Spitzer Irac Observations of White Dwarfs. I. Warm Dust at Metal-rich Degenerates

This paper presents the results of a Spitzer IRAC 3−8 μm photometric search for warm dust orbiting 17 nearby, metal-rich white dwarfs, 15 of which apparently have hydrogen dominated atmospheres (type DAZ). G166-58, G29-38, and GD 362 manifest excess emission in their IRAC fluxes and the latter two are known to harbor dust grains warm enough to radiate detectable emission at near-infrared wavelengths as short as 2 μm. Their IRAC fluxes display differences compatible with a relatively larger amount of cooler dust at GD 362. G166-58 is presently unique in that it appears to exhibit excess flux only at wavelengths longer than about 5 μm. Evidence is presented that this mid-infrared emission is most likely associated with the white dwarf, indicating that G166-58 bears circumstellar dust no warmer than T ∼ 400 K. The remaining 14 targets reveal no reliable midinfrared excess, indicating the majority of DAZ stars do not have warm debris disks sufficiently opaque to be detected by IRAC. Subject headings: circumstellar matter— infrared: stars– minor planets, asteroids— planetary systems – stars: abundances— stars: evolution— stars: individual (G166-58, G29-38, GD 362, PG 0235+064)— white dwarfs