Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared excesses in the spectral energy distributions of HE 0110 - 5630 , GD 61 , and HE 1349 - 2305 .
All three of these stars have helium-dominated atmospheres , and their infrared emissions are consistent with warm dust produced by the tidal destruction of ( minor ) planetary bodies .
This study brings the number of metal-polluted , helium and hydrogen atmosphere white dwarfs surveyed with IRAC to 53 and 38 respectively .
It also nearly doubles the number of metal-polluted helium-rich white dwarfs found to have closely orbiting dust by Spitzer .
From the increased statistics for both atmospheric types with circumstellar dust , we derive a typical disk lifetime of \log [ t _ { \mathrm { disk } } \mathrm { ( yr ) } ] = 5.6 \pm 1.1 ( ranging from 3 \times 10 ^ { 4 } -5 \times 10 ^ { 6 } yr ) .
This assumes a relatively constant rate of accretion over the timescale where dust persists , which is uncertain .
We find that the fraction of highly metal-polluted helium-rich white dwarfs that have an infrared excess detected by Spitzer is only 23 per cent , compared to 48 per cent for metal-polluted hydrogen-rich white dwarfs , and we conclude from this difference that the typical lifetime of dusty disks is somewhat shorter than the diffusion time scales of helium-rich white dwarf .
We also find evidence for higher time-averaged accretion rates onto helium-rich stars compared to the instantaneous accretion rates onto hydrogen-rich stars ; this is an indication that our picture of evolved star-planetary system interactions is incomplete .
We discuss some speculative scenarios that can explain the observations .