The new class of dusty DAZ white dwarfs

Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescope and the IRAC imager has revealed an infrared excess associated with the white dwarf WD 2115-560 naturally explained by circumstellar dust. This object is the fourth white dwarf observed to have circumstellar dust. All four are DAZ white dwarfs, i.e., they have both photospheric Balmer lines and photospheric metal lines. We discuss these four objects as a class, which we abbreviate "DAZd," where the "d" stands for "dust." Using an optically thick, geometrically thin disk model analogous to Saturn's rings, we find that the inner disk edges are at ≳ 0.1-0.2 R⊙ and that the outer disk edges are ~0.3-0.6 R⊙. This model naturally explains the accretion rates and lifetimes of the detected WD disks and the accretion rates inferred from photospheric metal abundances.