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The cool white dwarf SDSS J124231.07+522626.6 exhibits photosphericabsorption lines of 8 distinct heavy elements in medium resolution opticalspectra, notably including oxygen. The Teff = 13000 K atmosphere ishelium-dominated, but the convection zone contains significant amounts ofhydrogen and oxygen. The four most common rock-forming elements (O, Mg, Si, andFe) account for almost all the accreted mass, totalling at least 1.2e+24 g,similar to the mass of Ceres. The time-averaged accretion rate is 2e+10 g/s,one of the highest rates inferred among all known metal-polluted white dwarfs.We note a large oxygen excess, with respect to the most common metal oxides,suggesting that the white dwarf accreted planetary debris with a water contentof ~38 per cent by mass. This star, together with GD 61, GD 16, and GD 362,form a small group of outliers from the known population of evolved planetarysystems accreting predominantly dry, rocky debris. This result strengthens thehypothesis that, integrated over the cooling ages of white dwarfs, accretion ofwater-rich debris from disrupted planetesimals may significantly contribute tothe build-up of trace hydrogen observed in a large fraction of helium-dominatedwhite dwarf atmospheres.

Likely detection of water-rich asteroid debris in a metal-polluted white dwarf