THE MASS OF THE CLASSICAL CEPHEID S-MUSCAE

A good determination of the mass-luminosity relation for evolved stars on blue loops can determine the degree of excess mixing in the interi ors of their main-sequence companions. In this study we determine the dynamical mass of the Cepheid binary S Muscae. This can be combined wi th its known luminosity and be fitted on evolutionary tracks to determ ine the amount of mixing in the main-sequence progenitor. Using the Go ddard High Resolution Spectrograph on the Hubble Space Telescope we ha ve measured the orbital radial velocity changes for the companion of t he Cepheid S Mus. Spectra taken at minimum and maximum orbital velocit ies were cross-correlated. The velocity difference was measured to be 30.6+/-0.4 km s(-1). The difference for the orbital velocities of the Cepheid for the same phases was determined to be 26.9 km s(-1)+/-0.4 k m s(-1). This gives a velocity ratio of 1.14+/-0.02, which gives us th e mass ratio for the companions. Adding possible centering errors of t he target in the entrance apertures the error limit would be increased to +/-0.06. The derived spectral types of the companion S Mus B range from B3 V to B5 V depending on the criterion used to determine it, wi th an average spectral type B3.8 V. Using the average spectral type an d the main-sequence mass-spectral type relation from Andersen & Harman ec we find for S Mus B a mass of 5.2+/-0.2 M.. With the newly determin ed mass ratio the mass for the Cepheid S Mus A comes out to be 5.9(-0. 6)(+0.7) M.. Taking the mass of 5.9 M. at face value and adopting the absolute visual magnitude of M(upsilon)=-4.29 (log L/L.=3.62) for the Cepheid S Mus this indicates mixing in its main-sequence progenitor sl ightly in excess of the one assumed for the Maeder and Meynet evolutio nary tracks. The present uncertainties in mass and luminosity prevent, however, a firm conclusion about the exact degree of mixing.