A Dusty Disk around WD 1150−153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks

We report the discovery of excess K-band radiation from a metal-rich DAVwhite dwarf star, WD1150-153. Our near infrared spectroscopic observations showthat the excess radiation cannot be explained by a (sub)stellar companion, andis likely to be caused by a debris disk similar to the other DAZ white dwarfswith circumstellar debris disks. We find that the fraction of DAZ white dwarfswith detectable debris disks is at least 14%. We also revisit the problem ofexplaining the metals in white dwarf photospheres by accretion from theinterstellar medium (ISM). We use the observed interstellar column densitiestoward stars in close angular proximity and similar distance as DAZ whitedwarfs to constrain the contribution of accretion from the ISM. We find nocorrelation between the accretion density required to supply metals observed inDAZs with the densities observed in their interstellar environment, indicatingthat ISM accretion alone cannot explain the presence of metals in nearby DAZwhite dwarfs. Although ISM accretion will certainly contribute, our analysisindicates that it is not the dominant source of metals for most DAZ whitedwarfs. Instead, the growing number of circumstellar debris disks around DAZssuggests that circumstellar material may play a more dominant role in pollutingthe white dwarf atmospheres.Comment: ApJ, in pres