THE EARLY ULTRAVIOLET SPECTRAL EVOLUTION OF NOVA CYGNI-1992

We report on the high resolution ultraviolet line profile evolution of Nova Cygni 1992 using the International Ultraviolet Explorer satellite and the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope. Our observations cover the period from discovery in 1992 February through 1993 April. The initial expansion velocity, derived from P Cyg profiles on Mg II and other resonance lines, was about 4500 km s-1. All resonance transitions displayed P Cyg profiles around the time of UV maximum. We first discuss the spectral development as the initially optically thick atomic absorption curtain became optically thin. We then present the interpretation of the nebular spectral stage. The high resolution line profiles show that the shell rapidly developed nearly symmetric knots, or filaments, of emission. The optically thin transitions show an emission weighted expansion velocity of about 1800 km s-1, consistent with the velocity inferred from the radio and infrared data. The emission lines display a symmetric set of filaments that appeared as soon as the line profiles became optically thin. The GHRS observations demonstrate that the filaments were located in the inner, slower moving, parts of the ejecta. These probably record an initial low wave number instability imposed on the ejecta at the time of outburst. We suggest that the likely cause is a Rayleigh-Taylor instability. The emission at later stages is dominated by the filaments and, since this phenomenon is seen in virtually all novae, nebular line formation codes will have to account for the density inhomogeneities in order to adequately determine abundances for nova ejecta. The derived ejecta mass exceeds 10(-4)Y-1/2 M. where Y is the helium abundance enhancement factor, consistent with the long UV rise time. No simulations of accretion onto massive ONeMg white dwarfs can eject such massive envelopes; there is a serious disagreement between the observations and current theory.