The frequency of planetary debris around young white dwarfs

Abstract

(Abridged) We present the results of the first unbiased survey for metal pollution among H-atmosphere (DA) white dwarfs with cooling ages of 20-200 Myr and 17000K < Teff < 27000K, using HST COS in the far UV between 1130 and 1435 A. The atmospheric parameters and element abundances are determined using theoretical models, which include the effects of element stratification due to gravitational settling and radiative levitation. We find 48 of the 85 DA white dwarfs studied, or 56% show traces of metals. In 25 stars, the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%) must be currently accreting. Together with previous studies, we find no accretion rate trend in cooling age from ~40 Myr to ~2 Gyr. The median, main sequence progenitor of our sample corresponds to a star of ~2 Msun, and we find 13 of 23 white dwarfs descending from 2-3 Msun late B- and A-type stars to be currently accreting. Only one of 14 targets with Mwd > 0.8 Msun is found to be currently accreting, which suggests a large fraction are double-degenerate mergers, and the merger discs do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two white dwarf Hyads are currently accreting rocky debris. At least 27%, and possibly up to ~50%, of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris. At Teff > 23000K, the luminosity of white dwarfs is likely sufficient to vaporize circumstellar dust, and hence no stars with strong metal-pollution are found. However, planetesimal disruption events should occur in this cooling age and Teff range as well, and likely result in short phases of high mass transfer rates. It appears that the formation of rocky planetary material is common around 2-3 Msun late B- and A-type stars.