Probing the temperature gradient in the core boundary layer of stars with gravito-inertial modes: the case of KIC$\,$7760680

TL;DR

This study uses gravito-inertial modes to probe the near-core structure of a B-type star, KIC 7760680, revealing insights into its temperature gradient and mixing processes through asteroseismic analysis.

Contribution

It demonstrates the effectiveness of period spacings of high-order modes as diagnostics and compares different modeling prescriptions and statistical methods for asteroseismic fitting.

Findings

Period spacings outperform frequencies in diagnostics.

Models with convective boundary mixing fit better, but simpler models are statistically preferred.

Low vertical envelope mixing is indicated in KIC 7760680.

Abstract

Aims: We investigate the thermal and chemical structure in the near-core region of stars with a convective core by means of gravito-inertial modes. We do so by determining the probing power of different asteroseismic observables and fitting methodologies. We focus on the case of the B-type star KIC$\,$7760680, rotating at a quarter of its critical rotation velocity. Methods: We compute grids of 1D stellar structure and evolution models for two different prescriptions of the temperature gradient and mixing profile in the near-core region. We determine which of these prescriptions is preferred according to the prograde dipole modes detected in 4-yr $\textit{Kepler}$ photometry of KIC$\,$7760680. We consider different sets of asteroseismic observables and compare the outcomes of the regression problem for a $\chi^2$ and Mahalanobis Distance merit function, where the latter takes into account realistic uncertainties for the theoretical predictions and the former does not. Results: Period spacings of modes with consecutive radial order offer a better diagnostic than mode periods or mode frequencies for asteroseismic modelling of stars revealing only high-order gravito-inertial modes. We find KIC$\,$7760680 to reveal a radiative temperature gradient in models with convective boundary mixing, but less complex models without such mixing are statistically preferred for this rotating star, revealing extremely low vertical envelope mixing. Conclusions: Our results strongly suggest the use of measured individual period spacing values for modes of consecutive radial order as an asteroseismic diagnostic for stellar modelling of B-type pulsators with gravito-inertial modes.