Hot planetary nebulae nuclei and related objects
The hottest central stars found in planetary nebulae excite the O VI atomic transition in their atmospheres. Presumably the central stars of all planetary nebulae become O VI stars during some phase of their evolution. Observational studies show that all post-asymptotic giant branch (P-AGB) stars (excluding white dwarfs) that undergo nonradial pulsations are O VI stars. A full understanding of O VI stars is therefore crucial before we can develop a complete picture of P-AGB evolution. In this thesis we study O VI central stars of planetary nebulae, with particular attention to obtaining their evolutionary paths, masses and an understanding of the physical processes of the associated nebulae. Our approach is to use analyses of nebular and stellar spectral features as diagnostics of the physical conditions that prevail in and around these objects. We obtain stellar magnitudes, Zanstra temperatures, extinction constants, abundances, and other diagnostics. By placing the central stars on the HR diagram, we estimate their masses and evolutionary timescales. Correlations between spectral characteristics and observable stellar properties suggest that narrow-line and broad-line O VI central stars belong to different mass-groups. We also calculate the nebular electron densities and distances for a larger sample of planetary nebulae and demonstrate the existence of correlations between the different density diagnostics, in substantial agreement with leading hydrodynamical models. We perform theoretical studies of instability against nonradial oscillations of hydrogen-depleted P-AGB stars and white dwarfs. Our tests of instability, which are non-adiabatic and are based on stellar surface models combined with evolutionary models, are in agreement with nonradial oscillations found from photometric measurements in O VI central stars and other hydrogen-depleted evolved stars.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign