OPTICAL SPECTRAL EVOLUTION OF NOVA-CYGNI-1992=V-1974-CYGNI

We report the results of the optical spectrophotometric observations o f N Cyg 1992 = V 1974 Cyg performed at Asiago between 6 and 670 days a fter its visual maximum (1992 February 22, m(v) similar to 4.4). On 19 92 February 28, during the first decline the spectrum of the nova was characterized by the presence over a strong continuum of wide emission lines of HI (Balmer), FeII, NaI, CaII, flanked by two systems of P Cy g absorptions with mean radial velocities of -1250 and -1900 km/s. The transition phase was reached at the beginning of April with a gradual fading of the lines of lower ionization potential and the emerging of the HeI, NII, NIII lines. P Cygni absorption features with radial vel ocity higher than in February were still perceptible. The nova entered the nebular stage some weeks later, at the end of April. The presence in the spectrum, besides [OIII]lambda lambda 4959, 5007, of strong fo rbidden lines of [NeIII] at lambda lambda 3869, 3968, left no doubt of its classification as a 'neon nova' similar to QU Vul. The evolution of the spectrum in the next months was characterized by the progressiv e growth of the ionization degree. Following the forbidden lines of [N eIII] and [OIII], also those of [NeIV], [FeVI], [FeVII] and [NeV] beca me outstanding. The highest degree of ionization was attained on 1993 July, about 500 days from visual maximum, when the coronal lines of [F eX], [FeXI], [AX] reached their highest strength. The steadily decline of the degree of ionization began about one-hundred days later, with the rapid disappearance of the coronal lines and the progressive fadin g of the lines of highest ionization potential. The intensities of the emission lines during the nova evolution are reported in Tables 2 and 3. They are represented in Fig. 3, which shows the temporal developme nt of the line fluxes corrected for extinction. The discussion concern s the determination of: 1. Color excess (E(B)-V similar to 0.17), abso lute magnitude and distance (2.8 kpc) of the nova. 2. Effective temper ature of the contracting photosphere (from 170 000 to 650 000 K). 3. A bundance in the ejecta of He, O, N, and Ne, relative to H. It has been found that these elements, and particularly neon, are overabundant co mpared to the solar values.