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J. Dupuis (UCB), S. Vennes (ANU), P. Chayer (UVIC/JHU)
The hot DA white dwarf GD 246 is one of the brightest sources detected during the Extreme Ultraviolet Explorer (EUVE) survey. The extreme-UV emission from GD 246 is understood in terms of thermal radiation from its highly pure hydrogen photosphere. With \rm Teff ~55,000K and \log g ~7.8, the atmosphere of GD~246 is expected to contain detectable abundance of elements heavier than hydrogen on the basis of diffusion and radiative levitation theory. This expectation is strengthened by the detection of photospheric features associated to C, Si, and P in the FUSE and IUE spectra of GD 246 and by the failure of pure hydrogen synthetic white dwarf spectra to reproduce the EUVE spectrum. It has been suggested that the combined effect of trace abundances of iron-group elements may explain the strong flux deficiency observed in the EUV range. A lack of spectral resolution and inaccuracies in atomic line data have forbidden a definitive identification of the missing opacity sources in the EUVE spectra of GD 246.
To address this problem further, we have requested a spectrum of GD~246 with the CHANDRA LETG. With a better sensitivity and a spectral resolution about 5 times better than EUVE, CHANDRA may potentially resolve this problem. We observed GD~246 with CHANDRA on January 14, 2000 for a duration of 39950 s. We will present a preliminary analysis of the LETG spectrum of GD~246 and a comparison with the EUVE spectrum. Flux measurements, spectral line identifications and abundance measurements in both spectroscopic sets are critically analyzed and compared. We report the possible detection of Fe~VI absorption features. This indicates that iron may play a key role in explaining the missing EUV opacity.
This work is supported by NASA CHANDRA guest observer grant GO0-1083X and by NASA LTSA grant NAG-6551. S.V. is a QE II of the Australian Research Council.