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Discovery of Spectroscopic Variations in the DAB White Dwarf GD 323
We report the discovery of spectroscopic variations in GD 323, theprototypical DAB white dwarf. Simultaneous optical spectroscopicobservations over five consecutive nights of GD 323 and PG 1234+482, anonvariable comparison DA white dwarf of similar brightness, are used toreveal quasi-periodic variations in both the hydrogen and heliumabsorption lines over a timescale of hours. The amplitude of thevariation of the equivalent width of Hβ is ~30%. Moreover, thestrength of the hydrogen lines is shown to vary in opposite phase fromthat of He I λ4471. These results suggest that the modelcurrently thought to be the most viable to account for the simultaneouspresence of hydrogen and helium lines in GD 323, namely, a staticstratified atmosphere, may need to be reexamined. Instead, a model withan inhomogeneous surface composition, resulting perhaps from thedilution of a thin hydrogen atmosphere with the underlying heliumconvection zone, may be a better representation of GD 323. The observedvariation timescale of ~3.5 hr is consistent with the slow rotation rateof white dwarf stars.
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