Probing the physics in the core boundary layers of the double-lined B-type binary KIC 4930889 from its gravito-inertial modes

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: mathias.michielsen@kuleuven.be
² Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL Nijmegen, The Netherlands
³ Max Planck Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg, Germany

Received: 6 June 2022
Accepted: 15 September 2023

Abstract

Context. Stellar evolution models of B-type stars are still uncertain in terms of internal mixing properties, notably in the area between the convective core and the radiative envelope. This impacts age determination of such stars in addition to the computation of chemical yields produced at the end of their life.

Aims. We investigated the thermal and chemical structure and rotation rate in the near-core boundary layer of the double-lined B-type binary KIC 4930889 from its four-year Kepler light curve, ground-based spectroscopy, and Gaia astrometry.

Methods. We computed grids of 1D stellar structure and evolution models for different mixing profiles and prescriptions of the temperature gradient in the near-core region. We examined the preferred prescription and the near-core rotation rate using 22 prograde dipole modes detected by Kepler photometry of KIC 4930889. We employed a Mahalanobis distance merit function and considered various nested stellar model grids, rewarding goodness of fit but penalising model complexity.

Results. We were able to constrain the near-core rotation rate of the pulsator to Ω_rot = 0.73_−0.06^+0.02 d⁻¹. Furthermore, we found a preference for either an exponentially decaying mixing profile in the near-core region or absence of additional near-core mixing, but found no preference among the various options for the temperature gradient in this region. The frequency (co)variances of our theoretical predictions are much larger than the errors on the observed frequencies. This forms the main limitation on further constraining the individual parameters of our models. A combination of spectroscopic, astrometric, binary, and asteroseismic information was used to achieve these constraints. Additionally, non-adiabatic pulsation computations of our best models indicate a need for opacity enhancements to accurately reproduce the observed mode excitation.

Conclusions. The eccentric close binary system KIC 4930889 proves to be a promising target to investigate additional physics in close binaries by developing new modelling methods with the capacity to include the effect of tidal interactions for full exploitation of all detected oscillation modes.