NON-LTE MODEL ATMOSPHERE ANALYSIS OF NOVA CYGNI 1992

We use spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres to analyze the IUE and optical spectra of Nova Cygni 1992 during the early phases of its outburst. We find that the first IUE s pectrum obtained just after discovery on 1992 February 20, is best rep roduced by a model atmosphere with a steep density gradient and homolo gous expansion, whereas the IUE and optical spectra obtained on Februa ry 24 show an extended, optically thick, wind structure. Therefore, we distinguish two phases of the early evolution of the nova photosphere : the initial, rapid, ''fireball'' phase and the subsequent, much long er, optically thick ''wind'' phase. The importance of line-blanketing in nova spectra is demonstrated. Our preliminary abundance analysis im plies that hydrogen is depleted in the ejecta, corresponding to abunda nce enhancements of Fe by a factor of almost-equal-to 2 and of CNO by more than a factor of 10 when compared to solar abundances. The synthe tic spectra reproduce both the observed pseudo-continua as well as mos t of the observed features from the UV to the optical spectral range a nd demonstrate the importance of obtaining nearly simultaneous UV and optical spectra for performing accurate analyses of expanding stellar atmospheres (for both novae and supernovae).