Switch shocks in molecular clouds. I. Plane-parallel magnetohydrodynamic analysis

Magnetohydrodynamic shock waves in molecular clouds are discussed. Plane-parallel flows with oblique magnetic fields are analyzed with particular regard to switch shocks. Switch-on shocks occur in the limit as the flow and field directions coincide; a transverse field is then switched on by the shock. In molecular clouds these become continuous or C-type, here termed C-switches. Their existence rules out the alternative J-type hydrodynamic flows. J-switches are only possible at low Alfvén numbers even in strongly radiative shocks which possess an equilibrium temperature. C-switches yield both strong compression and large swings in the magnetic field direction. They arise under a wide range of general conditions. One requirement is that the Alfvén speed exceeds the sound speed. The other requirement, to keep the flow supersonic, is that A2/?0 is small, where A is the Alfvén number and ?0 is a ratio of heating to cooling time scales. For transverse-field shocks the weaker restraint is on A/?0. Upstream and downstream flow behavior is examined; weak downstream subshocks are more probable in C-switches than in transverse shocks. The temperature profile across an oblique shock is derived using the cool C-shock method. A formula is presented which determines the transverse shock of equivalent excitation. C-switch behavior influences flows within an angle ?1/A of the field direction. Switch-off C-shocks, in which the field pressure generated by the switch-on shock is converted into thermal pressure as the transverse field disappears, are also discussed. An approximate isothermal solution is found.