared spectroscopy, and the desorbing molecules were measured by quadrupole mass spectrometry in the gas phase. Theoretical Monte Carlo simulations were performed for interpretation of the experimental results and extrapolation to the astrophysical and planetary conditions. Results. H2S2 formation was observed during irradiation at 8 K. Molecules H2Sx with x > 2 were also identified and found to desorb during warm-up, along with S2 to S4 species. Larger Sx molecules up to S8 are refractory at room temperature and remained on the substrate forming a residue. Monte Carlo simulations were able to reproduce the molecules desorbing during warming up, and found that residues are chains of sulfur consisting of 6–7 atoms. Conclusions. Based on the interpretation of the experimental results using our theoretical model, it is proposed that S+ in translucent clouds contributes notoriously to S depletion in denser regions by forming long S chains on dust grains in a few times 104 yr. We suggest that the S2 to S4 molecules observed in comets are not produced by fragmentation of these large chains. Instead, they probably come either from UV photoprocessing of H2S-bearing ice produced in molecular clouds or from short S chains formed during the translucent cloud phase.

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Section: Monte Carlo Simulationsmentioning

confidence: 85%

Photoprocessing of H₂S on dust grains

Cazaux

Carrascosa

Muñoz

et al. 2022

A&A

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“…While such binding energies are a good approximation for an environment subject to thermal and photo processes (hot cores, photon-dominated regions (PDRs)), they do wash away the structure of the ices and may not be correct for shielded and cold environments such as molecular clouds or starless cores . In such environments, species sticking to the icy surface may have binding energies much lower than the one derived by TPD.…”

Section: Limitations and Frontiersmentioning

confidence: 97%

Thermal Desorption of Interstellar Ices: A Review on the Controlling Parameters and Their Implications from Snowlines to Chemical Complexity

Minissale

Aikawa

Bergin

et al. 2022

ACS Earth Space Chem.

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The evolution of star-forming regions and their thermal balance are strongly influenced by their chemical composition, which, in turn, is determined by the physicochemical processes that govern the transition between the gas phase and the solid state, specifically icy dust grains (e.g., particle adsorption and desorption). Gas–grain and grain–gas transitions as well as formation and sublimation of interstellar ices are thus essential elements of understanding astrophysical observations of cold environments (e.g., prestellar cores) where unexpected amounts of a large variety of chemical species have been observed in the gas phase. Adsorbed atoms and molecules also undergo chemical reactions that are not efficient in the gas phase. Therefore, the parametrization of the physical properties of atoms and molecules interacting with dust grain particles is clearly a key aspect to interpret astronomical observations and to build realistic and predictive astrochemical models. In this consensus evaluation, we focus on parameters controlling the thermal desorption of ices and how these determine pathways toward molecular complexity and define the location of snowlines, which ultimately influence the planet formation process. We review different crucial aspects of desorption parameters both from a theoretical and experimental points of view. We critically assess the desorption parameters (the binding energies, E b, and the pre-exponential factor, ν) commonly used in the astrochemical community for astrophysically relevant species and provide tables with recommended values. The aim of these tables is to provide a coherent set of critically assessed desorption parameters for common use in future work. In addition, we show that a nontrivial determination of the pre-exponential factor ν using transition state theory can affect the binding energy value. The primary focus is on pure ices, but we also discuss the desorption behavior of mixed, that is, astronomically more realistic, ices. This allows discussion of segregation effects. Finally, we conclude this work by discussing the limitations of theoretical and experimental approaches currently used to determine the desorption properties with suggestions for future improvements.

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“…The different mechanisms used in this model are accretion, diffusion, sublimation, chemical reactions and photo-dissociation. These different mechanisms (accretion, diffusion and sublimation) occur at rates which have been described in a previous work (Cazaux et al 2018). The accretion rate in number of molecules per second is…”

Section: Monte Carlo Simulationsmentioning

confidence: 85%

Photoprocessing of H2S on dust grains: building S chains in translucent clouds and comets

Cazaux,

Carrascosa,

Caro

et al. 2021

Preprint

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Context. Sulfur is a biogenic element used as a tracer of the evolution from interstellar clouds to stellar systems. However, most of the expected sulfur in molecular clouds remains undetected. Sulfur disappears from the gas phase in two steps. One first depletion occurs during the translucent phase, reducing the gas phase sulfur by 7-40 times, while the following freeze-out step occurs in molecular clouds, reducing it by another order of magnitude. This long-standing dilemma awaits an explanation. Aims. The aim of this study is to understand under which form the missing sulfur is hiding in molecular clouds. The possibility that sulfur is depleted onto dust grains is considered. Methods. Experimental simulations mimicking H 2 S ice UV-photoprocessing in molecular clouds were conducted at 8 K under ultrahigh vacuum. The ice was subsequently warmed up to room temperature. The ice was monitored using infrared spectroscopy and the desorbing molecules were measured by quadrupole mass spectrometry in the gas phase. Theoretical Monte Carlo simulations were performed for interpretation of the experimental results and extrapolation to the astrophysical and planetary conditions. Results. H 2 S 2 formation was observed during irradiation at 8 K. Molecules H 2 S x with x > 2 were also identified and found to desorb during warm-up, along with S 2 to S 4 species. Larger S x molecules up to S 8 are refractory at room temperature and remained on the substrate forming a residue. Monte Carlo simulations were able to reproduce the molecules desorbing during warming up, and found that residues are chains or sulfur consisting of 6-7 atoms. Conclusions. Based on the interpretation of the experimental results using our theoretical model, it is proposed that S + in translucent clouds contributes notoriously to S depletion in denser regions by forming long S-chains on dust grains in few times 10 4 years. We suggest that the S 2 to S 4 molecules observed in comets are not produced by fragmentation of these large chains. Instead, they probably come either from UV-photoprocessing of H 2 S-bearing ice produced in molecular clouds or from short S chains formed during the translucent cloud phase.

show abstract

Monte Carlo Simulations of the Formation and Morphology of Interstellar Ices

Cited by 3 publications

References 55 publications

Photoprocessing of H₂S on dust grains

Photoprocessing of H₂S on dust grains

Thermal Desorption of Interstellar Ices: A Review on the Controlling Parameters and Their Implications from Snowlines to Chemical Complexity

Photoprocessing of H2S on dust grains: building S chains in translucent clouds and comets

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