Hot Molecular Cores — A Case for Accretion

Abstract

Models of dense interstellar clouds should include accretion of gas-phase species onto dust grains. By modelling the chemistry of a hot molecular core we demonstrate that accretion does occur and that reactions on the surface with hydrogen and deuterium atoms are important. Observations of hot cores show enhanced abundances of ammonia, methanol and deuterium-bearing species. These enhancements cannot be reproduced by gas-phase chemistry alone. We explain the abundances by allowing accretion to occur as a cold cloud undergoes an isothermal free-fall collapse. Atoms and radicals are saturated with hydrogen on the surface, and the processed mantle material is then returned to the gas-phase as a result of grain heating caused by the formation of a nearby star. Support is thereby given to the idea that mantle material is returned to the gas-phase sporadically, on a timescale of a million years, as a result of star formation, shocks etc., rather than by a continually acting desorption mechanism.