Photospheric soft X-ray emission from hot DA white dwarfs

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Emission Spectra, Photosphere, Stellar Spectrophotometry, White Dwarf Stars, X Ray Spectra, Abundance, H Beta Line, Iue, Proportional Counters, Stellar Evolution, Stellar Temperature, Ultraviolet Spectra

Scientific paper

Soft X-radiation (≡150 eV) from four hot DA white dwarfs (EG 187, Gr 288, Gr 289, and LB 1663) has been detected with the imaging proportional counter of the Einstein Observatory. The observed pulse-height spectra are very soft and suggest that the emission arises from the hot photosphere with implied effective temperatures in the range 30,000 - 60,000K. IUE ultraviolet spectra and Hβ line profiles have been obtained for all four stars, and fitted, along with the X-ray fluxes, with a grid of hot, high-gravity, homogeneous model atmospheres of mixed hydrogen-helium composition. The authors have also examined the limits imposed on the hot white dwarf population by the Einstein Medium Sensitivity Survey and derived constraints on the space densities of DA stars with effective temperatures exceeding 40,000 and 60,000K.

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