Globular cluster metallicities and distances from disentangling their RR Lyrae light curves

TL;DR

This study derives homogeneous metallicities and distances for 39 globular clusters using Fourier analysis of RR Lyrae light curves, highlighting differences between RRab and RRc stars and validating results with Gaia and Hubble data.

Contribution

It provides the first homogeneous Fourier-based metallicity and distance measurements for globular clusters, clarifying the RR Lyrae $M_V$--[Fe/H] relation and the utility of RRc stars as distance indicators.

Findings

RRab stars show a non-linear $M_V$--[Fe/H] relation influenced by horizontal branch structure.

RRc stars exhibit a linear, tight $M_V$--[Fe/H] relation with a shallow slope.

Photometric metallicities and distances agree well with spectroscopic and Gaia/Hubble measurements.

Abstract

We present mean horizontal branch absolute magnitudes and iron abundances for a sample of 39 globular clusters. These quantities were calculated in an unprecedented homogeneous fashion based on Fourier decomposition of ligt curves of RR Lyrae cluster members. Zero points for the luminosity calibrations are discussed. Our photometrically derived metallicities and distances compare very well with spectroscopic determinations of [Fe/H] and accurate distances obtained using {\sl Gaia} and {\sl Hubble Space Telescope} data. The need to distinguish between the results for RRab and RRc stars for a correct evaluation of the $M_V$--[Fe/H] relation is discussed. For RRab stars, the relation is non-linear, and the horizontal branch structure plays a significant role. For RRc stars, the relation remains linear and tight, and the slope is very shallow. Hence, the RRc stars seem better indicators of the parental cluster distances. Systematic time-series CCD imaging performed over the last 20 years enabled to discover and classify 330 variables in our sample of globular clusters.