An insight into Capella (alpha Aurigae): from the extent of core overshoot to its evolutionary history

The binary star alpha Aurigae (otherwise known as Capella) is extremely important to understand the core hydrogen and helium burning phases of the stars, as the primary star is likely evolving through the core helium burning phase, and the masses of the two components are 2.5 Msun and 2.6 Msun, which fall into a mass range for which the extention of the core overshoot during the main sequence phase is uncertain. We aim at deriving the extent of the core overshoot experienced during the core burning phases and testing the efficiency of the convective transport of energy in the external envelope, by comparing results from stellar evolution modelling with the results from the observations. We consider evolutionary tracks calculated on purpose for the present work, for the primary and secondary star of Capella. We determine the extent of the extra-mixing from the core during the main sequence evolution and the age of the system, by requiring that the effective temperatures and surface gravities of the model stars reproduce those derived from the observations at the same epoch. We further check consistency between the observed and predicted surface chemistry of the stars. Consistency between results from stellar evolution modelling and the observations of Capella is found when extra-mixing from the core is assumed, the extent of the extra-mixed zone being of the order of 0.25 H_P. The age of the system is estimated to be 710 Myr. These results allow to nicely reproduce the observed surface chemistry, particularly the recent determination of the 12C/13C ratio based on LBT (Large Binocular Telescope) and VATT (Vatican Advanced Technology Telescope) observations