CCD Photometry of the globular cluster NGC 5466. RR Lyrae light curve decomposition and the distance scale

TL;DR

This study uses CCD photometry and Fourier analysis of RR Lyrae stars in NGC 5466 to refine distance and metallicity estimates, recalibrate the M_V-[Fe/H] relation, and discuss implications for cluster age.

Contribution

It provides new Fourier-based calibrations of RR Lyrae luminosity-metallicity relation and accurate distance and metallicity measurements for NGC 5466.

Findings

Distance to NGC 5466 is 16.0 ± 0.6 kpc.

Metallicity [Fe/H] is -1.91 ± 0.19.

Recalibrated M_V-[Fe/H] relation predicts M_V=0.58 at [Fe/H]=-1.5.

Abstract

We report the results of CCD V and r photometry of the globular cluster NGC 5466. The difference image analysis technique adopted in this work has resulted in accurate time series photometry even in crowded regions of the cluster enabling us to discover five probably semi-regular variables. We present new photometry of three previously known eclipsing binaries and six SX Phe stars. The light curves of the RR Lyrae stars have been decomposed in their Fourier harmonics and their fundamental physical parameters have been estimated using semi-empirical calibrations. The zero points of the metallicity, luminosity and temperature scales are discussed and our Fourier results are transformed accordingly. The average iron abundance and distance to the Sun derived from individual RR Lyrae stars, indicate values of [Fe/H]=-1.91 +- 0.19 and D = 16.0 +- 0.6 kpc, or a true distance modulus of 16.02 +- 0.09 mag, for the parent cluster. These values are respectively in the Zinn & West metallicity scale and in agreement with recent luminosity determinations for the RR Lyrae stars in the LMC. The M_V-[Fe/H] relation has been recalibrated as M_V=+(0.18+-0.03)[Fe/H]+(0.85+-0.05) using the mean values derived by the Fourier technique on RR Lyrae stars in a family of clusters. This equation predicts M_V=0.58 mag for [Fe/H]=-1.5, in agreement with the average absolute magnitude of RR Lyrae stars calculated from several independent methods. The M_V-[Fe/H] relationship and the value of [Fe/H] have implications on the age of the globular clusters when determined from the magnitude difference between the horizontal branch and the turn off point (HB-TO method). The above results however would not imply a change in the age of NGC 5466, of 12.5+-0.9 Gyr, estimated from recent isochrone fitting.