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

TL;DR

This paper demonstrates, for the first time in a globular cluster, that seismic masses of overtone RR Lyrae stars (RRc) can be directly compared with those of proximal red horizontal-branch stars within the same population. Using high-precision K2 light curves for M4, it identifies low-amplitude, high-degree non-radial modes (l ≈ 8, 9) in RRc stars and models their frequencies with linear pulsation theory, yielding RRc masses of 0.636–0.667 $M_$ (average 0.648 ± 0.028 $M_$). Parallel asteroseismic scaling for red giants provides independent masses, with rHB stars averaging 0.657 ± 0.034 $M_$; the two methods agree within ~0.01 $M_$, supporting the viability of RRc seismic masses via f61-type modes. Comparing these results with mass relations and BaSTI/MIST models reveals both consistency and limitations, notably in the assumed mass loss along the HB, and underscores the potential of cluster seismology to constrain late-stage stellar evolution and envelope mass loss.

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.

Figures (17)

• Figure 1: Corrected K2 light curves of the five RRc stars that were targeted by the mission.
• Figure 2: Color-magnitude diagram of M4 in Johnson passbands using photometry from Stetson-2014stetson-2019. Stars targeted by our study are identified by the colored star symbols, and the stars in Howell-2022 are shown as colored points. V6 & V42 overlap in the plot, which is indicated by the dual colored marker. The photometry has been corrected for dust using the Alonso-Garcia-2012 and pancino-2024 maps. The Gaia membership sample (grey) is from Vasiliev-2021_GalacticGC_memberships.
• Figure 3: Left: Fourier spectra of the five RRc stars. Here we removed the main pulsation frequency and its harmonics belonging to the first overtone to reveal the low-amplitude extra modes. Modes are labeled with red, and combination frequencies with black. Right: light curves folded with the first overtone period.
• Figure 4: Petersen diagram of the $f_{61}$-type modes. Here we plot signals detected in M4 (in red) over existing literature data. Frequency peaks fall onto the main ridges discovered in the OGLE data. Positions of the $f_{61}$ and $f_{63}$ ridges are labelled respectively.
• Figure 5: Positions of the seismically determined $T_{\rm eff}$ and $\log\, g$ values for the five RRc stars in M4, against the spectroscopically observed RRc stars in the $T_{\rm eff}$--$\log g$ plane, as collected by Molnar-2023.