Mode coupling coefficients between the convective core and radiative envelope of $\gamma$Doradus and slowly pulsating B stars

Abstract

Signatures of coupling between an inertial mode in the convective core and a gravito-inertial mode in the envelope have been found in four-year Kepler light curves of 16 rapidly rotating $\gamma$Doradus ($\gamma$Dor) stars. This makes it possible to obtain a measurement of the rotation frequency in their convective core. Despite their similar internal structure and available data, inertial modes have not yet been reported for slowly pulsating B (SPB) stars. We aim to provide a numerical counterpart of the recently published theoretical expressions for the mode-coupling coefficients, $\epsilon$ and $\tilde{\epsilon }$. These coefficients represent the two cases of a continuous and a discontinuous Brunt-Väisälä frequency profile at the core-envelope interface, respectively. We used asteroseismic forward models of two samples consisting of 26 SPB stars and 37 $\gamma$Dor stars to infer their numerical values of $\epsilon$. The asteroseismically inferred values of $\epsilon$ for the two samples are between 0.0 and 0.34. While $\epsilon$ is most strongly correlated with the near-core rotation frequency for $\gamma$Dor stars, the fractional radius of the convective core instead provides the tightest correlation for SPB stars. We find $\epsilon$ to decrease mildly as the stars evolve. Our asteroseismic results for the mode coupling support the theoretical interpretation and reveal that young, fast-rotating $\gamma$Dor stars are most suitable for undergoing couplings between inertial modes in the rotating convective core and gravito-inertial modes in the radiative envelope. The phenomenon has been found in 2.4\% of such pulsators with detected period spacing patterns, whereas it has not been seen in any of the SPB stars so far. (shortened abstract to meet the arXiv limits)

Manuscript in press at A&A, 9 pages, 8 figures

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