A linear analysis of the stability of isothermal C-type shocks propagating perpendicular to a magnetic field, explicitly including advection and the detailed structure of the steady shock front, shows that many C-type shocks are unstable. The mechanism is analogous to the Parker instability of gas supported against gravity by a magnetic field. The most important parameter determining the stability of a shock is the Alfven number: shocks with Alfven numbers greater than five are unstable, the growth rate rising rapidly with increasing shock strength. The fastest-growing modes form clumps on a scale of about half the shock thickness-about 0.01 pc for parameters typical of molecular clouds. These results are not expected to be qualitatively affected by the inclusion of frictional heating and radiative cooling or by an oblique magnetic field. Implications for molecular-cloud structure and the interpretation of observations of shocked regions in terms of steady-shock models is briefly discussed. The clumps are expected to be cooler than the unperturbed flow in more realistic, radiative-shock models. Low mass star formation may be triggered by the instability.
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 1990