Spitzer Observations of the Oldest White Dwarfs in the Solar
  Neighborhood
  We present Spitzer 5-15 micron spectroscopy of one cool white dwarf and 3.6-8
micron photometry of 51 cool white dwarfs with T_eff < 6000 K. The majority of
our targets have accurate BVRIJHK photometry and trigonometric parallax
measurements available, which enables us to perform a detailed model atmosphere
analysis using their optical, near- and mid-infrared photometry with state-
of-the-art model atmospheres. We demonstrate that the optical and infrared
spectral energy distributions of cool white dwarfs are well reproduced by our
grid of models. Our best fit models are consistent with the observations within
5% in all filters except the IRAC 8 micron band, which has the lowest signal-
to-noise ratio photometry. Excluding the ultracool white dwarfs, none of the
stars in our sample show significant mid-infrared flux deficits or excesses.
The non-detection of mid-infrared excess flux around our 2-9 Gyr old targets
constrain the fraction of cool white dwarfs with warm debris disks to 0.8%
(+1.5% -0.8%).
