THE ULTRAVIOLET AND X-RAY VIEW OF THE DEMISE OF NOVA V1974 CYGNI

We present new data on the variations of the ultraviolet nitrogen line s during the late, optically thin stages of the outburst of V1974 Cygn i. These show that, similar to 500 days after discovery, the ejecta re ached maximum ionization and then started to recombine, coincident wit h the X-ray turnoff observed with ROSAT We derive densities for the ej ecta at this stage and use these to discuss the development of the eje cta. The decline with time of the UV emission lines, especially He II lambda 1640, shows that the ejecta must have a linear velocity structu re. This agrees with models for the ejection by an explosion and model s that we have previously published for the line profiles. We then mod el the variations of the X-rays from this nova. We show that the ROSAT rise can be modeled by assuming a constant-luminosity central source, at approximately the Eddington limit for a massive white dwarf and an effective temperature of similar to 4 x 10(5) K, using only a decreas ing X-ray optical depth within the ejecta, as previously noted by Krau tter et al. This model can be generalized to explain the absence of X- ray emission during the early outburst stages of any nova. Last, we sh ow that the final decline in the X-rays requires a substantial decreas e in both the luminosity and temperature of the central star, in agree ment with expectations for thermonuclear burnout on the surface of the white dwarf.