VERY HIGH-PRECISION ORBIT OF CAPELLA BY LONG-BASE-LINE INTERFEROMETRY

The orbital elements of the double-lined spectroscopic binary Capella have been determined with unprecedented accuracy from observations wit h the Mark III optical interferometer. We have measured a magnitude di fference, m(Aa)-m(Ab), of -0.05m +/- 0.05m, 0.15m +/- 0.05m, and 0.28m +/- 0.10m at lambdalambda800, 550, and 450 nm, respectively. We confi rm previous observations that the more massive spectroscopic primary c omponent Aa is also the cooler and visually fainter of the two stars. Measured stellar radii are R(Aa) = (12.2 +/- 0.2)R. and R(Ab) = (9.2 /- 0.4)R.. There is some evidence in our data for a departure of the b rightness distribution of component Ab from a simple limb-darkened dis k, possibly related to star spots. Using spectroscopy by Barlow et al. [PASP, 105, 476 (1993)], we derive component masses of M(Aa) = (2.69 +/- 0.06) M. and M(Ab) = (2.56 +/- 0.04) M.. Comparison with recent st ellar evolution models (Schaerer et al. 1993, A&AS, 98, 523) yields a stellar age of 5.25 x 10(8) yr for a metallicity of Z = 0.008. In this scenario, component Aa has undergone helium ignition.