The ultraviolet and X-ray view of the demise of nova V1974 Cygni

We present new data on the variations of the ultraviolet nitrogen lines during the late, optically thin stages of the outburst of V1974 Cygni. These show that, similar to 500 days after discovery, the ejecta reached maximum ionization and then started to recombine, coincident with the X-ray turnoff observed with ROSAT We derive densities for the ejecta at this stage and use these to discuss the development of the ejecta. The decline with time of the UV emission lines, especially He II lambda 1640, shows that the ejecta must have a linear velocity structure. This agrees with models for the ejection by an explosion and models that we have previously published for the line profiles. We then model 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 decreasing X-ray optical depth within the ejecta, as previously noted by Krautter 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 show that the final decline in the X-rays requires a substantial decrease in both the luminosity and temperature of the central star, in agreement with expectations for thermonuclear burnout on the surface of the white dwarf.