HST observations of the DAB white dwarf HS 0209+0832: testing explanations for the ''DB gap''

Details HST observations of the DAB white dwarf HS 0209+0832: testing explanations for the ''DB gap'' HST observations of the DAB white dwarf HS 0209+0832: testing explanations for the ''DB gap''

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7473j&link_type=abstract

HST Proposal ID #7473

Physics

Hst Proposal Id #7473 Hot Stars And Stellar Corpses

Scientific paper

White dwarfs are basically divided into two spectral classes, the hydrogen-rich DAs and the helium-rich non-DAs. However, between 28 000 and 45 000 K not a single non-DA star exists, although there are many helium-rich stars above and below this ``DB gap''. Currently, the only scenario to explain this gap is the assumption that every non-DA must contain or accrete a small amount of hydrogen, which, during the evolution to lower temperatures, floats up to the surface and transforms the stars into DAs. At 28 000 K these objects become convectively unstable and the hydrogen layer can disappear by dilution, if it is very thin { 10^-14 Msun}; in this case the star would become a helium-rich white dwarf {spectral type DB} again. HS 0209+0832 is the first white dwarf at DB gap temperatures in which helium { 2 resolution and high S/N UV observations with HST would enable us to accurately determine the atmospheric parameters, especially the helium and metal content} of this unique object. The results will be important tests for the ultra-thin layer hypothesis and the theories of accretion from the interstellar medium.

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