Shocks in dense clouds. IV. Effects of grain-grain processing on molecular line emission

摘要

Grain-grain processing has been shown to be an indispensable ingredient ofshock modelling in high density environments. For densities higher than\sim10^5 cm-3, shattering becomes a self-enhanced process that imposes severechemical and dynamical consequences on the shock characteristics. Shattering isaccompanied by the vaporization of grains, which can directly release SiO tothe gas phase. Given that SiO rotational line radiation is used as a majortracer of shocks in dense clouds, it is crucial to understand the influence ofvaporization on SiO line emission. We have developed a recipe for implementingthe effects of shattering and vaporization into a 2-fluid shock model,resulting in a reduction of computation time by a factor \sim100 compared to amulti-fluid modelling approach. This implementation was combined with anLVG-based modelling of molecular line radiation transport. Using this model wecalculated grids of shock models to explore the consequences of differentdust-processing scenarios. Grain-grain processing is shown to have a stronginfluence on C-type shocks for a broad range of magnetic fields: they becomehotter and thinner. The reduction in column density of shocked gas lowers theintensity of molecular lines, at the same time as higher peak temperaturesincrease the intensity of highly excited transitions compared to shocks withoutgrain-grain processing. For OH the net effect is an increase in lineintensities, while for CO and H2O it is the contrary. The intensity of H2emission is decreased in low transitions and increased for highly excitedlines. For all molecules, the highly excited lines become sensitive to thevalue of the magnetic field. Although vaporization increases the intensity ofSiO rotational lines, this effect is weakened by the reduced shock width. Therelease of SiO early in the hot shock changes the excitation characteristics ofSiO radiation.