Spectrophotometry of RS Oph during the nebular phase⋆

Abstract

We present blue-wavelength spectroscopic observations of RS Oph, 201 day after the 1985 outburst. An analysis of the TiO bands indicates a spectral type M4III for the secondary and a distance of 2 kpc to the system. The system of forbidden lines is excited by a shock wave, whereas Balmer emission is mainly produced by photoionization. The source of photons is probably residual thermonuclear burning occurring at the surface of the white dwarf. The shell mass and filling factor at day 201 were approximately |$3.8\times10^{-5}\enspace M_\odot$| and 0.16, respectively. We deduce that the nova explosion ejected around |$10^{-6}\enspace M_\odot$|⁠, and estimate a wind mass-loss rate of |$1.8\times10^{-6}\enspace M_\odot$| yr−1 for the secondary. The last estimate is two orders of magnitude larger than what is observed in most giants, but it is argued that this is not unexpected in a close binary system. Finally, the helium to hydrogen abundance is 0.17, twice as large as the cosmic value.