Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster Omega Centauri as standard candles

Abstract

New photometry of RRab and RRc stars in Omega Cen is used to calibrate their absolute magnitudes M_V as a function of: a) metallicity; and b) the Fourier parameters of light curves in the V band. The zero point of both calibrations relies on the distance modulus to the cluster derived earlier by the CASE project based on observations of the detached eclipsing binary OGLE GC17. For RRab variables we obtained a relation of M_V=(0.26\pm 0.08)[Fe/H]+(0.91\pm 0.13). A dereddened distance modulus to the LMC based on that formula is $\mu_{0}=18.56\pm 0.14$ mag. The second calibration of M_V, which is based on Fourier coefficients of decomposed light curves, results in the LMC distance of $\mu_{0}=18.51\pm 0.07$ mag.