SDSSJ084539.17+225728.0: the first DBZ white dwarf with a metal-rich gaseous debris disc

We report the discovery of a third white dwarf hosting a gaseous debris disc, SDSS J084539.17+225728.0. The typical double-peaked Ca II 8498,8542,8662Å emission lines can be modelled in terms of a Keplerian gas disc with a radial extent from ∼ 0.5R⊙ to ∼ 1.0R⊙. The effective temperature of SDSS 0845+2257, Teff ≃ 18600 ± 500K, is comparable to the two other white dwarfs with gaseous discs, SDSS 1043+0855 and SDSS 1228+1040, and hence substantially hotter than the bulk of white dwarfs where dusty debris discs were identified through the presence of infrared excess flux. This may suggest that the conditions to produce emission lines from debris discs in the optical wavelength range are only met for a relatively narrow range in Teff . The observed asymmetry in the line profiles indicates a substantial eccentricity in the disc. Two spectra obtained four years apart reveal a significant change in the shapes and equivalent widths of the line profiles, implying that the circumstellar disc evolves on relatively short time scales. In contrast to SDSS 1043+0855 and SDSS 1228+1040, SDSS 0845+2257 has a helium-dominated atmosphere. We detect photospheric absorption lines of He, Ca, Mg, and Si in the Sloan Digital Sky Survey spectrum, and hence classify SDSS 0845+2257 as DBZ white dwarf. The abundances for the three metals determined from model atmosphere fits are Ca/He ≃ 1.3× 10, Mg/He ≃ 6.0×10, and Si/He ≃ 8.0×10. From the non-detection of Hα we derive H/He < 3 × 10, which implies that the hydrogen-to-metal abundance ratio of the circumstellar material is & 1000 times lower than in the Sun. This lends strong support to the hypothesis that the gaseous and dusty debris discs found around roughly a dozen white dwarfs originate from the disruption of rocky planetary material.