On the Origin of Metals in Some Hot White Dwarf Photospheres
  We have searched for evidence for dust and gas disks at a sample of hot DA white dwarfs 20 000K < Teff < 50 000K, without success. Although their atmospheres are polluted with heavy elements, we cannot yet convincingly and conclusively show that any of these objects is accreting metals from surrounding material derived from disrupted minor planets in an old solar system. 
M.R. Burleigh, M.A. Barstow, J. Farihi, N.P. Bannister, N. Dickinson,   P.R. Steele, P.D. Dobbie, F. Faedi, B.T. G\"ansicke
astro-ph.EP astro-ph.SR