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Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6463d&link_type=abstract
HST Proposal ID #6463
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Hst Proposal Id #6463 Hot Stars
Scientific paper
The PG 1159 stars form a new class of hydrogen deficient prewhite dwarfs (PWD) representing the hottest stage of PWDevolution. Quantitative spectral analyses of most knownPG 1159 stars have been carried out by the proposers fromoptical, UV (IUE, HST) and EUV (EUVE) observations. It hasbeen shown that these stars have atmospheres dominated by Cand He. In the hottest stars also a significant amount of O ispresent. The proposed targets, HS2324+3944, NGC7094, and Abell43define a small group of peculiar PG 1159 stars. Unlikeall other PG 1159 stars hydrogen is detected in their spectra.Stellar parameters derived by the investigators from opticalspectra suggest that, as a consequence of a lower mass, theywent through a less drastic mass-loss phase than the ordinaryPG 1159 stars. In order to verify that hypothesis a moreprecise determination of the effective temperature isrequired. This is not possible from optical analyses alone. Wedemonstrate that a great improvement on the determination ofthe effective temperatures can be achieved by HST-GHRSspectra. The spectra will be analyzed with new highlysophisticated NLTE line blanketed model atmospheres, whichallow to include many elements up to the iron group self-consistently. A precise placement of HS2324+3944, NGC7094, and Abell43in the HR-diagram holds the key for understanding theevolutionary history of the PG 1159 class.
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