Very High Precision Orbit of Capella by Long Baseline Interferometry

The orbital elements of the double-lined spectroscopic binary Capella have been determined with unprecedented accuracy from observations with the Mark III optical interferometer. We have measured a magnitude difference, m_Aa - m_Ab, of -0 center dot 05 m +/- 0 center dot 05 m, 0 center dot 15 m +/- 0 center dot 05 m, and 0 center dot 28 m +/- 0 center dot 10 m at lambda lambda 800, 550, and 450 nm, respectively. We confirm previous observations that the more massive spectroscopic primary component Aa is also the cooler and visually fainter of the two stars. Measured stellar radii are R_Aa = (12.2 +/- 0.2) solar radius and R_Ab = (9.2 +/- 0.4) solar radius. There is some evidence in our data for a departure of the brightness distribution of component Ab from a simple limb-darkened disk, possibly related to star spots. Using spectroscopy by Barlow et al. (1993), we derive component masses of M_Aa = (2.69 +/- 0.06) solar mass and M_Ab = (2.56 +/- 0.04) solar mass. Comparison with recent stellar evolution models (Schaerer et al. 1993) yields a stellar age of 5.25 x 10⁸ yr for a metallicity of Z = 0.008. In this scenario, component Aa has undergone helium ignition.