Abstract

Several dozen white dwarfs have close-in dusty discs that are visible as an infrared excess over the photospheric emission of the white dwarf. In a small number of systems an additional gaseous component is detected by double-peaked line emission emission in the CaII 8600A triplet. The dynamical analysis of the line profile shape shows that the radial extent of the gas overlaps with that of the dust, both residing within the tidal disruption radius of the white dwarf. The origin of these gaseous discs is not well understood: on the one hand, sublimation of the dust by irradiation from the white dwarf is insufficient to heat the gas to the temperatures implied by the CaII emission, and on the other hand most white dwarfs with dusty discs do not show evidence for a gaseous component. Here we present time-series spectroscopy of the DA white dwarf SDSS J1617+1620 obtained from 2006 to 2013, which shows the transient appearance of strong, double-peaked CaII emission lines in 2008, and only very weak lines before and after that epoch. Possible explanations for this unprecedented variability include the impact of a small planetesimal onto an existing circumstellar dust disc, or the periodic passage of a debris tail in an eccentric orbit.

Presentation

Poster