Detection of Weak Circumstellar Gas around the DAZ White Dwarf WD 1124-293: Evidence for the Accretion of Multiple Asteroids

Abstract

Single metal polluted white dwarfs with no dusty disks are believed to be actively accreting metals from a circumstellar disk of gas caused by the destruction of asteroids perturbed by planetary systems. We report, for the first time, the detection of circumstellar Ca~II gas in absorption around the DAZ WD~1124-293, which lacks an infrared excess. We constrain the gas to $>$7 $R_{\rm WD}$ and $<$32000~AU, and estimate it to be at $\sim$54~R$_{\rm WD}$, well within WD~1124-293's tidal disruption radius. This detection is based on several epochs of spectroscopy around the Ca~II H and K lines ($λ$=3968Å, 3933Å) with the MIKE spectrograph on the Magellan/Clay Telescope at Las Campanas Observatory. We confirm the circumstellar nature of the gas by observing nearby sightlines and finding no evidence for gas from the local interstellar medium. Through archival data we have measured the equivalent width of the two photospheric Ca lines over a period of 11 years. We see $<$ 5-7\% epoch-to-epoch variation in equivalent widths over this time period, and no evidence for long term trends. The presence of a circumstellar gas implies a near edge-on inclination to the system, thus we place limits to short period transiting planetary companions with R $>$ R$_{\rm \oplus}$ using the WASP survey. The presence of gas in orbit around WD~1124-293 implies that most DAZs could harbor planetary systems. Since 25-30\% of white dwarfs show metal line absorption, the dynamical process for perturbing small bodies must be robust.