Abstract

We will present the results of a first unbiased Spitzer survey of a homogeneous and well-defined sample of 140 white dwarfs in search of infrared excesses compatible with the presence of circumstellar dust discs formed from the disruption of planetesimals. The stars were selected without any regard to atmospheric metal content but chosen to have 1) hydrogen rich atmospheres, 2) 17000 < Teff < 25000 K and correspondingly young cooling ages of 20-120 Myr, and 3) UV brightness for a corresponding HST COS Snapshot survey. We find that four white dwarfs host a dust disc, three of which are previously known and one reported here for the first time, yielding a nominal 3% of white dwarfs with infrared excess in this range of post-main sequence ages. These stars largely descend from 2-3 solar mass, A-type stars and thus 3% is a firm lower limit on the fraction of such main-sequence stars that host rocky exoplanetary systems. Furthermore, our complementary HST observations indicate this fraction is likely to be roughly a factor of 10 higher, with the bulk of all circumstellar discs at white dwarfs remain hidden from current infrared detection methods. The presence of narrow and attenuated dust rings, and especially atmospheric pollution, corroborate this interpretation.

Presentation

Poster