Metallicity of Galactic RR Lyrae from Optical and Infrared Light Curves: I. Period-Fourier-Metallicity Relations for Fundamental Mode RR Lyrae

TL;DR

This paper develops new period-$_{31}$-[Fe/H] relations for fundamental mode RR Lyrae stars using optical and mid-infrared light curves, enabling more accurate metallicity estimates across a broad range.

Contribution

It provides the first mid-infrared period-$_{31}$-[Fe/H] relation for RR Lyrae stars, calibrated with the largest dataset covering extensive metallicity and light curve parameters.

Findings

The new optical relation minimizes systematic metallicity trends.

Photometric metallicities are consistent with spectroscopic measurements.

Using optical and infrared data together reduces intrinsic scatter.

Abstract

We present newly-calibrated period-$\phi_{31}$-[Fe/H] relations for fundamental mode RR Lyrae stars in the optical and, for the first time, mid-infrared. This work's calibration dataset provides the largest and most comprehensive span of parameter space to date with homogeneous metallicities from $-3<\textrm{[Fe/H]}<0.4$ and accurate Fourier parameters derived from 1980 ASAS-SN ($V$-band) and 1083 WISE (NEOWISE extension, $W1$ and $W2$ bands) RR Lyrae stars with well-sampled light curves. We compare our optical period-$\phi_{31}$-[Fe/H] with those available in the literature and demonstrate that our relation minimizes systematic trends in the lower and higher metallicity range. Moreover, a direct comparison shows that our optical photometric metallicities are consistent with both those from high-resolution spectroscopy and globular clusters, supporting the good performance of our relation. We found an intrinsic scatter in the photometric metallicities (0.41 dex in the $V$-band and 0.50 dex in the infrared) by utilizing large calibration datasets covering a broad metallicity range. This scatter becomes smaller when optical and infrared bands are used together (0.37 dex). Overall, the relations derived in this work have many potential applications, including large-area photometric surveys with JWST in the infrared and LSST in the optical.