Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993hst..prop.4263h&link_type=abstract
HST Proposal ID #4263
Other
Hst Proposal Id #4263
Scientific paper
The PG1159 stars form a new class of hydrogen deficient pre white dwarfs (PWD) representing the hottest stage (Teff>=100,000 K) of PWD evolution. First quan- titative spectral analyses have been carried out by the proposers. The proto- type PG1159-035 has been analysed from the HST-FOS spectrum with unprecedented precision. It has been shown that these stars have atmospheres dominated by carbon and helium with a significant amount of oxygen present. Extreme mass loss has removed the H rich and most of the He rich layers. Such a mass loss event has been observed in Lo4. About every other member of the PG1159 stars is a non- radial, multi-periodic g-mode pulsator driven by cyclic ionisation of C and O. Modelling of the pulsations allows the stellar mass to be determined very accurately and the stellar interior to be probed (asteroseismology). A prerequisite for such modelling is a precise determination of the photospheric parameters. We propose to obtain UV observations of three additional PG1159 stars (one pulsator and two non-pulsators) in addition to the (pulsating) prototype in order to trace differences in photospheric parameters and composition. The spectra will be analysed with new highly-sophisticated NLTE line blanketed model atmospheres, which allow the synthetic spectrum to be calculated and line blending of iron group elements to be included self-consistently. The results will allow us to determine the position of the new instability strip (defined by the pulsating PG1159 stars) in the HRD.
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