| Introduction Haloes around Planetary Nebulae (PNe) are a quite common phenomenon: among the elliptical PNe for which adequately deep imaging was obtained, the halo detection rate is about 70% (cf. R. Corradi, this conference). Morphologically, a halo is defined as an outer round structure whose surface brightness is much lower (typically <= 10 -3) than the bright inner parts of the PN proper, while its size is much larger. Physically, one has to distinguish between different kind of haloes. AGB haloes consist of photo-ionized matter still containing information about the mass-loss history during the last thermal pulse cycle on the Asymptotic Giant Branch. A second kind of halo may develop when recombination sets in as a consequence of the fast stellar luminosity drop during the advanced post-AGB evolution. These recombination halos, which can be confused with a real AGB halo on a pure morphological basis (Corradi et al. 2000, A&A 354, 1071), are not a signature of an AGB mass-loss episode. As a case study, we compare the properties of the rather big halo of the double-shell planetary NGC 6826 with the results of hydrodynamical simulations, as part of an effort to extract information about the final mass-loss episode on the AGB from modern deep imaging of PN haloes. |
Hydrodynamical AGB wind modeling
Mass loss variation and resulting wind structure
Computed and observed halo structure
| Conclusions Hydrodynamical modeling of AGB wind envelopes and PN evolution reveals that PN haloes do indeed contain information about the mass-loss history at the end of the AGB. Our halo study of NGC 6826 indicates that — mass loss must increase substantially towards the end of the AGB phase (l~ r-7 p~ r-4) — the outer edge of the halo of NGC 6826 is defined by the the final helium-shell flash on the AGB — time elapsed since the last helium-shell flash = halo kinematical age (~ 33 000 yrs, for d ~ 1.1 kpc, vhalo ~ km/s) — the central star of NGC 6826 is most likely still powered by hydrogen burning The structure of the AGB halo depends not only on the mass loss history but also on outflow velocity (dust properties!). The derivation of the mass-loss history at the end of the AGB evolution from observed surface brightness profiles of PN haloes therefore requires detailed hydrodynamical simulations. |
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Statistical properties of the PN haloes |
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Location of the CSs in the H-R diagram (top-left), radii (top-right), SBs relative to the rims (bottom-left), and kinematical ages after substraction of the inner nebula ages (bottom-right). The meaning of each symbol is given within the inset boxes in the upper plots. | |