Metal abundances in very hot DA white dwarfs -- a test of diffusion theory

Details Metal abundances in very hot DA white dwarfs -- a test of diffusion theory Metal abundances in very hot DA white dwarfs -- a test of diffusion theory

Jul 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8636n&link_type=abstract

HST Proposal ID #8636

Physics

Hst Proposal Id #8636 Hot Stars

Scientific paper

The photospheric abundances of white dwarfs are controlled by diffusion processes like gravitational settling and radiative levitation. UV, EUV and X-ray observations revealed a large metallicity spread from star to star, which cannot be explained by state-of-the-art stationary diffusion calculations. Additional processes, e.g. a weak stellar wind, have been invoked. A contrasting pair of extremely hot DA white dwarfs, recently discovered by the HS survey, has very similar temperature and gravity, but probably very different metal abundances. HS 0505+0112 is unique because it is the only H-rich hot white dwarf known to show optical C lines. Balmer line fitting yields further evidence that the heavy element abundances of HS0505+0112 must be high. In contrast HS 0615+6535 displays Balmer lines only and fitting their line profiles is consistent with an unusually low amount of metals. We propose to determine abundances of C, N, O, Si and Fe and Ni directly from STIS UV spectra by means of fully metal line blanketed NLTE model atmospheres. Indications of mass loss will also be searched for providing us with an important check of the diffusion theory in white dwarfs. Both stars are also ideally suited for a test, whether the C, N, O line opacity can account for the Balmer line discrepancy noted for the analysis of very hot H-rich stars.

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