The water ice distribution in Taurus determined by gas–grain chemistry

Abstract

The detection of water ice in interstellar grain mantles by observation of absorbed radiation from field stars through the Taurus molecular cloud complex has revealed that the column density of water ice increases linearly,or nearly linearly with visual extinction except at the lowest values (below A(V) approximate to 3). Previous explanations of the small or negligible abundance of ice at low extinction have focused on the destruction and desorption of ice by radiation, and have ignored the efficiency of formation of ice. To understand the dependence of the ice column density on A(V) in more detail and to understand what it tells us about the Taurus dark cloud, we have run gas-grain chemical models under a variety of physical conditions. For higher values of extinction, our models predict that much of the elemental abundance of available oxygen is in the form of ice for evolutionary times greater than about 10(5) yr, and this leads to the observed near-linear relationship between ice column density and A(V). For lower extinction, our models show that if any significant amount of ice is still present, its existence can be attributed to great cloud age, to our poor understanding of dust-grain surfaces, or to cloud clumpiness. The first and third explanations pertain only if photodesorption of ice is inefficient.