High resolution spectroscopy of symbiotic stars. II. RW Hydrae: orbit, eclipses, and stellar parameters.

With IUE spectra and published optical photometry, we show that the symbiotic star RW Hya is an eclipsing binary. We use a series of high resolution optical spectra to determine the orbital elements and basic parameters of the stellar components in the system. We measure the orbital velocity amplitude of the red star and determine the binary mass function of RW Hya. Absorption line fitting yields the rotation velocity of the cool star, and, assuming co-rotation, its radius and luminosity which are found to be R_r=60_R_sun_ and L_r=600_L_sun_, respectively. The masses of the red giant and the hot companion turn out to be M_r=1.6_M_sun_ and M_h=0.5_M_sun_, respectively. The red giant only fills a small fraction of its Roche lobe and the presence of an accretion disk around the hot companion is highly unlikely. We present a series of Hα line profiles obtained at various orbital phases. Their variation and the eclipse light curve in the Stroemgren u and v filters indicate, that the symbiotic nebulosity is a small compact region located close to the red giant's surface. A broad pedestal Hα emission disappears at times when the hot companion passes behind the red giant. This may be an indication that a fast stellar wind streams off the hot star. The Hα profiles also indicate that at least in RW Hya, the for symbiotic stars typical double-peak structure is due to self-absorption. The fact that the symbiotic nebula has only dimensions of the order of ~0.5AU has important consequences in relation to Zanstra techniques designed to measure the luminosity and temperature of the hot companion star.