ON THE INTERPRETATION OF THE ULTRAVIOLET-SPECTRA OF SYMBIOTIC STARS AND RECURRENT NOVAE .2. THE 1985 OUTBURST OF RS OPHIUCHI

We discuss the 1985 outburst of the symbiotic recurrent nova RS Oph us ing spectra obtained with the International Ultraviolet Explorer satel lite and contemporaneous ground-based optical spectra. The low-resolut ion integrated fluxes show that a short-lived constant bolometric lumi nosity phase existed in the early outburst. If this reached the Edding ton luminosity, it implies a minimum mass for the white dwarf of 1.2 M .. This and the lack of evidence for eclipses in ultraviolet low-resol ution spectra obtained in quiescence support the orbital solution of D obrzycka & Kenyon (1994), in particular the low inclination of the sys tem and the low mass, 0.5 M., of the red giant. The high-resolution op tical and ultraviolet line profiles showed the development of two sepa rate contributors. One was a broad-line component produced by emission from the high-velocity ejecta. The other was a narrow-line component produced in the portion of the red giant wind that was ionized by the UV pulse from the explosion and by radiation from the shock produced b y the passage of the ejecta through the wind. The expanding Stromgren sphere had two effects. The strength of the line absorption from the o verlying wind decreased against the ejecta and the ionized emitting ga s, and the emission measure increased within the expanding H II region . We model these effects using the techniques developed in Shore & Auf denberg (1993) to show how the column density of the cool material dec reased with time. We show that the increased ionization of the wind ac counts for changes in the absorption line components in the ultraviole t and use this decrease and the narrow emission-line ratios to determi ne the physical parameters for the line-forming region in the wind. Fr om this, we derive the mass-loss rate for the red giant. In addition, we find evidence for a nitrogen overabundance in the wind using the ti me development of the N v 1240 Angstrom doublet. The primary source fo r ionizing the red giant wind was radiation produced by the shocked ej ecta as they traversed the stellar wind. The shock-generated emission also produced and powered the coronal species until about 100 days aft er ourburst at which time shock breakout occurred. The ionized wind su bsequently recombined, although a hot source was still present on the white dwarf on the basis of continued visibility of the ultraviolet O III fluorescence lines and the IR He I 1.08 mu m line. Finally, we dis cuss how many of the techniques developed in this study of a photoioni zing pulse propagating into a dense environment can be applied to the analysis of active galactic nuclei.