Chemical evolution in protostellar envelopes: Cocoon chemistry

We have modeled the chemistry that occurs in the envelopes surrounding newborn stars as they are gradually heated by the embedded protostar and the ice mantles of dust grains evaporate, resulting in a hot molecular core. We consider two dynamical scenarios: (1) a cloud undergoing the inside-out gravitational collapse calculated by Shu and (2) a quasi-stationary envelope. The radial distribution of dust temperature means that differences in surface binding energies result in distinct spatial zones with specific chemistries, as more volatile species (e.g., H2S) are evaporated before more tightly bound species (e.g., H2O). We use our results to identify chemical features that depend on the nature of the collapse and so determine observational tests that may be able to distinguish between different dynamical models of the star formation process. We show that the observed molecular abundances in massive hot cores can be explained only if these objects are supported against collapse.