The Effects of Grain Size and Temperature Distributions on the Formation of Interstellar Ice Mantles

TL;DR

This study explores how grain size and temperature variations influence the formation and composition of interstellar ice mantles, revealing that temperature differences significantly affect ice build-up across grain sizes.

Contribution

It introduces a detailed model considering a distribution of grain sizes and temperatures, improving understanding of ice formation in interstellar environments.

Findings

Temperature differences significantly influence ice composition.

Small grains dominate ice accumulation during cloud collapse.

Ice layers per grain are limited to about 40 monolayers.

Abstract

Computational models of interstellar gas-grain chemistry have historically adopted a single dust-grain size of 0.1 micron, assumed to be representative of the size distribution present in the interstellar medium. Here, we investigate the effects of a broad grain-size distribution on the chemistry on dust-grain surfaces and the subsequent build-up of molecular ices on the grains, using a three-phase gas-grain chemical model of a quiescent dark cloud. We include an explicit treatment of the grain temperatures, governed both by the visual extinction of the cloud and the size of each individual grain-size population. We find that the temperature difference plays a significant role in determining the total bulk ice composition across the grain-size distribution, while the effects of geometrical differences between size populations appear marginal. We also consider collapse from a diffuse to a dark cloud, allowing dust temperatures to fall. Under the initial diffuse conditions, small grains are too warm to promote grain-mantle build-up, with most ices forming on the mid-sized grains. As collapse proceeds, the more abundant, smallest grains cool and become the dominant ice carriers; the large population of small grains means that this ice is distributed across many grains, with perhaps no more than 40 monolayers of ice each (versus several hundred assuming a single grain size). This effect may be important for the subsequent processing and desorption of the ice during the hot-core phase of star-formation, exposing a significant proportion of the ice to the gas phase, increasing the importance of ice-surface chemistry and surface-gas interactions.