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Circumstellar Disks at White Dwarfs: Observations

The current picture painted by the observations of circumstellar dust at white dwarfs, and the consequent atmospheric pollution, is of a surviving planetary system. This chapter recounts in detail both the discovery and empirical characterization of dust disks at single white dwarfs, including all observational data available up to early 2011. Observations of the disks themselves and the accreted heavy elements within the stellar photospheres are consistent with tidally disrupted asteroid analogs. The observed chemistry supports objects that condensed within the inner regions of main-sequence stars and thus far appear similar in constitution to the terrestrial planets and their building blocks.