Infrared Signatures of Disrupted Minor Planets at White Dwarfs

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with Teff . 20, 000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457−086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt & 3 × 10 g s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 10 g s in the solar system zodiacal cloud and 10 g s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets. Subject headings: circumstellar matter— infrared: stars– minor planets, asteroids— planetary systems – stars: abundances— stars: evolution— stars: individual (GD 16, PG 1457−086)— stars: low-mass, brown dwarfs — white dwarfs