Physical conditions in dense and cold regions of interstellar clouds favour the formation of ice mantles on the surfaces of interstellar grains. It is predicted that most of the gaseous species heavier than H2 or He will adsorb onto the grains and wi
ll disappear from the gas-phase, changing its chemistry, within 10^9/n_H years. Nonetheless, many molecules in molecular clouds are not completely depleted in timescales of 10^5 yr. Several speculative mechanisms have been proposed to explain why molecules stay in the gas phase, but up to now none are fully convincing. At the same time, these mechanisms are not mutually exclusive and we can still explore the effects of other possible processes. We speculate on the consequences of H2 coating of grains on the evaporation rates of adsorbed species. More experiments and simulations are needed to calculate the evaporation rate Eevap(X-H2).
HH 211 is a highly collimated jet originating from a nearby young Class 0 protostar. Here is a follow-up study of the jet with our previous observations at unprecedented resolution up to ~ 0.3 in SiO (J=8-7), CO (J=3-2), and SO (N_J=8_9-7_8). SiO, CO
, and SO can all be a good tracer of the HH 211 jet, tracing the internal shocks in the jet. Although the emissions of these molecules show roughly the same morphology of the jet, there are detailed differences. In particular, the CO emission traces the jet closer to the source than the SiO and SO emissions. In addition, in the better resolved internal shocks, both the CO and SO emission are seen slightly ahead of the SiO emission. The jet is clearly seen on both sides of the source with more than one cycle of wiggle. The wiggle is reflection-symmetric about the source and can be reasonably fitted by an orbiting source jet model. The best-fit parameters suggest that the source itself could be a very low-mass protobinary with a total mass of ~ 60 M_Jup and a binary separation of ~ 4.6 AU. The abundances of SiO and SO in the gas phase are found to be highly enhanced in the jet as compared to the quiescent molecular clouds, even close to within 300 AU from the source where the dynamical time scale is <10 yrs. The abundance enhancements of these molecules are closely related to the internal shocks. The detected SiO is either the consequence of the release of Si-bearing material from dust grains or of its formation via gas chemistry in the shocks. The SO, on the other hand, seems to form via gas chemistry in the shocks.
