X-ray and Infrared Observations of Two Externally-Polluted White Dwarfs

With XMM-Newton and the Spitzer Space Telescope, we obtain upper bounds to the X-ray fluxes from G29-38 and GD 362, and the 70 micron flux from G29-38. These data provide indirect evidence that G29-38 is accreting from a tidally-disrupted asteroid: it is neither accreting large amounts of hydrogen and helium nor is its surrounding dusty disk being replenished from a reservoir of cold grains experiencing Poynting-Robertson drag. The upper bound to the X-ray flux from GD 362 is consistent with the estimated rate of mass accretion required to explain its pollution by elements heavier than helium. GD 362 also possesses 0.01 of an Earth's mass of hydrogen, an anomalously large amount for a white dwarf with a helium-dominated atmosphere. One possibility is that before the current disk was formed, this hydrogen was accreted from either about 100 Ceres-like asteroids or one large object. An alternative scenario which simultaneously explains all of GD 362's distinctive properties is that we are witnessing the consequences of the tidal-destruction of a single parent body that had internal water and was at least as massive as Callisto and probably as massive as Mars.