The ice threshold in molecular clouds: A diagnostic of the infrared radiation field

Abstract

The threshold extinction at which ice is deposited on dust in molecular clouds is observed to show wide variation from cloud to cloud. The desorption of H2O molecules following absorption at 3 μm is proposed to account for the ice deposition threshold extinction. However, this mechanism is effective only up to H2O monolayer coverage and does not restrict the subsequent ice mantle growth, once started. It is shown that the threshold extinction is a measure of the local infrared radiation field near 3 μm in each cloud.