  Early-type galaxies possess a dilute hot (2-10E6 K) gas that is probably the
thermalized ejecta of the mass loss from evolving stars. We investigate the
processes by which the mass loss from orbiting stars interacts with the
stationary hot gas for the case of the mass ejected in a planetary nebula
event. Numerical hydrodynamic simulations show that at first, the ejecta
expands nearly symmetrically, with an upstream bow shock in the hot ambient
gas. At later times, the flow past the ejecta creates fluid instabilities that
cause about half of the ejecta to separate and the other half to flow more
slowly downstream in a narrow wake. When radiative cooling is included, most of
the material in the wake (>80%) remains below 1E5 K while the separated ejecta
is hotter (1E5-1E6 K). The separated ejecta is still less than one-quarter the
temperature of the ambient medium and the only way it will reach the
temperature of the ambient medium is through turbulent mixing (after the
material has left the grid). These calculations suggest that a significant
fraction of the planetary nebula ejecta may not become part of the hot ambient
material. This is in contrast to our previous calculations for continuous mass
loss from giant stars in which most of the mass loss became hot gas. We
speculate that detectable OVI emission may be produced, but more sophisticated
calculations will be required to determine the emission spectrum and to better
define the fraction of cooled material.
