Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

TL;DR

This study uses microscopic simulations to model the formation of methanol and formaldehyde ices in cold dense interstellar environments, comparing results with observations and prior models.

Contribution

It extends previous models to include formation of both species under interstellar conditions, incorporating freeze-out and hydrogenation processes.

Findings

H2CO and CH3OH form efficiently in cold dense cores

Ice mantles have layered structures with CH3OH on top

CO and H2CO are mainly in lower ice layers

Abstract

Methanol and its precursor formaldehyde are among the most studied organic molecules in the interstellar medium and are abundant in the gaseous and solid phases. We recently developed a model to simulate CO hydrogenation via H atoms on interstellar ice surfaces, the most important interstellar route to H2CO and CH3OH, under laboratory conditions. We extend this model to simulate the formation of both organic species under interstellar conditions, including freeze-out from the gas and hydrogenation on surfaces. Our aim is to compare calculated abundance ratios with observed values and with the results of prior models. Simulations under different conditions, including density and temperature, have been performed. We find that H2CO and CH3OH form efficiently in cold dense cores or the cold outer envelopes of young stellar objects. The grain mantle is found to have a layered structure with CH3OH on top. The species CO and H2CO are found to exist predominantly in the lower layers of ice mantles where they are not available for hydrogenation at late times.