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

Globular clusters offer a powerful way to test the properties of stellar populations and the late stages of low-mass stellar evolution. For this paper we studied 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 determined the frequency composition of five RRc stars and modeled their physical parameters from linear pulsation models. We were 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 ± 0.028 M⊙ for RR Lyrae stars and 0.657 ± 0.034 M⊙ 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 ℓ = 8 and 9 nonradial 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.