Observational tests and predictive stellar evolution

We compare 18 binary systems with precisely determined radii and masses from 23 to 1.1 M_⊙ and stellar evolution models produced with our newly revised code TYCHO. "Overshooting" and rotational mixing were suppressed in order to establish a baseline for isolating these and other hydrodynamic effects. Acceptable coeval fits are found for 16 pairs without optimizing for heavy-element or helium abundance. The precision of these tests is limited by the accuracies of the observed effective temperatures. High-dispersion spectra and detailed atmospheric modeling should give more accurate effective temperatures and heavy-element abundances. PV Cas, a peculiar early A system, EK Cep B, a known post-T Tauri star, and RS Cha, a member of a young OB association, are matched by pre-mainsequence models. Predicted mass loss agrees with upper limits from IUE for CW Cep A and B. Relatively poor fits are obtained for binaries having at least one component in the mass range 1.7 < M/M_⊙ < 2.6, whose evolution is sensitive to mixing. These discrepancies are robust and consistent with additional mixing in real stars. The predicted apsidal motion implies that massive star models are systematically less centrally condensed than the real stars. If these effects are due to overshooting, then the overshooting parameter α_ov increases with stellar mass. The apsidal motion constants are controlled by radiative opacity under conditions close to those directly measured in laser experiments, making this test more stringent than possible before.