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

Abstract

We present measurements of the dipole mode asymptotic period spacing ($\hat{I}”{\hat{I}}_1$), the coupling factor between p- and g- modes ($q$), the g-mode phase offset ($\hat{I}{\mu }_g$), and the mixed-mode frequency rotational splitting ($\hat{I}´\hat{I}{½}_{\mathrm{rot}}$) 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 $\hat{I}”{\hat{I}}_1$ 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 $\hat{I}{\mu }_g$ up the lower giant branch up to before the luminosity bump, and find no significant trends in $\hat{I}{\mu }_g$ or $\hat{I}´\hat{I}{½}_{\mathrm{rot}}$ 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.

Accepted in the Astrophysical Journal. Code repository is at https://github.com/jsk389/BOChaMM and associated peakbagging data is publicly available at https://zenodo.org/record/7888633