Optical spectral evolution of Nova Cygni 1992 = V 1974 Cygni

We report the results of the optical spectrophotometric observations of N Cyg 1992 = V 1974 Cyg performed at Asiago between 6 and 670 days after its visual maximum (1992 February 22, m_v approximately 4.4). On 1992 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 Cyg 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 velocity 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) λλ 4959, 5007, of strong forbidden lines of (NeIII) at λλ 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 progressive growth of the ionization degree. Following the forbidden lines of (NeIII) and (OIII), also those of (NeIV), (FeVI), (FeVII) and (NeV) became outstanding. The highest degree of ionization was attained on 1993 July, about 500 days from visual maximum, when the coronal lines of (FeX), (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 fading of the lines of highest ionization potential. The intensities of the emission lines during the nova evolution are reported. The discussion concerns the determination of: color excess, absolute magnitude and distance (2.8 kpc) of the nova; effective temperature of the contracting photosphere; and abundance in the ejecta of He, O, N, and Ne, relative to H