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 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.