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Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infraredexcesses in the spectral energy distributions of HE 0110-5630, GD 61, and HE1349-2305. All three of these stars have helium-dominated atmospheres, andtheir infrared emissions are consistent with warm dust produced by the tidaldestruction of (minor) planetary bodies. This study brings the number ofmetal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRACto 53 and 38 respectively. It also nearly doubles the number of metal-pollutedhelium-rich white dwarfs found to have closely orbiting dust by Spitzer. Fromthe increased statistics for both atmospheric types with circumstellar dust, wederive a typical disk lifetime of log[t_{disk} (yr)] = 5.6+-1.1 (ranging from3*10^4 - 5*10^6 yr). This assumes a relatively constant rate of accretion overthe timescale where dust persists, which is uncertain. We find that thefraction of highly metal-polluted helium-rich white dwarfs that have aninfrared excess detected by Spitzer is only 23 per cent, compared to 48 percent for metal-polluted hydrogen-rich white dwarfs, and we conclude from thisdifference that the typical lifetime of dusty disks is somewhat shorter thanthe diffusion time scales of helium-rich white dwarf. We also find evidence forhigher time-averaged accretion rates onto helium-rich stars compared to theinstantaneous accretion rates onto hydrogen-rich stars; this is an indicationthat our picture of evolved star-planetary system interactions is incomplete.We discuss some speculative scenarios that can explain the observations.

Constraints on the Lifetimes of Disks Resulting from Tidally Destroyed Rocky Planetary Bodies