A vindication of the RR Lyrae Fourier light curve decomposition for the calculation of metallicity and distance in globular clusters

TL;DR

This study validates the Fourier decomposition method of RR Lyrae light curves for accurately determining metallicity and distance in globular clusters, highlighting its consistency with spectroscopic and Gaia/HST data.

Contribution

It provides a homogeneous calibration approach for RR Lyrae stars, distinguishing between RRab and RRc types, and explores the metallicity dependence of the horizontal branch luminosity.

Findings

Good agreement with spectroscopic metallicities.

Linear correlation for RRc stars' luminosity.

Non-linear calibration for RRab stars' luminosity.

Abstract

We report the mean metallicity and absolute magnitude of RR Lyrae stars in a sample of 37 globular clusters, calculated via the Fourier decomposition of their light curves and ad hoc semiempirical calibrations, in an unprecedented homogeneous approach. This enabled a new discussion of the metallicity dependence of the horizontal branch (HB) luminosity, as a fundamental distance indicator. The calibration for the RRab and RRc stars should be treated separately. For the RRab the dispersion is larger and non-linear. For the RRc stars the correlation is less steep, very tight and linear. The relevance of the HB structural parameter L, is highlighted and offer a non-linear calibration of the form MV ([Fe/H],L). Excellent agreement is found between values of [Fe/H] and MV from the light curve decomposition with spectroscopic values and distances obtained via Gaia-DR3 and HST. The variable stars census in 35 clusters includes 326 stars found by our program.