High resolution spectroscopy of symbiotic stars I. SY Muscae: orbital elements, M giant radius, distance

We have obtained a series of high resolution optical spectra of the eclipsing symbiotic system SY Mus. We measured the radial velocity curve of the M star component, and determined its rotation velocity. Assuming co-rotation we obtain for its radius 86R_sun_. We determine M4.5 for the spectral type, i.e. its temperature is 3500K. The resulting luminosity is 1000L_sun_. The cool component is thus a giant star with a luminosity slightly larger than average. Comparison with evolutionary tracks yield an actual mass of 1.30M_sun_. From the binary mass function we obtain 0.43M_sun_ for the hot companion. The separation of the stellar components is 1.72AU and the distance from the center of the red giant to the inner Lagrange point L_1_ is 1.05AU. The red giant extends to less than 40% of the distance to L_1_ and therefore, the SY Mus binary system is well detached. We present line profiles of Hα and the Raman scattered line at λ6825 at various orbital phases. The Hα profile is often double-peaked and shows rapid intensity variations near quadrature. This indicates that the bulk of the Hα emission is produced in a small high density region close to the red star's surface facing the hot companion. The non-variability of the Raman scattered line at λ6825 indicates that the Raman scattering zone is large compared to the size of the red giant. We do not observe any change in the absorption spectrum of the red giant as a function of phase, i.e. there is no observable irradiation effect.