Dynamical evolution of planetary nebulae - I. Formation of shells in an accelerating wind in protoplanetary nebulae

Evolutionary calculations of central stars of planetary nebulae show a time-dependent increase in surface temperature. The high-speed winds, which these stars emit, expand into a previously ejected ‘superwind’ and are most likely driven by radiation pressure. Hence a model explaining the dynamical evolution of the surrounding planetary nebulae must take into account that the wind speed increases and the mass-loss rate decreases with time. We show that at an early stage a shell will form and that the flow changes from momentum to energy driven, once the wind speed is in excess of 150 km s – 1. Radii and velocities of these shells are generally lower in an accelerating wind model than in the simple multiple-winds model. Another important difference is the occurrence of Rayleigh–Taylor instabilities in the shell when the flow is energy driven.