Spectroscopic Evidence from HST and Archival IUE Studies of Dwarf Novae as Past Classical Novae

Abstract

I present recent spectroscopic evidence suggesting the past occurences of thermonuclear runaways on the accreting white dwarfs in certain dwarf novae both above and below the orbital period gap. This information is based upon synthetic spectral analyses of HST and IUE archival spectra obtained when the underlying white dwarf photosphere is detected spectroscopically or when peculiar N V/C IV emission line ratios, associated with the accretion disk or accretion column, appear. Hubble STIS observations of VW Hydri 2 days and 7 days after the end of a superoutburst reveal a heated white dwarf with deep broad Lymanα, narrow metallic absorption features and evidence of a hotter Keplerian‐broadened component. These spectra confirm a large N/C ratio and the existence of enhanced abundances of odd‐numbered nuclear species P, Mn and Al as well as a N/C ratio indicative of proton capture by even‐numbered nuclear species outside the CNO by‐cycle during the CNO H‐burning thermonuclear processing. These STIS observations confirm that a past (pre‐historic?) thermonuclear runaway has occured on the white dwarf in VW Hyi. It is expected that the thermonuclear runaway would be strong enough to produce a nova outburst. Therefore, these two classes of close binaries, namely dwarf novae and classical novae, are linked and can overlap. Additional evidence for such a link to novae is also discussed for the dwarf nova U Gem, nova‐like variables in low optical brightness states of little or no accretion and in dwarf novae with very peculiar N V/C IV emission line ratios.