2023ApJ...944L..51B

RR Lyrae variables are excellent Population II distance indicators thanks to their well-defined period-luminosity relations (PLRs) at infrared wavelengths. We present results of near-infrared (NIR) monitoring of Galactic globular clusters to empirically quantify the metallicity dependence of NIR PLRs for RR Lyrae variables. Our sample includes homogeneous, accurate, and precise photometric data for 964 RR Lyrae variables in 11 globular clusters covering a large metallicity range (Δ[Fe/H] ∼ 2 dex). We derive JHK_s-band period-luminosity-metallicity (PLZ) and period-Wesenheit-metallicity (PWZ) relations anchored using 346 Milky Way field RR Lyrae stars with Gaia parallaxes, and simultaneously solved for independent distances to globular clusters. We find a significant metallicity dependence of ∼0.2 mag dex^–1 in the JHK_s-band PLZ and PWZ relations for RR Lyrae stars independent of the adopted metallicity scale. The metallicity coefficients and the zero-points of the empirical PLZ and PWZ relations are in excellent agreement with the predictions from the horizontal branch evolution and pulsation models. Furthermore, RR Lyrae-based distances to our sample of globular clusters are also statistically consistent with other independent measurements in the literature. Our recommended empirical JHK_s-band PLZ relations for RR Lyrae stars with periods of fundamental mode pulsation (P_f) are:\begin{eqnarray*}\begin{array}{rcl}{M}_{J} & = & -0.44\,(\pm 0.03)-1.83\,(\pm 0.02)\mathrm{log}({P}_{{\rm{f}}})+0.20\,(\pm 0.02)\,[\mathrm{Fe}/{\rm{H}}]\,(\sigma =0.05\,\mathrm{mag})\\ {M}_{H} & = & -0.74\,(\pm 0.02)-2.29\,(\pm 0.02)\mathrm{log}({P}_{{\rm{f}}})+0.19\,(\pm 0.01)[\mathrm{Fe}/{\rm{H}}]\,(\sigma =0.05\,\mathrm{mag})\\ {M}_{{K}_{s}} & = & -0.80\,(\pm 0.02)-2.37\,(\pm 0.02)\mathrm{log}({P}_{{\rm{f}}})+0.18\,(\pm 0.01)\,[\mathrm{Fe}/{\rm{H}}]\,(\sigma =0.05\,\mathrm{mag}).\end{array}\end{eqnarray*}