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

Abstract

Context. The binary star α Aurigae (otherwise known as Capella) is extremely important to understanding the core hydrogen and helium burning phases of stars, as its primary star is likely evolving through the core helium burning phase. Moreover, the masses of the star’s two components are ∼2.5 M⊙ and ∼2.6 M⊙, meaning they fall into a mass range in which the extension of the core overshoot during the main sequence phase is uncertain. Aims. We aim to derive the extent of the core overshoot experienced during the core burning phases and to test the efficiency of the convective transport of energy in the external envelope of Capella by comparing results from stellar evolution modelling with the results from observations. Methods. We considered evolutionary tracks calculated expressly for the present work, that is, for the primary and secondary stars of Capella. We determined 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 checked the consistency between the observed and predicted surface chemistry of the stars. Results. The 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 HP. The age of the system is estimated to be 710 Myr. These results allow the observed surface chemistry to be well reproduced, particularly the recent determination of the 12C/13C ratio based on Large Binocular Telescope and Vatican Advanced Technology Telescope observations.