On the initial phase of interaction between expanding stellar envelopes and surrounding medium (Q1076575)

The initial stages of deceleration in the circumstellar medium of a stellar envelope, thrown off by a shock wave, are investigated. The equations of spherical-symmetric adiabatic hydrodynamics are shown to have a similarity solution in the case of the density of the expanding envelope being approximated by a reasonable power low. The overall flow pattern has such a form that the stellar material is decelerated in the internal shock wave while another shock propagates through the circumstellar matter. Between the shocks there is a contact discontinuity separating the circumstellar and stellar matter. The characteristics of the similarity solution are calculated for various exponents in the density laws of an expanding envelope and circumstellar matter and for two values of the adiabatic index. Some parts of the flow exhibit Rayleigh-Taylor instability. Special attention is paid to the validity of the hydrodynamics. In full agreement with \textit{V. F. Dyachenko}, \textit{V. S. Imshennik} and \textit{V. V. Paleychik} [Astron. Zh. 46, 739 ff. (1969)], we conclude that the kinetic and collisionless processes are of great importance if the initial stages of stellar envelope deceleration are to be properly monitored.