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 П K line strength. At the two best-observed epochs, we find that the Ca line equivalent width (EW) = 165 ̶ 4 mǺ (in 1996.885) and 280 ± 8 mǺ (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 \u3c1 \u3emǺ to these measurements. Calcium line strengths at other observational epochs support variations with timescales as short as 2 weeks. These Ca EW variations indicate that the metal accretion process inG29-38, 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 yr