SiO line emission from interstellar jets and outflows: silicon-containing mantles and non-stationary shock waves^*

¹ Physics Department, The University, Durham DH1 3LE, UK e-mail: antoine.gusdorf@durham.ac.uk
² Institut d'Astrophysique Spatiale (IAS), Bâtiment 121, 91405 Orsay, France
³ LERMA (UMR 8112 du CNRS), Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
⁴ Université Paris-Sud 11 and CNRS (UMR 8617), France

Received: 23 June 2008
Accepted: 29 August 2008

Abstract

Context. 
We study the production and emission of SiO and H in the gas phase of molecular outflows, extending previous work in which we considered steady-state C-type shock waves and assumed the silicon to be present only in the cores of silicate grains.

Aims. We place constraints on the physical parameters of the pre-shock region, using recent observations of SiO and observations of molecular hydrogen. We show the effects of introducing SiO-containing mantles and of varying the age of the shock wave. We consider simultaneously the emission of SiO and H from the young L1157 outflow.

Methods. The molecular outflows are studied by means of a code that can generate stationary C- and J-type shock models and approximate non-stationary solutions, which combine these two types of shock wave. The emission of molecular hydrogen is computed by this code, whereas the SiO emission is computed by means of a separate LVG model, which uses the calculated physical and chemical profiles. A grid of models has been computed, with shock speeds in the range 10 35 km s^-1 and pre-shock gas densities 10⁴ 10⁶ cm^-3. A wide range of magnetic field strengths has been investigated, from 45 μG to about 600 μG.

Results. We illustrate our results by means of observational data obtained on the blue lobe of the L1157 outflow. Given the combinations of pre-shock densities and shock velocities necessary to fit the H observations, we find that the erosion only of the silicate material in the grains cores cannot account for the observed SiO line intensities. We investigate the possiblity that a fraction of the SiO is present initially in the grain mantles, and we succeed in constraining this fraction. Introducing even a few percent of the silicon (as SiO) into the mantles is sufficient to increase the SiO line widths and fluxes by an order of magnitude. With this assumption, it is possible to find a non-stationary shock model that provides a reasonable fit of the observations of both H and SiO.

Conclusions. With a few percent of the silicon present initially in the grain mantles, good agreement is obtained with recent observations of SiO line integrated line intensities for a pre-shock density n_H = 10⁴ cm^-3 and a shock speed v_s = 20 km s^-1. The magnetic field strength and the shock age are not well constrained by the observations of either H or SiO. We show that CO observations (in particular, with the Herschel satellite) could provide further discrimination between the models.