Mass-loss during evolution off of the main-sequence results in reduced dynamical stability at the white dwarf stage, increasing the probability of this scenario occurring (Veras & Gänsicke 2015). The now established scenario is that perturbations to the trajectories of small planetary objects such as asteroids can occasionally push them on to grazing orbits with the degenerate star (Debes & Sigurdsson 2002). 2015), typically through double-peaked emission lines of Ca ii and Fe ii. Gaseous components to these discs have also been identified at a handful of young metal-polluted white dwarfs (e.g. The infrared flux is interpreted to come from circumstellar dust discs that have thermally reprocessed the light incident from the close-by white dwarf (Graham et al. The first indicator pointing towards a planetary origin for metal pollution came from the detection of white dwarfs with infrared excesses (Zuckerman & Becklin 1987 Becklin et al. This astonishing realization has led to white dwarfs becoming the primary tool for directly probing the bulk compositions of rocky planetary material (Zuckerman et al. In the last two decades, it has become clear that these accreted metals originate from rocky debris that has survived the post-main-sequence evolution of its host star (Graham et al. 2003 Koester, Gänsicke & Farihi 2014) of white dwarfs requires recent or ongoing accretion of metal-rich material (Vauclair, Vauclair & Greenstein 1979). Therefore, the observed atmospheric contamination by metals at 25–50 per cent (Zuckerman et al. Due to gravitational settling, metals are expected to sink below the observable photosphere on time-scales many orders of magnitude shorter than the white dwarf cooling age (Koester 2009). These are classified as spectral types DAZ, DBZ and DZ, depending on whether hydrogen, helium or no lines are present in the spectrum in addition to those from metals (Sion et al. White dwarfs are often found to have traces of metals polluting their otherwise pristine atmospheres of hydrogen or helium (van Maanen 1917 Weidemann 1960 Hintzen & Tapia 1975 Shipman, Greenstein & Boksenberg 1977 Aannestad & Sion 1985 Zuckerman & Reid 1998 Koester & Kepler 2015).
Planets and satellites: composition, stars: abundances, stars: atmospheres, white dwarfs 1 INTRODUCTION
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