Abstract

Almost all main-sequence stars, including the Sun, will eventually become white dwarfs. During the giant-stages of the stellar evolution, planetary objects within about 1 AU will be engulfed, while those further out will survive to the white dwarf stage. Rocky-planetesimals (i.e. asteroids/minor planets) that are accreted onto the central degenerate-star, lead to white dwarf atmospheres enriched with metals, rather than the pure hydrogen/helium composition expected from gravitational settling of elements. The spectra of these “metal-polluted” white dwarfs therefore provide a unique tool for analysing the bulk compositions of rocky material outside of the Solar-system.

Using SDSS spectra, we have identified 231 white dwarfs that are both old (cooling ages 1—8 Gyr), and have atmospheres rich in metallic elements. Abundance analyses of the spectra reveal wide ranging compositions from crust-like to core-like, with the most extreme cases indicating highly differentiated planetesimals. Additionally, the old ages of the systems we identify have allowed us to probe remnant planetary system evolution. We find compelling evidence that white dwarfs deplete their planetesimal reservoirs on a ~1 Gyr e-folding timescale, an effect not seen in other studies which have predominantly focussed on much younger white dwarfs.