A Homogenized Catalogue of Variable Stars in the Globular Cluster M22: Membership, Physical Parameters, and Distance

Abstract

We present a photometric analysis of the variable stars in the field of the globular cluster M22 (NGC 6656), using time-series photometry from the literature and from OGLE and Gaia-DR3 data. From a sample of 666 variable stars in the field, we identify 94 as cluster members, including 10 RRab, 17 RRc, 2 Type II Cepheids, 15 SX Phe, 15 SR stars and 22 eclipsing binaries. Fourteen of these members are not included in the Catalogue of Variable Stars in Globular Clusters. Through the Fourier decomposition of the RR Lyrae light curves, we derived $[\mathrm{Fe/H}] = -1.73 \pm 0.17$ dex and $d = 3.27 \pm 0.14$ kpc. Among the RRc stars, V15 shows an additional frequency with a frequency ratio $f_{0.61}/f_\mathrm{1O} = 0.6152$, consistent with the RR$_{0.61}$ phenomenon. Using P-L relations for member SX Phe stars, we obtained $d = 3.31 \pm 0.17$ kpc, consistent with the RR Lyrae distance. We constructed a CMD comprising only cluster members, which allows us to confirm the evolutionary status of the cluster's RR Lyrae stars. None of the fitted isochrones satisfactorily reproduces the CMD morphology, which is consistent with the presence of multiple stellar populations in M22.