The Mystery Deepens:SpitzerObservations of Cool White Dwarfs

We present 4.5$\mu$m and 8$\mu$m photometric observations of 18 cool whitedwarfs obtained with the Spitzer Space Telescope. Our observations demonstratethat four white dwarfs with T_eff< 6000 K show slightly depressed mid-infraredfluxes relative to white dwarf models. In addition, another white dwarf with apeculiar optical and near-infrared spectral energy distribution (LHS 1126) isfound to display significant flux deficits in Spitzer observations. Thesemid-infrared flux deficits are not predicted by the current white dwarf modelsincluding collision induced absorption due to molecular hydrogen. We postulatethat either the collision induced absorption calculations are incomplete orthere are other unrecognized physical processes occuring in cool white dwarfatmospheres. The spectral energy distribution of LHS 1126 surprisingly fits aRayleigh-Jeans spectrum in the infrared, mimicking a hot white dwarf witheffective temperature well in excess of 10$^5$ K. This implies that the sourceof this flux deficit is probably not molecular absorption but some otherprocess.Comment: 17 pages, 4 figures, ApJ in press, 10 May 200