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Adding more mysteries to the DA white dwarf GD 394

We present spectroscopic and timing analyses of the hot DA white dwarf GD 394 showing abundance inhomogeneities across its surface. Lick Hamilton echelle, International Ultraviolet Explorer (IUE), HST GHRS, and Hopkins Ultraviolet Telescope (HUT) spectra show dominant Si III and Si IV features, while Extreme Ultraviolet Explorer (EUVE) spectra show evidence of a host of trace element opacities. We report the detection of Si III ?4552 triplet with a measured radial velocity of 27 km s-1 in agreement with far ultraviolet (FUV) line velocities, but indicating a silicon abundance (Si/H=2 x 10-5) a factor of 10 larger than measured in the FUV range [Si/H=(2-7) x 10-6]. Effective temperature measurements based on Lyman continuum (EUVE) and Lyman line series (HUT) are systematically cooler (?T~-4000 K) than measurements based on medium-dispersion Balmer line spectroscopy, an effect attributed to yet unidentified opacities. A timing analysis of EUVE deep-survey and scanner time series, as well as spectrometer data, shows GD 394 to be variable in the extreme ultraviolet (EUV) with an amplitude of 25% and a period of 1.150+/-0.003 days. The EUV variability suggests abundance inhomogeneities in the atmosphere, and we explore different models to explain its origin.

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