Bolometric Evolution of Nova Cygni 1992 and Prediction of the EUV Flux from the Hot Remnant

We report on combined ultraviolet and optical observations of the early stages of the outburst of Nova Cygni 1992, the brightest Galactic nova in 17 years. The initial optical rise and decline are produced by a dramatic increase in UV line opacity and its gradual subsequent lifting as the ejecta expand. The source of the t₃-M_V relation is easily understood in light of these low resolution IUE spectra. The combined data confirm the theoretical prediction that a nova evolves at a constant bolometric luminosity in the early stages of outburst. The present data verifies this prediction for Nova Cygni for at least the first 60 days of the outburst. The discovery of far-UV (900-1300 Angstroms) radiation with the Voyager Ultraviolet Spectrometer and the detection of an increasing soft X-ray flux with ROSAT imply that the constant bolometric luminosity phase lasted for more than 6 months. Assuming that the phase of constant L_bol extends through the first year of the outburst, the observed UV decline indicates a steady rise in T_eff of the remnant of about 2% per month. These data predict that the temperature of the hot remnant is now T> 500,000 K. Given the estimated log N_H<6times 10(20) cm(-2) , it will be easily detected by the EUVE Short Wavelength spectrometer. Measurement of EUV fluxes from Nova Cygni will test the constant bolometric luminosity model for an ONeMg nova. It will constrain the white dwarf mass and provide the first substantial test of the theory of the post-explosion evolution of a nova remnant. EUVE observations of Nova Cygni are scheduled sometime in Summer 1993.