Nuclear turnoff times for ONeMG novae determined using ultraviolet spectral evolution

Classical novae go through a phase of constant bolometric luminosity as any remaining hydrogen in the accreted layers on the white dwarf is converted to helium. The length of this phase is dependent on many factors, including the amount of mass left on the white dwarf after the explosive phase of the outburst, the white dwarf's mass, and the efficiency of any secondary mass-loss mechanisms, such as radiation pressure-driven winds. Observations show that the end of nuclear burning is accompanied by a decline (or "turnoff") in the X-ray output of the novae, but this turnoff has been directly observed in the X-ray light curves of only two novae, GQ Mus and V1974 Cygni. In 1996, Shore, Starrfield, & Sonneborn showed that for V1974 Cyg the nuclear turnoff can also be indirectly obtained from the evolution of the ultraviolet emission lines. In this study, we analyze the IUE archive for novae that displayed a UV-line evolution like that of V1974 Cyg. We determine UV decline times and estimate nuclear turnoff times for the following novae: V693 Corona Austrinae 1981, QU Vul 1984, LMC 1990 No. 1, V838 Herculis 1991, and V351 Puppis 1991. The turnoff times of these novae, all of which occurred on ONeMg white dwarfs, were very rapid. Finally, the turnoff times are correlated with speed class such that the fastest novae show the shortest turnoff times.