THE HYDRODYNAMICS OF A SUPERNOVA SHOCK IN THE INTERSTELLAR-MEDIUM

The interactions of a supernova blast wave with the inhomogeneities of the interstellar medium have been observed both in the young supernova remnants, with ages approximately 10(2) yr, and in the middle-aged supernova remnants older than approximately 10(4) yr. Low mass supernovae produce remnants interacting with an interstellar medium unaffected by the progenitor star, in which the density inhomogeneities mainly appear in the form of clouds. The encounter of the supernova shock with a cloud affects the propagation of the main shock and compresses and accelerates the cloud itself. In the adiabatic approximation, the cloud embedded by the blast wave becomes hydrodynamically unstable under the action of Rayleigh-Taylor and Kelvin-Helmoltz instabilities, adding mass to the intercloud medium. The thermal evaporation and radiative cooling further contribute to the energy and mass exchange between the cloud and the interstellar medium. Massive stars, such as progenitors of the supernovae, reprocess the interstellar medium, either ionizing the interstellar gas or by adding mass and kinetic energy via stellar winds. The supernova shock will move in a stratified medium and the remnant may be "muffled" by the circumstellar material disposed around the progenitor. Here we shall review the results of two-dimensional numerical hydrodynamics both in a cloudy and in a stratified medium comparing them with the observations in optical and X-ray bands for young (CAS-A) and evolved remnants (Cygnus Loop).