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

Metals in the photospheres of white dwarfs with TeA between 12,000 and 25,000 K should gravitationally settle out of these atmospheres in 1Y2 weeks. Temporal variations in the line strengths of these metals could provide a direct measurement of episodic metal accretion. Using archival VLTand Keck spectroscopy, we find evidence that the DAZd white dwarf G29-38 shows significant changes in its Ca iiK line strength. At the two best-observed epochs, we find that the Ca line equivalent width (EW) 1⁄4 165 4 m8 (in 1996.885) and 280 8 m8 (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 m8 to these measurements. Calcium line strengths at other observational epochs support variations with timescales as short as 2 weeks. These Ca EWvariations indicate that themetal 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. Subject headingg s: accretion, accretion disks — stars: individual (G29-39) — white dwarfs