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 spectrum obtained just after discovery on 1992 February 20, is best reproduced by a model atmosphere with a steep density gradient and homologous expansion, whereas the IUE and optical spectra obtained on February 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 longer, optically thick ''wind'' phase. The importance of line-blanketing in nova spectra is demonstrated. Our preliminary abundance analysis implies that hydrogen is depleted in the ejecta, corresponding to abundance 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 synthetic spectra reproduce both the observed pseudo-continua as well as most of the observed features from the UV to the optical spectral range and demonstrate the importance of obtaining nearly simultaneous UV and optical spectra for performing accurate analyses of expanding stellar atmospheres (for both novae and supernovae).