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K.M. Vanlandingham, G.J. Schwarz, S. Starrfield (Arizona State University), S.N. Shore (Indiana University South Bend)
Hydrodynamical models of classical novae predict that stable nuclear burning should continue on the surface of the white dwarf for up to 100 years after the initial outburst. X-ray observations of GQ Mus and Nova Cyg 92 show much shorter turn-off times implying additional mass loss from the white dwarf must occur. Unfortunately, these two novae are the only ones for which accurate turn-off times have been measured. An indirect method for determining novae turn-off times using UV observations has been proposed by Shore, Starrfield & Sonneborn (1996). Shore et al.\ show that the slope of the flux ratio NV 1240Å/NIV] 1486Å\ and the slope HeII 1640Å\ line flux as a functions of time indicate the temporal evolution of the X-ray illumination by the central source. When the white dwarf returns to quiescence, there is no longer an ionization source. This means that the expansion of the ejecta, which produces a drop in the electron density, controls the recombination rate. Therefore, the ionization fractions are `frozen' into the ejecta. We have applied this method to the following ONeMg novae from the IUE archives: V693 Corona Austrinae 1981, V838 Herculis 1991, QU Vul 1984, and V351 Puppis 1991. Future monitoring of novae with AXAF will provide direct observational evidence of novae turn-off times.
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