CCD time-series photometry of variable stars in globular clusters and the metallicity dependence of the horizontal branch luminosity

TL;DR

This study uses CCD time-series photometry and difference image analysis to discover and analyze variable stars in globular clusters, deriving their properties and exploring the metallicity dependence of the horizontal branch luminosity.

Contribution

It provides a homogeneous calibration of cluster distances and metallicities using Fourier decomposition of RR Lyrae light curves and investigates the luminosity-metallicity relation of the horizontal branch.

Findings

Discovered approximately 250 variable stars in 23 globular clusters.

Calibrated the P-L relation for SX Phe stars for independent distance estimates.

Analyzed the $M_V$-[Fe/H] relation, finding it should be considered separately for RRab and RRc stars.

Abstract

We describe and summarize the findings from our CCD time-series photometry of globular clusters (GCs) program and the use of difference image analysis (DIA) in the extraction of very precise light curves even in the crowded central regions down to $V\sim 19$ mag. We have discovered approximately 250 variable stars in a sample of 23 selected GCs and have, therefore, updated the census of variables in each system and in some cases we have actually completed it down to a certain magnitude. The absolute magnitude and [Fe/H] for each individual RR Lyrae is obtained via the Fourier decomposition of the light curve. An average of these parameters leads to the distance and metallicity of the host GCs. We have also calibrated the P-L relation for SX Phe stars which enables an independent calculation of the cluster distance. We present the mean [Fe/H], $M_V$ and distance for a group of selected GCs based exclusively on the RR Lyrae light curve Fourier decomposition technique and set on a rather unprecedented homogeneous scale. We also discuss the luminosity dependence of the horizontal branch (HB) via the $M_V$-[Fe/H] relation. We find that this relation should be considered separately for the RRab and RRc stars.