Institute of Astronomy

Evolved planetary systems around very cool and old white dwarfs

SpeakerTalk DateTalk Series
Mark Hollands28 July 2014Across HR 2014 Posters


Metal-pollution of white dwarfs (WDs) are the signposts of evolved planetary systems. Rocky planetesimals venturing too close to the WD are tidally disrupted, forming a dust disc that is subsequently accreted by the WD. The photospheric abundances provide powerful insight into the chemical composition of the parent bodies. We present a sample of 59 cool and old, strongly metal-polluted WDs identified from Sloan Digital Sky Survey (SDSS) spectroscopy. These stars evaded prior detection as the strong absorption lines in the blue part of their spectra dramatically alter their colours, mixing them into the colour space of intermediate-redshift quasars. In the atmospheres of most of these WDs we identify calcium, magnesium, sodium and iron, however their relative abundances vary significantly. The minimum masses that have been accreted regularly exceed those of the largest solar system asteroids. The coolest of these objects is below 5000 K, corresponding to a cooling age in excess of 7 Gyr. This provides a lower limit for how long ago rocky material began forming within the Galaxy, and, more generally, studying these old systems offers insight into terrestrial planet formation. Major challenges in analysing these stars are their cool, dense atmospheres, where non-ideal effects become important, as well as the extremely broad (100s of A) line profiles that require new numerical treatment. We have obtained a first set of ground-based ultraviolet spectroscopy in order to test, and further develop the atmosphere models.