The Effects of Subsurface Chemistry in the Grain Mantles on the Deuterium Chemistry in Molecular Clouds

TL;DR

This study investigates how subsurface chemistry in grain mantles influences deuterium enrichment in molecules within molecular clouds, highlighting the role of mantle processes and cosmic-ray effects on deuteration over time.

Contribution

It introduces a kinetic chemistry model that includes gas, surface, and mantle pore reactions, revealing how mantle chemistry affects molecular deuteration in star-forming regions.

Findings

Surface reactions enhance deuteration of molecules.

Deuteration in mantle molecules decreases over 10 million years.

Implication that molecule freeze-out should not exceed 10 Myr before evaporation.

Abstract

The deuterium enrichment in molecules in dark molecular cloud cores and starforming regions is usually attributed to gas-phase chemistry. Here we examine the effects of surface and mantle chemical reactions on the deuteration of species. We use a simple kinetic chemistry model that includes gas, surface and mantle pore phase reactions of deuterated species. The mantle is assumed to be partially reactive due to pores with sufficient surface area for chemical reactions, that are continuously transformed by cosmic-rays. Calculation results show that surface reactions generally enhance the deuteration for at least several molecules. However, once they are buried and become mantle molecules, they lose their deuteration over a timescale of 10 million years due to processes in the mantle. If deuterated species in young star-forming regions come from grain mantles, a cautious conclusion is that the freeze-out of molecules, perhaps, should not occur more than 10 Myr before the mantle evaporates to the gas phase.