Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993mnras.264...16b&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 264, NO. 1/SEP1, P. 16, 1993
Astronomy and Astrophysics
Astronomy
100
Stars: Atmospheres - White Dwarfs - Ultraviolet: Stars - X-Rays: Stars
Scientific paper
We present here a detailed study of a sample of hot I)A white dwarfs detected in the EUV and soft X-ray bands by ROSAT during its all-sky survey. The survey data are combined with additional ROSAT pointed-phase observations, earlier EXOSAT results and the available optically determined effective temperatures and gravities. We show that the spectra of those stars with temperatures below ≍40 000 K are well described by nearly pure H atmospheres, with only very small traces of additional opacity due to He allowed by the data. Above ≍40 000 K, however, neither homogeneous nor stratified H + He models can, in general, explain the observed EUV and X-ray fluxes. We conclude that additional sources of opacity in the form of trace metals must be present in the photospheres of these stars. The disappearance of this material when the stars cool below 40 000-50 000 K is in agreement with theoretical radiative levitation calculations. We propose that these results are direct observational evidence that the composition of most DA white dwarf atmospheres is dominated by the balance between gravitational and radiative forces. Limits on the H-layer mass obtained for the cooler stars indicate that He makes only a minimal, if any, contribution to the EUV/soft X-ray opacity in the phosphere.
Barstow Martin Adrian
Diamond C. J.
Finley David S.
Fleming Thomas Anthony
Holberg M. C.
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