Nova Cygni 1978 - I. The nebular phase

Abstract

UV observations of Nova Cygni 1978, which were obtained using IUE on 17 dates between 1978 September 12 and 1979 July 8 inclusive, will be discussed in the present series of papers. Paper I is mainly concerned with the interpretation, during the nebular stage, of the UV observations together with optical and IR data from other observers. The adopted value of the reddening, E(B – V) = 0.4 ± 0.1 from the 2200 Å feature, is in satisfactory agreement with values deduced by other methods. The total observed flux, F(obs), is obtained from UV, optical and IR observations as a function of D, the number of days after outburst. The maximum value, F(max), of F(obs) occurs on D = 6 and between D = 13 and D = 27, F(obs) has an approximately constant value of 0.30 F(max); subsequently F(obs) declines. The estimated Eddington luminosity of the remnant is L(E) = 5 × 104L⊙. The luminosity L(max) obtained from F(max) is equal to L(E) for a distance of d = 2.2 kpc. This value is adopted; it is in good agreement with the value obtained by Duerbeck et al. from the relation between reddening and distance in the direction to the nova, but less than the value of d = 3.6 kpc deduced from the relation between decay time and maximum absolute visual magnitude. The UV spectra show emission lines of: He II; CII, III and IV; N II, III, IV and V; O I, II, III, IV and V; and Mg II. The lines C IIλ 1335, C IIIλ 2297 and N IVλ 1718 are produced by dielectronic recombination via low-lying autoionizing states. Electron temperatures are deduced from the flux ratios (C IIλ 1335)/(C III] λ 1908), (C IIIλ 2297)/(C IVλ 1549) and (N IVλ 1718)/ (N Vλ 1240). Ion abundances relative to He2+ are obtained for three ionization stages of C, four of N, and four of O. Abundances relative to H+ are obtained using optical data of Klare et al. and IUE observations of the H I Balmer continuum. Element abundances relative to H, by number, are 0.12 for He, 0.008 for C, 0.021 for N and 0.017 for O. They are enhanced, relative to the Sun, by factors of 20 for total CNO and 200 for N, indicating that the nova was produced by a thermonuclear runaway. The best observations for the UV continuum, obtained on D = 70, are interpreted as due to recombination and free–free emission of ions of H, He, C, N and O and radiation from a stellar remnant with a temperature of 1.5 × 105 K and a radius of 0.13 R⊙. There is reasonable agreement between values of the ionized ejected mass, Mi, estimated using electron densities deduced from forbidden lines and estimated using measured expansion velocities. The value adopted is Mi = 1.1 × 1029 g and the corresponding kinetic energy is 6 × 1044 erg. The ionized mass may be equal to the total ejected mass, but this is not certain. The onset of thermal infrared emission is probably due to pre-existing dust heated by absorption of radiation in UV resonance lines.