THE MASSES AND RADII OF THE ECLIPSING BINARY ZETA-AURIGAE

We present a full determination of the fundamental stellar and orbital parameters of the eclipsing binary zeta Aurigae (K4 Ib + B5 V) using recent observations with the Hubble Space Telescope Goddard High Resol ution Spectrograph (GHRS) and the Mark III long-baseline optical inter ferometer. The information obtained from spectroscopic and interferome tric measurements is complementary, and the combination permits a comp lete determination of the stellar masses, the absolute semimajor axis of the orbit, and the distance. A complete solution requires that both components be visible spectroscopically, and this has always been dif ficult for the zeta Am systems. The zeta Aur K star primary presents n o difficulty, and accurate radial velocities are readily obtainable in the optical. However, the B star secondary is more problematic. Groun d-based radial velocity measurements are hampered by the difficulty of working with the composite spectrum in the blue-violet region, the sm all number of suitable lines in the generally featureless optical spec trum of the B star, and the great width of the few available lines (th e Balmer lines of hydrogen and a few weak He I lines) due to rapid rot ation. We avoid the worst of these problems by using GHRS observations in the ultraviolet, where the K star flux is negligible and the intri nsic B star spectrum is more distinctive, and obtain the most accurate determination of the B star radial velocity amplitude to date. We als o analyze published photometry of previous eclipses and near-eclipse p hases of zeta Aur in order to obtain eclipse durations, which fix the length of the eclipse chord and therefore determine the orbit inclinat ion. The long-baseline interferometry (LBI) yields, in conjunction wit h the spectroscopic solution, the distance to the system and thus the absolute stellar radius of the resolved K supergiant primary star, zet a Am A. The secondary is not resolved by LBI, but its angular (and abs olute) radius is found by fitting the model stellar flux plus an inter stellar extinction model to the flux-calibrated GHRS data. We find M(K ) = 5.8 +/- 0.2 M(Theta), M(B) = 4.8 +/- 0.2 M(Theta), R(K) = 148 +/- 3 R(Theta), and R(B), = 4.5 +/- 0.3 R(Theta) for the masses and radii of the zeta Am stars. We determine the distance to zeta Am to be 261 /- 3 pc. Additionally, we refine the stellar parameters of the B star secondary presented in the 1995 spectroscopic study of Bennett, Brown, & Linsky. We also determine the effective temperature of the K star p rimary using values of the bolometric flux, angular diameter, and inte rstellar extinction derived in this study. The positions of the zeta A m stars on the theoretical H-R diagram are compared to current evoluti onary model tracks, and the resulting good agreement provides a strong check of the internal selfconsistency of this analysis and the accura cy of the theoretical models. The zeta Aurigae stars are confirmed to be coeval with an age of 80 +/- 15 Myr.