THE DYNAMIC INSTABILITY OF ADIABATIC BLAST WAVES

Adiabatic blast waves, which have a total energy injected from the center, E proportional-to t(q), and propagate through a preshock medium with a density P(E) proportional-to tau-omega, are described by a family to similarity solutions. Previous work has shown that adiabatic blast waves with increasing or constant postshock entropy behind the shock front are susceptible to an oscillatory instability, caused by the difference between the nature of the forces on the two sides of the dense shell behind the shock front. This instability sets in if the dense postshock layer is sufficiently thin. In this paper we consider the stability of adiabatic blast waves with a decreasing postshock entropy. Such blast waves, if they are decelerating, always have a region behind the shock front which is subject to convection. Some accelerating blast waves also have such a region, depending on the values of q, omega, and gamma, where gamma is the adiabatic index. However, since the shock interface stabilizes dynamically induced perturbations, blast waves become convectively unstable only if the convective zone is localized around the origin or a contact discontinuity far from the shock front. On the other hand, the contact discontinuity of accelerating blast waves is subject to a strong Rayleigh-Taylor instability. The frequency spectra of the nonradial, normal modes of adiabatic blast waves have been calculated. We discuss the implications of this work for two astrophysical interests, the instability of shocks propagating through supernova envelopes and the evolution of planetary nebulae.