Temporal evolution of magnetic molecular shocks

I. Moving grid simulations

¹ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK e-mail: lesaffre@ast.cam.ac.uk
² University of Oxford, Department of Astrophysics, Oxford OX1 3RH, UK
³ CEA/DAPNIA/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
⁴ LERMA, UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁵ IAS, UMR-8617 du CNRS, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
⁶ LUTH, UMR-8102 du CNRS, Observatoire de Paris, 92190 Meudon Cedex, France 


Received: 15 December 2003
Accepted: 4 June 2004

Abstract

We present time-dependent 1D simulations of multifluid magnetic shocks with chemistry resolved down to the mean free path. They are obtained with an adaptive moving grid implemented with an implicit scheme. We examine a broad range of parameters relevant to conditions in dense molecular clouds, with preshock densities 10³ cm cm^-3, velocities 10 km s km s^-1, and three different scalings for the transverse magnetic field: .
We first use this study to validate the results of Chièze et al. (1998, MNRAS, 295, 672), in particular the long delays necessary to obtain steady C-type shocks, and we provide evolutionary time-scales for a much greater range of parameters.
We also present the first time-dependent models of dissociative shocks with a magnetic precursor, including the first models of stationary CJ shocks in molecular conditions. We find that the maximum speed for steady C-type shocks is reached before the occurrence of a sonic point in the neutral fluid, unlike previously thought. As a result, the maximum speed for C-shocks is lower than previously believed.
Finally, we find a large amplitude bouncing instability in J-type fronts near the H₂ dissociation limit (km s^-1), driven by H₂ dissociation/reformation. At higher speeds, we find an oscillatory behaviour of short period and small amplitude linked to collisional ionisation of H. Both instabilities are suppressed after some time when a magnetic field is present.
In a companion paper, we use the present simulations to validate a new semi-analytical construction method for young low-velocity magnetic shocks based on truncated steady-state models.