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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 . 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[sub 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[sup 5] yr, and this leads to the observed near-linear relationship between ice column density and A[sub 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. [ABSTRACT FROM AUTHOR]

Hot cores are rich in saturated molecules that are believed to arise from the evaporation of molecular ices on dust grains. It is usually assumed that the ices are evaporated instantaneously when a nearby star is switched on. We have developed a new model in which the grain temperature rises over a time-scale determined by the switch-on time of the star. This time-scale is likely to be comparable to the lifetime of the hot cores. In consequence, evaporation of different species occurs at different epochs, leading to chemical differentiation in time and space within the hot core. By computing qualitative models of hot cores, we show that observations of hot cores may be able to constrain the rise time of hot stars to the main sequence. [ABSTRACT FROM AUTHOR]

ABSTRACT From observations of sulphur-bearing and other molecular species and chemical models it has been established that elemental sulphur is roughly two orders of magnitude more depleted in the detectable parts of such regions than are elemental carbon, nitrogen and oxygen. It seems surprising that sulphur is so depleted but not entirely depleted. We suggest that the fact that much of the sulphur is in S[sup +] in translucent clumps with hydrogen number densities of less than 10[sup 3] cm[sup -3] plays a significant role in determining why it is so depleted in denser sources. Ions collide more rapidly with grains and may stick more efficiently to them than neutrals; so, as a clump collapses, sulphur may become depleted in it more rapidly than elements that are not primarily ionized in translucent material. Eventually in the collapse, gas-phase sulphur will become contained mostly in neutral species, which in our picture leads to a large decrease in its depletion rate and a remnant gas-phase elemental fractional abundance high enough for sulphur-bearing species in dense cores to be detectable. [ABSTRACT FROM AUTHOR]