An IUE and HST Archival Study of the Hot White Dwarf in the Symbiotic Variable RW Hydrae

We have carried out an ultraviolet spectroscopic study of the hot white dwarf in the eclipsing S-type symbiotic variable RW Hydrae (P_orb=370.2 days). Our study used orbital phase-resolved IUE archival low-resolution and high-resolution spectra and Hubble Space Telescope archival spectra, including all available images obtained at inferior conjunction (when the accreting hot white dwarf is in front of the red giant). This system is one of less than a handful of symbiotic variables in which the photospheric continuum from the hot white dwarf is the dominant source of light in the far-UV. We have found N V features characteristic of an outflow with velocity of approximately 170 km s^-1, almost certainly associated with outflow from the white dwarf, as they are present only in those phases in which the white dwarf is visible. We have also performed the first model atmosphere synthetic spectral fit to the white dwarf-dominated far-UV continuum and found that the best fit was provided by a model with solar abundances, logg=8, and a temperature of 41,000 K, considerably cooler than estimates of the temperature based on Zanstra techniques. We cannot exclude the possibility that the hot component is hidden by an accretion curtain and that our temperature does not characterize the accreting white dwarf's true photospheric temperature. For a measured red giant wind mass-loss rate of 1×10^-7 M_solar yr^-1 and a wind accretion efficiency of 7%, we estimate M=7×10^-9 M_solar yr^-1. The heating at this rate of accretion cannot maintain the white dwarf surface temperature even close to what we have derived. At the surface luminosity implied by our fitting, the required accretion rate for steady nuclear burning is a factor of 20 lower than the above accretion rate. Based on these results, we conclude that the hot component of RW Hya is a young (lower limit age of 1.6 Myr), accretion-heated white dwarf, very likely with nuclear shell burning, on its final cooling track.