DUST EVAPORATION IN PROTOSTELLAR CORES

We investigate sublimation and chemical sputtering of refractory grains. The general time-dependent treatment of these two physical processes, as well as the limit in which the time-independent description can be used, are discussed. We follow the evaporation of dust in the physical environment characteristic of the central regions of the outer protostellar core, as envisaged by current hydrodynamical models. We find that destruction of silicate grains occurs via sublimation and proceeds under quasi-equilibrium conditions. Evaporation of carbon grains is on the opposite dominated by chemisputtering by H atoms (at T less than or equal to 1000 K) and H2O molecules (at T > 1000 K). The effectiveness of these mechanisms may receive a further boost from the high surface density of active sites characteristic of amorphous materials. Because the destruction of carbon grains takes place under nonequilibrium conditions, dust disappearance is coupled to the hydrodynamical evolution 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, decreases by a factor of 4 around 1100 K, because of the destruction of carbon grains, and by a factor of about 500 at 1300 K, where the evaporation of silicates marks the disappearance of the bulk of dust. We additionally model the sublimation of aluminum oxide grains which, if indeed present, would dominate the opacity at higher T, until they also disappear around 1720 K.