DUST EVAPORATION IN PROTOSTELLAR CORES

We investigate sublimation and chemical sputtering of refractory grain s. The general time-dependent treatment of these two physical processe s, as well as the limit in which the time-independent description can be used, are discussed. We follow the evaporation of dust in the physi cal environment characteristic of the central regions of the outer pro tostellar core, as envisaged by current hydrodynamical models. We find that destruction of silicate grains occurs via sublimation and procee ds under quasi-equilibrium conditions. Evaporation of carbon grains is on the opposite dominated by chemisputtering by H atoms (at T less th an or equal to 1000 K) and H2O molecules (at T > 1000 K). The effectiv eness of these mechanisms may receive a further boost from the high su rface density of active sites characteristic of amorphous materials. B ecause the destruction of carbon grains takes place under nonequilibri um conditions, dust disappearance is coupled to the hydrodynamical evo lution of the system in which it occurs. Even more critical however is the interplay between processes of growth/destruction of dust grains and gas-phase chemical reactions. The Rosseland mean opacity K, decrea ses by a factor of 4 around 1100 K, because of the destruction of carb on grains, and by a factor of about 500 at 1300 K, where the evaporati on of silicates marks the disappearance of the bulk of dust. We additi onally model the sublimation of aluminum oxide grains which, if indeed present, would dominate the opacity at higher T, until they also disa ppear around 1720 K.