Extreme-ultraviolet spectrophotometry of HD 15638 and HR 8210 (IK Peg)

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Abstract
The main-sequence stars HR8210 (IKPeg) and HD 15638 have both been found to have DA white dwarf companions by observations with the IUE satellite, following their discovery as bright EUV sources by the ROSAT Wide Field Camera. Accurate estimates of the temperature and surface gravity of each white dwarf, based on the IUE data, were only possible when external constraints such as the distance to the primary were applied. We present Extreme Ultraviolet Explorer observations of each of these systems. From the resulting spectra we are able to measure the temperature and surface gravity of each white dwarf with much greater accuracy than possible with the earlier data. We demonstrate that the white dwarf atmospheres have H layers sufficiently thick that they appear as pure H DAs. There is no direct evidence for the presence of any additional material in the photospheres that might have come from interactions with the primaries. However, an inconsistency between the temperature of the HD 15638 white dwarf obtained from the EUVE spectrum, with a pure H model atmosphere, and that derived from the UV data might be explained by very small traces of heavy-element pollution. The distance we estimate for the white dwarf in HR 8210 is consistent with that to the A star, indicating that the A star has not departed from single-star evolutibn, nor has it evolved away from the main sequence. Measurements of the interstellar column densities yield He I/H I ratios consistent with cosmic abundances.

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