Deuterium enrichment of the interstellar grain mantle

TL;DR

This study uses Monte Carlo simulations to explore how deuterium enriches interstellar grain mantles across different cloud environments, revealing complex chemical processes and fractionation patterns influenced by physical conditions.

Contribution

It introduces a detailed simulation framework for deuterium enrichment in interstellar ices considering various physical conditions and charged particle interactions.

Findings

Deuterium enrichment varies significantly with cloud density and radiation fields.

High deuteration of methanol observed, low for water, contradicting some models.

Surface coverage shifts from photo-dissociation products to complex molecules with increasing density.

Abstract

We carry out Monte-Carlo simulation to study deuterium enrichment of interstellar grain mantles under various physical conditions. Based on the physical properties, various types of clouds are considered. We find that in diffuse cloud regions, very strong radiation fields persists and hardly a few layers of surface species are formed. In translucent cloud regions with a moderate radiation field, significant number of layers would be produced and surface coverage is mainly dominated by photo-dissociation products such as, C,CH_3,CH_2D,OH and OD. In the intermediate dense cloud regions (having number density of total hydrogen nuclei in all forms ~ 2 x 10^4 cm^-3), water and methanol along with their deuterated derivatives are efficiently formed. For much higher density regions (~ 10^6 cm^-3), water and methanol productions are suppressed but surface coverage of CO,CO_2,O_2,O_3 are dramatically increased. We find a very high degree of fractionation of water and methanol. Observational results support a high fractionation of methanol but surprisingly water fractionation is found to be low. This is in contradiction with our model results indicating alternative routes for de-fractionation of water. Effects of various types of energy barriers are also studied. Moreover, we allow grain mantles to interact with various charged particles (such as H^+, Fe^+,S^+ and C^+) to study the stopping power and projected range of these charged particles on various target ices.