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Session 86 - Rotating, Modeling and Blue Stars.
Oral session, Wednesday, January 15
Planetary Nebulae (PN) carry to the interstellar medium the dredged-up products of the stellar nucleo-synthesis. 99 out of 100 born stars go through this phase and as such PN play a major role in the chemical evolution of the Galaxy.
Of all PN, ``[WC]" PN are a rare subgroup whose hydrogen-deficient central stars' (CSPN) spectra mimic population I Wolf-Rayet WC stars. In particular, [WCLate] stars have low-ionization spectra (HeI, CII and OII dominate), and their PN are compact and often accompanied by substantial quantities of dust.
The mechanisms responsible for the ejection of PN is still not very clear. Nor is it clear what processes differentiate between H-deficient and H-rich: normal asymptotic giant branch evolution or more exotic evolutionary scenarios such as the rebirth of a PN after the star has already become a white dwarf.
In our study we tackle the problem from several directions: 1) we determine stellar and nebular physical parameters and chemistries from direct measurements of UV and optical spectra. To achieve this, new atomic physics methods are employed. 2) The stellar spectra are modelled by sophisticated Wolf-Rayet codes (Hillier, 1990, Aamp;A231, 111) and stellar parameters are derived and compared with the empirical ones. The temperature and luminosity are then used to place the stars on the HR diagram along with theoretical tracks. Massive Pop. I Wolf-Rayet stars are then compared to their central star counterparts and conclusions drawn on the Wolf-Rayet phenomenon. 3) Morphological classes and nebular ages are derived from HST images. These are then compared to theoretical evolutionary times. 4) Variability of the stellar spectra is interpreted in view of different evolutionary scenarios. 5) Connections to other types of H-deficient central stars are sought (e.g. weak emission line stars, PG1159-type white dwarfs and RCoronaeBorealis stars).
>From these results we draw conclusions on the evolutionary status of hydrogen--deficient CSPN.
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