A critical test of stellar evolution and convective core 'overshooting' by means of zeta Aurigae systems

Quantitative tests of late stellar evolution are presented by computing models with our evolutionary code to match the exact properties of certain zeta Aurigae eclipsing binaries and related non-eclipsing systems. Those binaries have a late-type giant or supergiant primary and their orbital inclination is well determined from either eclipses or speckle orbits. They provide the only direct measurements of masses for such evolved stars, together with ether well-determined physical parameters. In the computations all effects of enhanced mixing beyond the convective cores during central hydrogen burning stages, e.g., core overshooting and any rotationally induced meridional mixing, are represented by a simple overshooting prescription. Its single parameter can be constrained to within 25 per cent of its value and leads to an overshooting length l(ov) of similar to 0.24 H-p (pressure scaleheights) for 2.5 M(circle dot), slightly increasing to similar to 0.32 H-p for 6.5 M(circle dot). Those values are required by our code to reproduce the well-determined luminosities of the giants in or at the end of their blue loop. This new method provides the currently most sensitive test of the overshooting issue.