THE DYNAMICAL MASS OF THE BEAT CEPHEID Y-CARINAE AND STELLAR OPACITIES

The beat Cepheid, Y Carinae A, has a B9 V companion, Y Car B, The prim ary period P-0 of the Cepheid is 3.64 days and the secondary, P-1, is 2.56 days. Its period ratio P-1/P-0 is thus 0.703. Y Car is the only b eat Cepheid known to be a binary and thus offers us the unique opportu nity to determine the dynamical mass for a beat Cepheid. We have deter mined 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 Hobble Space Telescope. When combined with the ground-based orbital velocity amplitude of the Cepheid and th e mass of the companion, the implied mass of the Cephieid is M=3.8+/-1 .2 M.. With the Cepheid luminosity given by the period-luminosity rela tion, this mass, taken at face value, indicates excess mixing in the m ain sequence progenitor corresponding to convective overshoot by about 0.9 pressure scale height, however, the large error bars prevent a fi rm conclusion. As shown by Simon the period ratio for beat Cepheids de pends sensitively on the opacities. For models calculated with Cox-Tab or opacities the period ratios for beat Cepheids indicate masses betwe en one and two M.. Models calculated with the new Livermore OPAL opaci ties on the ether hand indicate masses around 4 solar masses. The good agreement of the beat mass with the dynamical mass, determined here f or Y Car, provides a confirmation that the OPAL opacities are a signif icant improvement over the Cox-Tabor (1976) opacities. (C) 1997 Americ an Astronomical Society.