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

¹ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, H-1121 Budapest, Hungary
² ELTE Eötvös Loránd University, Institute of Physics and Astronomy, 1117 Pázmány Péter sétány 1/A, Budapest, Hungary
³ Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁴ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland
⁵ Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH 43210 USA
⁶ Center for Cosmology and Astroparticle Physics (CCAPP), The Ohio State University, 191 W. Woodruff Avenue, Columbus, OH 43210 USA
⁷ University of Wyoming, 1000 E University Ave, Laramie, WY USA

^⋆ Corresponding author: molnar.laszlo@csfk.org; lmolnar@konkoly.hu

Received: 5 September 2024
Accepted: 28 July 2025

Abstract

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.