Abstract

Given the extremely high surface gravity of white dwarfs, their atmospheres are expected to be composed of pure H, He, or a mixture of both. The detection of photospheric metals unambiguously signals recent, or ongoing accretion, and debris from tidally disrupted planetary bodies is the most plausible origin of the polluting circumstellar material. We report the discovery of one of the most metal-polluted white dwarfs, SDSSJ1242+5226. The atmosphere of this star is He-dominated, and our intermediate resolution spectroscopy (3100-8500A) reveals absorption features of H, O, Na, Mg, Si, Ca, and Fe. Photospheric abundances determined our state-of-the-art model atmosphere code show that the circumstellar debris is rich in refractory elements, as typical for heavily polluted white dwarfs. However, our analysis implies a very large mass of H in the white dwarf envelope, reminiscent of two other strongly-polluted white dwarfs, GD362 and GD16. We argue that the common characteristics of GD362, GD16, and SDSSJ1242+5226 may be related to the accretion of large water-rich planetary bodies.

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