Matching seismic masses for RR Lyrae-type and oscillating red horizontal-branch stars in M4

TL;DR

This study uses high-precision Kepler data to compare seismic masses of RR Lyrae stars and red horizontal-branch stars in globular cluster M4, providing insights into stellar evolution and pulsation modes.

Contribution

First direct comparison of seismic masses of RR Lyrae and red horizontal-branch stars within the same stellar population, revealing potential high-degree non-radial pulsation modes.

Findings

Seismic masses of RR Lyrae stars are approximately 0.648 M_sun.

Red horizontal-branch stars have seismic masses around 0.657 M_sun.

Results suggest RRc stars may exhibit high-degree non-radial pulsation modes.

Abstract

Globular clusters offer a powerful way to test the properties of stellar populations and the late stages of low-mass stellar evolution. In this paper we study oscillating giant stars and overtone RR Lyrae-type pulsators in the nearest globular cluster, M4, with the help of high-precision, continuous light curves collected by the Kepler space telescope in the K2 mission. We determine the frequency composition of five RRc stars and model their physical parameters from linear pulsation models. We are able, for the first time, to compare seismic masses of RR Lyrae stars directly to the masses of the very similar red horizontal branch stars in the same stellar population, independently determined from asteroseismic scaling relations. We find average seismic masses of $0.648\pm0.028\,M_\odot$ for RR Lyrae stars and $0.657\pm0.034\,M_\odot$ for red horizontal-branch stars. While the accuracy of our RR Lyrae masses still relies on the accuracy of evolutionary mass differences of neighboring horizontal branch subgroups, this result strongly indicates that RRc stars may indeed exhibit high-degree, $\ell = 8$ and 9 non-radial modes, and modeling these modes can provide realistic mass estimates. We compare the seismic masses of our red horizontal branch and RR Lyrae stars to evolutionary models and to theoretical mass relations and highlight the limitations of these relations.