STELLAR WIND PALEONTOLOGY - SHELLS AND HALOS OF PLANETARY-NEBULAE

The shells and halos of planetary nebulae (PNs) have generally been attributed to mass loss by the central star prior to the onset of a fast wind. Since they have yet to be impacted either directly or indirectly by the fast wind, it has commonly been believed that the density distribution of the shells will fall as r~2. Hence the emission measure will fall as θ-3, where θ is an angular coordinate on the sky. However, we find from CCD surface brightness observations of six PNs with generally simple (i.e., round) morphologies and bright, well-resolved shells that the emission measure decreases as - b·θ+1 in all cases, where b is a positive constant. This form of the emission measure departs radically from that expected for a steady wind. Consequently we find that we can exploit the emission measure distribution in order to explore the history of mass ejection and evolution. Specifically we can place constraints on (1) the form of the mass ejection that formed the shell, (2) the external pressure that forces its post-ejection hydrodynamic evolution to the present, and (3) evolutionary timescales. These goals are accomplished by assuming a two-wind model in which a dense "superwind" ejected by a preplanetary nebula interacts hydrodynamically with a far more tenuous "red giant wind" ejected during a previous epoch of evolution (e.g., as the star was ascending the AGB). We perform hydrodynamical simulations using this assumption to model the emission measure distribution of the shells and halos of PN. These simulations allow us to conclude that mass is ejected in pulse(s) whose duration is short compared to the AGB-PN evolutionary timescale, and that the linear form of the emission measure occurs about 5000 yr after the superwind ejection ceases.