On the interpretation of the ultraviolet spectra of symbiotic stars and recurrent novae. II. The 1985 outburst of RS Ophiuchi

We discuss the 1985 outburst of the symbiotic recurrent nova RS Oph using spectra obtained with the International Ultraviolet Explorer satellite and contemporaneous ground-based optical spectra. The low-resolution integrated fluxes show that a short-lived constant bolometric luminosity phase existed in the early outburst. If this reached the Eddington 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-resolution spectra obtained in quiescence support the orbital solution of Dobrzycka & Kenyon (1994), in particular the low inclination of the system and the low mass, 0.5 M_⊙, of the red giant. The high-resolution optical and ultraviolet line profiles showed the development of two separate 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 by the passage of the ejecta through the wind. The expanding Strömgren sphere had two effects. The strength of the line absorption from the overlying wind decreased against the ejecta and the ionized emitting gas, and the emission measure increased within the expanding H II region. We model these effects using the techniques developed in Shore & Aufdenberg (1993) to show how the column density of the cool material decreased with time. We show that the increased ionization of the wind accounts for changes in the absorption line components in the ultraviolet and use this decrease and the narrow emission-line ratios to determine the physical parameters for the line-forming region in the wind. From 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 time development of the N v 1240 Å doublet. The primary source for ionizing the red giant wind was radiation produced by the shocked ejecta as they traversed the stellar wind. The shock-generated emission also produced and powered the coronal species until about 100 days after ourburst at which time shock breakout occurred. The ionized wind subsequently 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 μm line. Finally, we discuss how many of the techniques developed in this study of a photoionizing pulse propagating into a dense environment can be applied to the analysis of active galactic nuclei.