Physics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6656p&link_type=abstract
HST Proposal ID #6656
Physics
Hst Proposal Id #6656 Hot Stars
Scientific paper
The study of white dwarf structure provides valuable insightsinto the final stages of stellar evolution. Recently, rapiddevelopments in UV spectroscopy have altered our understandingof these simplest of stars. It is now clear that hydrogen(DA) and helium (DB) white dwarfs follow differentevolutionary paths. While it appears that once a DA isformed, it remains a DA throughout its evolution, ourunderstanding of DB evolution is particularly sketchy. About20% of all DBs contain traces of hydrogen, and we observe agap in DB distribution between 30,000 rmK< rmT_eff<45,000rmK. Current theory predicts that, at these temperatures,hydrogen floats to the surface, masking the DB white dwarfbeneath a thin hydrogen photosphere. In 1992, HST obtainedhigh resolution spectra, centered on 1215 Angstrom, of theprototype DBV PG1645+325 (GD358), detecting broad shallowabsorption wings of hydrogen surrounding the geocoronalemission. Initial results indicate that PG1645+325, rmT_eff=27,000 K, does not have enough hydrogen to have been a DA inthe DB gap. We propose a search for hydrogen in hot DB whitedwarfs, to test theories of DB evolution and address thequestion of the uniqueness of PG1645+325.
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