Determination of photospheric helium abundances for the hot white dwarfs LB 1663 and CD -38 deg 10980 from EXOSAT soft X-ray photometry

Details Determination of photospheric helium abundances for the hot white dwarfs LB 1663 and CD -38 deg 10980 from EXOSAT soft X-ray photometry Determination of photospheric helium abundances for the hot white dwarfs LB 1663 and CD -38 deg 10980 from EXOSAT soft X-ray photometry

Nov 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987apj...322..315p&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 322, Nov. 1, 1987, p. 315-319. ZWO-supported research.

Astronomy and Astrophysics

Astronomy

7

Abundance, Exosat Satellite, Hot Stars, Photosphere, White Dwarf Stars, X Ray Astronomy, Data Reduction, Helium, Stellar Atmospheres, X Ray Imagery

Scientific paper

The photospheric soft X-ray emission of hot (Te> 20,000 K) DA white dwarfs is very sensitive to the composition of the stars' atmospheres. Very small amounts of helium in a hydrogen-dominated atmosphere, amounts which are difficult or impossible to measure at UV and optical wavelengths, can be detected in the soft X-ray band. In this paper we present a measurement of the photospheric helium abundance of LB 1663 and CD - 38°10980, based on three-color soft X-ray photometry obtained with the low-energy imaging telescopes on the EXOSAT observatory, using constraints on Te and absolute flux levels derived from optical and UV observations. We find for CD -38°10980 that n(He)/n(H) < 5 x 10-6, almost independent of the other spectral parameters. For LB 1663 we find n(He)/n(H) = (1-2) x 10-4, in agreement with, but considerably more accurate than, an earlier result obtained with the Einstein IPC.

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