SiO line emission from C-type shock waves : interstellar jets and outflows

TL;DR

This study models SiO emission from C-type shocks in molecular outflows, improving previous models by incorporating better grain coupling and initial conditions, and compares results with observations of specific outflows.

Contribution

It introduces refined shock models with enhanced grain coupling treatment and realistic initial abundances, providing better agreement with observed SiO emissions in outflows.

Findings

Good match with observed SiO line intensities in L1157 and L1448.

Revised models predict narrower shocks and delayed SiO formation.

Discrepancies in line widths suggest multiple unresolved shock regions.

Abstract

We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si--bearing material in refractory grain cores, which are taken to be olivine; we calculate also the rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady--state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models has been generated. We compare our results with those of an earlier study (Schilke et al. 1997). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si being released into the gas phase. More realistic assumptions concerning the initial fractional abundance of O2 lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG `slab' models. The fractional abundance of SiO is deduced. Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam.