Physical parameters of RR Lyrae stars in NGC 6171

Abstract

We present an analysis of VI CCD time series photometry of the globular cluster NGC 6171. The main goal is to determine individual physical parameters for single‐mode RR Lyrae stars present in the field of view of our images, by means of light curve Fourier decomposition and well‐established calibrations and zero points. This leads to the estimation of the mean values of the metallicity and distance for the parental cluster. From the RRab stars, we find [Fe/H]ZW = −1.33 ± 0.12 and distance d = 5.3 ± 0.3 kpc, and from the RRc stars we find [Fe/H]ZW = −1.02 ± 0.19 and distance d = 5.3 ± 0.2 kpc. Independent methods such as the P–L (period–luminosity) relations for RRab and SX Phe stars enable the estimation of a weighted average distance to the cluster of 5.4 ± 0.1 kpc. We confirm the amplitude modulations of the Blazhko type of five RRab and the non‐membership of V21 to the cluster. The color–magnitude diagram is consistent with an age of 11 Gyrs. The distribution of RRab and RRc stars seems well segregated around the first overtone red edge of the instability strip. This positions NGC 6171 among OoI‐type clusters where the pulsating modes are neatly separated in the horizontal branch. We report two new irregular variables of the Lb type.