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Discovery of Photospheric Calcium Line Strength Variations in the DAZd White Dwarf G29-38

Metals in the photospheres of white dwarfs with Teff between 12,000 and 25,000 K should gravitationally settle out of these atmospheres in 1-2 weeks. Temporal variations in the line strengths of these metals could provide a direct measurement of episodic metal accretion. Using archival VLT and Keck spectroscopy, we find evidence that the DAZd white dwarf G29-38 shows significant changes in its Ca II K line strength. At the two best-observed epochs, we find that the Ca line equivalent width (EW) = 165 +- 4 mA (in 1996.885) and 280 +- 8 mA (in 1999.653), which is an increase of 70%. We consider the effect that pulsation has on the Ca EWs for this known variable star, and find that it adds an error of < 1 mA to these measurements. Calcium line strengths at other observational epochs support variations with timescales as short as two weeks. These Ca EW variations indicate that the metal accretion process in G29-38, presumably from its debris disk, is episodic on timescales of a few weeks or less, and thus the accretion is not dominated by Poynting-Robertson drag from an optically thin, continuous disk, which has a timescale of ~1 year.