Abstract

Evolved planetary systems around very cool and old white dwarfs

Mark Hollands (University of Warwick)

Boris Gaensicke (University of Warwick) Detlev Koester (Univeristy of Kiel)

We have spectroscopically identifed a sample of 62 very cool (below 9000 K) and old (1-7 Gyr) DZ white dwarfs from the Sloan Digital Sky Survey. These stars have evaded prior detection as the extremely broad H and K lines in the blue part of their spectra dramatically alter their colours, mixing them into the colour-space of intermediate redshift quasars. In most of these stars we detect Ca, Mg, Fe and Na. However their relative abundances vary significantly. The coolest of these objects is below 5000 K and corresponds to a cooling age of approximately 7 Gyr. The only mechanism that can explain the large amounts of metal in the convection zones of these white dwarfs is accretion of planetary debris. Hence, these stars provide a lower limit on the onset of the formation of rocky material within the Milky Way, and, more generally, into the formation of early terrestrial planets. Additionally, we identify several of these objects to have strong (5-10 MG) magnetic fields. The origin these fields is not fully understood, but we notice that their typical field strengths appear to be substantially higher than those found among cool DAZ white dwarfs.

Mode of presentation: oral