A white dwarfaccreting planetary material determined from X-ray observations

The atmospheres of a large proportion of white dwarf stars are polluted by heavy elements' that are expected to sink out of visible layers on short timescales(2,3). This has been interpreted as a signature of ongoing accretion of debris from asteroids(4), comets(5) and giant planets(6). This scenario is supported bythe detection of debris discs(7) and transits of planetary fragments(8) around some white dwarfs. However, photospheric metals are only indirect evidence for ongoing accretion, and the inferred accretion rates and parent body compositions heavily depend on models of diffusion and mixing processes within the white dwarf atmosphere(9-11). Here we report a 4.4 sigma detection of X-rays from a polluted white dwarf, G29-38. From the measured X-ray luminosity, we derive an instantaneous accretion rate of M-x =1.63(-0.40)(+1.29) x 10(9) g s(-1), which is independent of stellar atmosphere models. This rate is higher than estimates from past studies of the photospheric abundances of G29-38, suggesting that convective overshoot may be needed to model the spectra of debris-accreting white dwarfs. We measure a low plasma temperature of k(B)T= 0.5 +/- 0.2 keV, corroborating the predicted bombardment solution for white dwarfs accreting at low accretion rates(12)(,13).

Automated summary

The atmospheres of white dwarf stars are often polluted by heavy elements, which are thought to come from debris accretion. This scenario is supported by the detection of debris discs and transits of planetary fragments. However, the presence of photospheric metals is only indirect evidence, and the actual accretion rates and parent body compositions depend on diffusion and mixing processes within the white dwarf atmosphere. In this study, X-ray emissions from a polluted white dwarf, G29-38, were detected and used to determine an instantaneous accretion rate that is independent of stellar atmosphere models. The measured rate is higher than previous estimates, suggesting the need to consider convective overshoot when modeling the spectra of debris-accreting white dwarfs. A low plasma temperature was also observed, supporting the predicted bombardment solution for white dwarfs accreting at low rates.