Mixed-mode Ensemble Asteroseismology of Low-Luminosity Kepler Red Giants

Abstract

We present measurements of the dipole mode asymptotic period spacing ($\Delta\Pi_1$), the coupling factor between p- and g- modes ($q$), the g-mode phase offset ($\epsilon_g$), and the mixed-mode frequency rotational splitting ($\delta\nu_{\mathrm{rot}}$) for 1,074 low-luminosity red giants from the Kepler mission. Using oscillation mode frequencies extracted from each star, we apply Bayesian optimization to estimate $\Delta\Pi_1$ from the power spectrum of the stretched period spectrum and to perform the subsequent forward modelling of the mixed-mode frequencies. With our measurements, we show that the mode coupling factor $q$ shows significant anti-correlation with both stellar mass and metallicity, and can reveal highly metal-poor stars. We present the evolution of $\epsilon_g$ up the lower giant branch up to before the luminosity bump, and find no significant trends in $\epsilon_g$ or $\delta\nu_{\mathrm{rot}}$ with stellar mass and metallicity in our sample. Additionally, we identify six new red giants showing anomalous distortions in their g-mode pattern. Our data products, code, and results are provided in a public repository.