标题： X射线和红外观测两颗外部污染白矮星 显示英文标题

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

摘要： 利用XMM-Newton和斯皮策空间望远镜，我们获得了来自G29-38和GD 362的X射线流量上限以及来自G29-38的70微米流量。 这些数据间接表明G29-38正在从潮汐破坏的小行星吸积物质：它既没有大量吸积氢气和氦气，其周围的尘埃盘也没有从经历光压-罗伯逊拖拽的冷颗粒储库中得到补充。 GD 362的X射线流量上限与解释其被比氦重元素污染所需的质量吸积率一致。 GD 362还拥有相当于地球质量0.01倍的氢气，对于一个以氦为主的大气白矮星来说，这是一个异常大的数量。 一种可能性是，在当前盘形成之前，这些氢气是从大约100个类似谷神星的小行星或一个较大的天体吸积而来的。 另一种同时解释GD 362所有独特性质的情景是，我们正在见证单一天体因潮汐破坏的结果，该天体内部有水且至少像木卫二一样大，并且可能像火星一样大。 显示英文摘要

摘要： 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.