CATEGORIES
Tight asteroseismic constraints on core overshooting and diffusive mixing in massive stars
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07.09.2015
Affiliation
Institute of Astronomy, KU Leuven
Main category
Natural Sciences (Astrophysics and Astrononmy)
Abstract
Thanks to the unprecedented high quality space photometry provided by CoRoT and Kepler missions, our view on stellar oscillations in B-type stars and the physics of the upper HRD is progressively improving. O- and B-type stars harbor a fully mixed convective cores, and a radiative envelope. However, the interface between these two layers -- the so-called overshooting layer -- is not understood from first principles. Thus, the width and the mixing efficiency of the overshooting layer is always treated by simplistic schemes like step-function or exponentially diffusive mixing prescriptions. We modelled two rich main sequence pulsators observed by Kepler and CoRoT which also turn out to be very slowly rotating pulsating B stars. They are KIC 10526294 and HD 50230, respectively. Based on forward seismic modelling, we derive the width of the overshooting layer on top of their receding cores. Additionally, we show that extra diffusive mixing of 100 to 10 000 cm$^2$ sec$^{-1}$ in the radiative envelope of these two stars is essential to better fit their observed g-mode frequencies. We also compare the classical step-function versus exponential diffusive overshoot. The diffusive overshooting prescription outperforms the other to fit the observed frequencies by a factor 2 to 3 (in $\chi^2$ sense). The derived values for the overshooting and diffusive mixing coefficients are weakly dependent on the choice of opacities and chemical mixtures, and are considered robust constraints.
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