Spatially Resolved Spectra of V1974 Cygni (Nova Cygni 1992) with the Goddard High Resolution Spectrograph

Abstract

We report the observation of spatially resolved low-resolution GHRS spectra of the shell of the bright Galactic ONeMg nova Cygni 1992. The spectra were obtained in 1995 September, after the X-ray turnoff of the nova. We have determined the dynamical structure of the ejecta and show the evolution of the ionization within the ejecta. Abundance limits have been found to agree with the published analyses based on earlier IUE data for key species in the ejecta, especially neon. The knot spectrum, obtained with the 025 aperture, shows an enhanced Ne/He ratio compared with the 2'' aperture observation. These data provide a detailed picture of physical conditions and abundances of the knots and larger scale ejecta that have not previously been available from UV spectra, including those taken before the COSTAR installation and IUE data. The knots were created during the initial stages of the explosion, and the differential mixing that may have resulted can be studied using the spatially resolved spectra. The 1995 spectra were compared with archival Faint Object Spectrograph data from 1994 and previous GHRS spectra from 1992 and 1993. We also obtained high signal-to-noise ratio (S/N) observations of the UV continuum of the hot central star in order to study the physical conditions in the post-thermonuclear runaway envelope. These show that the white dwarf had relaxed to a very low effective temperature, about 20,000 K, and a luminosity of about 30 L☉, within 2 years after the X-ray turnoff. We also used reprocessed archival IUE spectra to determine the decay time for the ultraviolet continuum flux for this nova, which we find to be about 1.7 years. The current temperature and luminosity do not fall on the white dwarf cooling tracks for any reasonable stellar mass.