The First Mid-Infrared Spectra of Cool White Dwarfs

We present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3.5–8 μ m photometry for 19 cool white dwarfs with 5000 K ⩽ Teff ⩽ 9000 K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-infrared photometry and 7.5-14.5 μm spectrum of WD 0018–267 are consistent with a Teff = 5720 K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar, with significant mid-infrared flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 μm range. Even though this deficit is attributed to collision-induced absorption (CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of the LHS 1126 spectral energy distribution is best fit with a power-law index of –1.99, identical to a Rayleigh-Jeans spectrum. This argues that the deficit may be due to an unrecognized graylike opacity source in the infrared.