Cosmic Rays and Radiative Instabilities
  In the absence of magnetic fields and cosmic rays, radiative cooling laws
with a range of dependences on temperature affect the stability of interstellar
gas. For about four and a half decades, astrophysicists have recognised the
importance of the thermal instablity for the formation of clouds in the
interstellar medium. Even in the past several years, many papers have concerned
the role of the thermal instability in the production of molecular clouds.
About three and a half decades ago, astrophysicists investigating radiative
shocks noticed that for many cooling laws such shocks are unstable. Attempts to
address the effects of cosmic rays on the stablity of radiative media that are
initially uniform or that have just passed through shocks have been made. The
simplest approach to such studies involves the assumption that the cosmic rays
behave as a fluid. Work based on such an approach is described. Cosmic rays
have no effect on the stability of initially uniform, static media with respect
to isobaric perturbations, though they do affect the stability of such media
with respect to isentropic perturbations. The effect of cosmic rays on the
stability of radiative shocked media depends greatly on the efficiency of the
conversion of energy in accelerated cosmic rays into thermal energy in the
thermalized fluid. If that efficiency is low, radiative cooling makes weak
shocks propagating into upstream media with low cosmic-ray pressures more
likely to be cosmic-ray dominated than adiabatic shocks of comparable strength.
The cosmic-ray dominated shocks do not display radiative overstability. Highly
efficient conversion of cosmic-ray energy into thermal energy leads shocked
media to behave as they do when cosmic rays are absent.
