INFRARED SIGNATURES OF DISRUPTED MINOR PLANETS AT WHITE DWARFS

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T eff ∼< 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 x 108 g s-1 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 106 g s-1 in the solar system zodiacal cloud and 1010 g s-1 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.