The Frequency of Debris Disks at White Dwarfs
  We present near- and mid-infrared photometry and spectroscopy from PAIRITEL,
IRTF, and Spitzer of a metallicity-unbiased sample of 117 cool,
hydrogen-atmosphere white dwarfs from the Palomar-Green survey and find five
with excess radiation in the infrared, translating to a 4.3+2.7-1.2% frequency
of debris disks. This is slightly higher than, but consistent with the results
of previous surveys. Using an initial-final mass relation, we apply this result
to the progenitor stars of our sample and conclude that 1-7Msol stars have at
least a 4.3% chance of hosting planets; an indirect probe of the
intermediate-mass regime eluding conventional exoplanetary detection methods.
Alternatively, we interpret this result as a limit on accretion timescales as a
fraction of white dwarf cooling ages; white dwarfs accrete debris from several
generations of disks for ~10Myr. The average total mass accreted by these stars
ranges from that of 200km asteroids to Ceres-sized objects, indicating that
white dwarfs accrete moons and dwarf planets as well as Solar System asteroid
analogues.
