A&A 482, 549-559 (2008)
DOI: 10.1051/0004-6361:20078054

Parametrization of C-shocks. Evolution of the sputtering of grains

I. Jiménez-Serra^{1, 2}, P. Caselli^{2, 3}, J. Martín-Pintado¹, and T. W. Hartquist<sup>2

1</sup>  Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia (CSIC), C/ Serrano 121, 28006 Madrid, Spain
    e-mail: izaskun@damir.iem.csic.es
²  School of Physics and Astronomy, University of Leeds LS2 9JT, Leeds, UK
³  INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

(Received 11 June 2007 / Accepted 30 January 2008)

Abstract
Context. The detection of narrow SiO line emission toward the young shocks of the L1448-mm outflow has been interpreted as a signature of the magnetic precursor of C-shocks. In contrast with the low SiO abundances (10^-12) derived from the ambient gas, the narrow SiO emission in the precursor component at almost ambient velocities reveals enhanced SiO abundances of ~10^-11. It has been proposed that this enhancement of the SiO abundance is produced by the sputtering of the grain mantles at the early stages of C-shocks. However, modelling of the sputtering of grains has usually averaged the SiO abundances over the dissipation region of C-shocks, which cannot explain the recent observations.
Aims. We model the evolution of the gas-phase abundances of molecules like SiO, CH₃OH, and H₂O, produced by the sputtering of the grain mantles and cores as the shock propagates through the ambient gas. We consider different initial gas densities and shock velocities.
Methods. We propose a parametric model to describe the physical structure of C-shocks as a function of time. Using the known sputtering yields for water mantles (with other minor constituents like silicon and CH₃OH) and olivine cores by collisions with H₂, He, C, O, Si, Fe, and CO, we follow the evolution of the abundances of silicon, CH₃OH, and H₂O ejected from grains along the evolution of the shock.
Results. The evolution of the abundances of the sputtered silicon, CH₃OH, and H₂O shows that CO seems to be the most efficient sputtering agent in low-velocity shocks. The velocity threshold for the sputtering of silicon from the grain mantles is appreciably reduced (by 5-10 kms^-1) by CO compared to other models. The sputtering by CO can generate SiO abundances of ~10^-11 at the early stages of low-velocity shocks, consistent with those observed in the magnetic precursor component of L1448-mm. Our model satisfactorily reproduces the progressive enhancement of SiO, CH₃OH, and H₂O observed in this outflow, suggesting that this enhancement may be due to the propagation of two shocks with  kms^-1 and  kms^-1 coexisting within the same region.
Conclusions. Our simple model can be used to estimate the time-dependent evolution of the abundances of molecular shock tracers like SiO, CH₃OH, H₂O, or NH₃ in very young molecular outflows.

Key words: ISM: clouds -- shock waves -- ISM: jets and outflows -- ISM: dust, extinction

© ESO 2008

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.