Solving the mystery of the heavy-element opacity in the DA white dwarf GD394

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Abstract
The white dwarf GD 394 is one of a very small number of bright hot DAs which have detectable quantities of highly ionized heavy elements in their spectra. Whether this material resides in the photosphere of GD 394 or is in the form of a circumstellar shell has been the subject of much debate, with the large discrepancy between the published radial velocity of the star and the measured velocities of the Si iii/iv lines having apparently settled the argument in favour of the latter idea. However, GD 394 is also the coolest DA white dwarf in the ROSAT sky survey showing convincing evidence for the presence of absorbing material which strongly suppresses the EUV continuum. This result has reopened the argument about the nature of the silicon lines. From a combined analysis of the IUE, HST and EUVE spectra, coupled with new measurements of the photospheric radial velocity from Hα data, we have established that the Si iii and Si iv lines are indeed photospheric. Furthermore, other heavy elements must be present to account for the EUV opacity, but at levels below the thresholds for detecting absorption lines in either far-UV or EUV bands.

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