A SPITZER SPACE TELESCOPE STUDY OF THE DEBRIS DISKS AROUND FOUR SDSS WHITE DWARFS

We present Spitzer Space Telescope data of four isolated white dwarfs that were previously known to harbor circumstellar gaseous disks. Infrared Array Camera photometry shows a significant infrared excess in all of the systems, SDSS0738+1835, SDSS0845+2257, SDSS1043+0855, and SDSS1617+1620, indicative of a dusty extension to those disks. The 4.5 mu m excesses seen in SDSS0738, SDSS0845, and SDSS1617 are 7.5, 5.7, and 4.5 times the white dwarf contribution, respectively. In contrast, in SDSS1043, the measured flux density at 4.5 mu m is only 1.7 times the white dwarf contribution. We compare the measured IR excesses in the systems to models of geometrically thin, optically thick disks, and find that we are able to match the measured spectral energy distributions to within 3 sigma of the uncertainties, although disks with unfeasibly hot inner dust temperatures generally provide a better fit than those below the dust sublimation temperature. Possible explanations for the dearth of dust around SDSS1043+0855 are briefly discussed. Including our previous study of SDSS1228+1040, all five white dwarfs with gaseous debris disks have significant amounts of dust around them. It is evident that gas and dust can coexist around these relatively warm, relatively young white dwarfs.