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

¹ 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 km s^-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  km s^-1 and  km s^-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.