Rotation of the convective core in $\gamma$ Dor stars measured by dips in period spacings of g modes coupled with inertial modes

TL;DR

This study uses dips in period spacings of g modes to measure the rotation of convective cores in $ ext{γ}$ Dor stars, revealing mostly uniform rotation with slightly faster cores in less evolved stars.

Contribution

It introduces a method to measure convective core rotation in $ ext{γ}$ Dor stars using resonance dips in period spacings, accounting for differential rotation.

Findings

Most $ ext{γ}$ Dor stars studied rotate nearly uniformly.

Convective cores tend to rotate slightly faster than the g-mode cavity in less evolved stars.

The resonance dip provides a diagnostic for core rotation and stellar parameters.

Abstract

The relation of period spacing ($\Delta P$) versus period ($P$) of dipole prograde g modes is known to be useful to measure rotation rates in the g-mode cavity of rapidly rotating $\gamma$ Dor and slowly pulsating B (SPB) stars. In a rapidly rotating star, an inertial mode in the convective core can resonantly couple with g modes propagative in the surrounding radiative region. The resonant coupling causes a dip in the $P$-$\Delta P$ relation, distinct from the modulations due to the chemical composition gradient. Such a resonance dip in $\Delta P$ of prograde dipole g modes appears around a frequency corresponding to a spin parameter $2f_{\rm rot}{\rm(cc)}/\nu_{\rm co-rot} \sim 8-11$ with $f_{\rm rot}$(cc) being the rotation frequency of the convective core and $\nu_{\rm co-rot}$ the pulsation frequency in the co-rotating frame. The spin parameter at the resonance depends somewhat on the extent of core overshooting, central hydrogen abundance, and other stellar parameters. We can fit the period at the observed dip with the prediction from prograde dipole g modes of a main-sequence model, allowing the convective core to rotate differentially from the surrounding g-mode cavity. We have performed such fittings for 16 selected $\gamma$ Dor stars having well defined dips, and found that the majority of $\gamma$ Dor stars we studied rotate nearly uniformly, while convective cores tend to rotate slightly faster than the g-mode cavity in less evolved stars.