Shocks in Molecular Clouds: Switches and Bows

Abstract

Some progress towards understanding the behaviour of shocks in clouds is reported. First, synthetic infrared maps of bow shocks are presented. The model discussed involves a J-type section over the bow’s leading edge which produces emission from hot ionized gas and an oblique C-shock section in which molecular hydrogen survives. The location and features of the intensity distributions indicate the bow speed and orientation to the line of sight. A problem encountered in the OMC-1 outflow, the constant S(13):O(7) H2 line ratio, is discussed in terms of allowed bow shapes.Second, MHD shock theory is discussed. A shock transition can be continious, controlled by ion-neutral streaming, whatever the direction of the magnetic field. A plane-parallel shock propagating directly along field lines in a molecular cloud can be a continous (C-type) wave rather than a switched on. Switch-type behaviour occurs for oblique shocks: for fields within an angle ~ 1/A of the shock normal, where A is the Alfvé waves; are predicted to be present in collimated molecular outflows and may also be apparent as weak jump shocks in atomic jets.