Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008a%26a...490..695g&link_type=abstract
Astronomy and Astrophysics, Volume 490, Issue 2, 2008, pp.695-706
Astronomy and Astrophysics
Astrophysics
30
Astrochemistry, Magnetohydrodynamics (Mhd), Molecular Processes, Ism: Jets And Outflows, Infrared: Ism, Radio Lines: Ism
Scientific paper
Context: We study the production and emission of SiO and H_2 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_2 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 ≤ v_s ≤ 35 km s-1 and pre-shock gas densities 104 ≤ n_H ≤ 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_2 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_2 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 nH = 104 cm-3 and a shock speed vs = 20 km s-1. The magnetic field strength and the shock age are not well constrained by the observations of either H_2 or SiO. We show that CO observations (in particular, with the Herschel satellite) could provide further discrimination between the models.
Appendices are only available in electronic form via http://www.aanda.org
Cabrit Silvie
Flower David R.
Gusdorf Antoine
Pineau des Forêts Guillaume
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