A New and Simple Approach to Determine the Abundance of Hydrogen Molecules on Interstellar Ice Mantles

Abstract

Water is usually the main component of ice mantles, which cover the cores of dust grains in cold portions of dense interstellar clouds. When molecular hydrogen is adsorbed onto an icy mantle through physisorption, a common assumption in gas-grain rate equation models is to use an adsorption energy for molecular hydrogen on a pure water substrate. However, at high density and low temperature, when H2 is efficiently adsorbed onto the mantle, its surface abundance can be strongly overestimated if this assumption is still used. Unfortunately, the more detailed microscopic Monte Carlo treatment cannot be used to study the abundance of H2 in ice mantles if a full gas-grain network is utilized. We present a numerical method adapted for rate-equation models that takes into account the possibility that an H2 molecule can, while diffusing on the surface, find itself bound to another hydrogen molecule, with a far weaker bond than the H2-water bond, which can lead to more efficient desorption. We label the ensuing desorption "encounter desorption". The method is implemented first in a simple system consisting only of hydrogen molecules at steady state between gas and dust using the rate-equation approach and comparing the results with the results of a microscopic Monte Carlo calculation. We then discuss the use of the rate-equation approach with encounter desorption embedded in a complete gas-grain chemical network. For both systems, the rate-equation model with encounter desorption reproduces the H2 granular coverage computed by the microscopic Monte Carlo model. The method is especially useful for dense and cold environments, and for time-dependent physical conditions, such as occur in the collapse of dense cores and the formation of protoplanetary disks. It is not significantly CPU time consuming, so can be used for example with complex 3D chemical-hydrodynamical simulations.

arXiv comment8 pages, 7 figures, language editing, accepted for publication in Astronomy & Astrophysics

References in corpus (33)

• Complex Chemistry in Star-Forming Regions: An Expanded Gas-Grain Warm-up Chemical Model
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• A KInetic Database for Astrochemistry (KIDA)
• Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments
• Evaporation of ices near massive stars: models based on laboratory TPD data
• Chemistry of Dark Clouds: Databases, Networks, and Models
• Photodesorption of Ices II: H2O and D2O
• Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
• Photodesorption of ices I: CO, N2 and CO2
• On the Formation of CO2 and Other Interstellar Ices
• Simulation of the Formation and Morphology of Ice Mantles on Interstellar Grains
• Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions
• Multilayer modeling of porous grain surface chemistry I. The GRAINOBLE model
• Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores
• Oxygen depletion in dense molecular clouds: a clue to a low O2 abundance?
• Water Abundance in Molecular Cloud Cores
• Discovery of Fulminic Acid, HCNO, in Dark Clouds
• First Astronomical Detection of the Cumulene Carbon Chain Molecule H_2C_6 in TMC1
• Search for anions in molecular sources: C4H- detection in L1527
• Modeling the Lukewarm Corino Phase - is L1527 unique?
• Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry
• Cold CO in circumstellar disks: On the effects of photodesorption and vertical mixing
• Water formation on bare grains: When the chemistry on dust impacts interstellar gas
• A Unified Microscopic-Macroscopic Monte Carlo Simulation of Gas-Grain Chemistry in Cold Dense Interstellar Clouds
• Survival of interstellar molecules to prestellar dense core collapse and early phases of disk formation
• Interstellar Simulations Using A Unified Microscopic-Macroscopic Monte Carlo Model with a full Gas-Grain Network including Bulk Diffusion in Ice Mantles
• Moment equations for chemical reactions on interstellar dust grains
• The depletion of NO in pre-protostellar cores
• Chemistry in the First Hydrostatic Core Stage By Adopting Three-Dimensional Radiation Hydrodynamic Simulations
• Beyond the pseudo-time-dependent approach: chemical models of dense core precursors
• Diffusion-limited reactions on a two-dimensional lattice with binary disorder
• Effects of H2 coating of grains on depletion of molecular species

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• The ALMA-PILS survey: First detection of the unsaturated 3-carbon molecules Propenal (C${}_2$H${}_3$CHO) and Propylene (C${}_3$H${}_6$) towards IRAS 16293$-$2422 B
• Oxygen fractionation in dense molecular clouds
• Modeling C-Shock Chemistry in Isolated Molecular Outflows
• Chemical nitrogen fractionation in dense molecular clouds
• A new model of the chemistry of ionizing radiation in solids: CIRIS
• Chemical complexity of phosphorous bearing species in various regions of the Interstellar medium
• The efficiency of photodissociation for molecules in interstellar ices
• Is there any linkage between interstellar aldehyde and alcohol?
• Importance of the H2 abundance in protoplanetary disk ices for the molecular layer chemical composition
• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• The connection between warm carbon chain chemistry and interstellar irradiation of star-forming cores
• Chemical evolution of the gas in C-type shocks in dark clouds
• Effect of binding energies on the encounter desorption
• Astrochemical models of interstellar ices: History matters
• The chemical structure of the Class 0 protostellar envelope NGC 1333 IRAS 4A
• The effect of multilayer ice chemistry on gas-phase deuteration in starless cores
• Effects of turbulent dust grain motion to interstellar chemistry
• On the Encounter Desorption of Hydrogen Atoms on Ice Mantle
• A multigrain-multilayer astrochemical model with variable desorption energy for surface species
• Effect of surface H${}_2$ on molecular hydrogen formation on interstellar grains
• A sensitivity analysis of interstellar ice chemistry in astrochemical models