The dynamical mass of the beat cepheid Y carinae and stellar opacities.

Abstract

The beat Cepheid, Y Carinae A, has a B9 V companion, Y Car B. The primary period P₀ of the Cepheid is 3.64 days and the secondary, P₁, is 2.56 days. Its period ratio P₁/P₀ is thus 0.703. Y Car is the only beat Cepheid known to be a binary and thus offers us the unique opportunity to determine the dynamical mass for a beat Cepheid. We have determined its mass by measuring the orbital velocity amplitude of the hot companion Y Car B using the Goddard High-Resolution Spectrograph (GHRS) with the G200M grating on the Hubble Space Telescope. When combined with the ground-based orbital velocity amplitude of the Cepheid and the mass of the companion, the implied mass of the Cephieid is M=3.8±1.2M_sun. With the Cepheid luminosity given by the period-luminosity relation, this mass, taken at face value, indicates excess mixing in the main sequence progenitor corresponding to convective overshoot by about 0.9 pressure scale height, however, the large error bars prevent a firm conclusion. As shown by Simon the period ratio for beat Cepheids depends sensitively on the opacities. For models calculated with Cox-Tabor opacities the period ratios for beat Cepheids indicate masses between one and two M_sun. Models calculated with the new Livermore OPAL opacities on the other hand indicate masses around 4 solar masses. The good agreement of the beat mass with the dynamical mass, determined here for Y Car, provides a confirmation that the OPAL opacities are a significant improvement over the Cox-Tabor (1976) opacities.