The interaction between the blast wave of a supernova remnant and interstellar clouds

Abstract

When an interstellar (or circumstellar) cloud is overtaken by a blast wave, a shock is driven into the cloud. The velocity of the shock is derived as a function of the cloud density, and the effects of thermal conduction between the blast wave and the shocked cloud are discussed. It is shown that many of the observed features of the quasi-stationary flocculi in Cas A can be accounted for by this model. Some aspects of the evolution of supernova remnants (SNRs) in a cloudy interstellar medium are discussed. It is argued that the optically observed filaments in older SNRs represent shocked interstellar clouds with a significantly lower velocity than the blast wave propagating in the intercloud medium. The dynamics of large SNRs are relatively unaffected by interstellar clouds. It is suggested that most optically observed SNRs are still in the adiabatic phase of their expansion. Application of results to the Cygnus Loop shows that Woltjer's interpretation of the optical and X-ray observations is self-consistent. Vela X and Shajn 147 are also briefly discussed.