Chemical Abundances of White Dwarfs in Cataclysmic Variables

Abstract

There is spectroscopic evidence suggesting the past occurences of thermonuclear runaways (TNRs) on the accreting white dwarfs in certain dwarf novae both above and below the orbital period gap. In the dwarf novae U Gem, VW Hyi and WZ Sge, synthetic spectral analyses of HST, FUSE and IUE archival spectra indicate supra-solar N and depleted C in their WD photospheres during dwarf nova quiescence. Sub-solar metal abundances (∼ 0.1to0.5× solar) have been reported in the white dwarf photospheres of 14 dwarf novae below the period gap but N abundances are lacking. Several CV white dwarfs reveal elevated abundances of odd-numbered nuclear species P, Mn and Al indicative of proton capture by even-numbered nuclear species outside the CNO by-cycle during the CNO H-burning thermonuclear processing. In addition, there are magnetic and non-magnetic systems having peculiar N V/C IV emission line ratios, associated with the accretion disk or accretion column. Possible nova shells have been detected around two dwarf novae reinforcing the evidence from chemical abundances that dwarf novae and classical novae are spectroscopically linked and can overlap.