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Session 97 - Planetary Nebulae.
Display session, Thursday, January 16
Metropolitan Ballroom,
It is commonly accepted that planetary nebulae (PNe) are formed by the current fast stellar winds sweeping up the previous slow winds. In this scenario, fast stellar winds are shocked to 10^6--10^7 K and emit X-rays. Such X-ray emission should be diffuse, with the highest emissivity expected near the inner wall of the visible shell.
Contrary to this expectation, ROSAT observations of PNe have shown all X-ray emission concentrated near the central star. Most of these X-ray sources are unresolved. Spectral analyses of the unresolved sources show high-temperature components (a few \times10^6 K). It has been suggested that this hot gas originates from wind-wind interactions, although the location of the emission is not close to the visible shell. Only Abell 30 and possibly NGC\,6543 have been resolved by ROSAT instruments. Our High Resolution Imager observation of Abell 30 shows X-ray emission peaking on the central star and on the surrounding H-deficient clumps. HST WFPC2 images and ground-based echelle spectra of A30 clearly reveal wind-clump interactions in the X-ray emission regions.
The distribution, temperature, and density of the X-ray emitting gas derived from ROSAT observations suggest that hot (10^6 K) gas is produced by interactions between the fast stellar wind and clumpy circumstellar material. The efficient mixing of shocked fast stellar winds and the ablated clump material may play a key role in determining the cooling rate of the interior of a PN. This effect should be taken into consideration in future hydrodynamic modelling of PNe.
