Spitzer Observations of the Oldest White Dwarfs in the Solar Neighborhood

We present Spitzer 5–15 μm spectroscopy of one cool white dwarf and 3.6–8 μm photometry of 51 cool white dwarfs with Teff < 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, nearand 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 μm 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 nondetection 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 %.