HIGH-RESOLUTION SPECTROSCOPY OF SYMBIOTIC STARS .2. RW HYDRAE - ORBIT, ECLIPSES, AND STELLAR PARAMETERS

With IUE spectra and published optical photometry, we show that the sy mbiotic star RW Hya is an eclipsing binary. We use a series of high re solution optical spectra to determine the orbital elements and basic p arameters of the stellar components in the system. We measure the orbi tal velocity amplitude of the red star and determine the binary mass f unction of RW Hya. Absorption line fitting yields the rotation velocit y of the cool star, and, assuming co-rotation, its radius and luminosi ty which are found to be Rr = 60 R. and L(r) = 600 L., respectively. T he masses of the red giant and the hot companion turn out to be M(r) = 1.6 M. and M(h) = 0.5 M., respectively. The red giant only fills a sm all fraction of its Roche lobe and the presence of an accretion disk a round the hot companion is highly unlikely. We present a series of H a lpha line profiles obtained at various orbital phases. Their variation and the eclipse light curve in the Stroemgren u and v filters indicat e, that the symbiotic nebulosity is a small compact region located clo se to the red giant's surface. A broad pedestal H alpha emission disap pears at times when the hot companion passes behind the red giant. Thi s may be an indication that a fast stellar wind streams off the hot st ar. The H alpha profiles also indicate that at least in RW Hya, the fo r symbiotic stars typical double-peak structure is due to self-absorpt ion. The fact that the symbiotic nebula has only dimensions of the ord er of similar to 0.5 AU has important consequences in relation to Zans tra techniques designed to measure the luminosity and temperature of t he hot companion star.