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