K2 data of M4 RR Lyraes and RGB parameters

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+/-0.028M_{sun}_ for RR Lyrae stars and 0.657+/-0.034M_{sun}_ 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 non-radial modes, and modeling these modes can provide realistic mass estimates.