Axions and the RR Lyrae Period-Luminosity Relation

We investigate the impact of axions on the period-luminosity relation (PLR) of RR Lyrae stars. During the red giant branch phase, emission through the electron coupling cools the core, increasing its mass at ignition. The larger core produces more luminous horizontal-branch stars, systematically shifting the zero point of the RR Lyrae PLR toward brighter magnitudes. We model the RR Lyrae population of the globular cluster M3 using stellar evolution and pulsation calculations tailored to its age and composition, and compare the resulting synthetic $K_s$-band PLR with observations. For ${\alpha }_{26}\equiv {10}^{26}{g}_{ae}^2\mathrm{/}4\pi =0.5$, the axion-induced shift in the predicted PLR is comparable to the current uncertainty in the absolute M3 zero-point calibration.