Palomar Transient Factory and RR Lyrae: Metallicity-Light Curve Relation Based on ab-Type RR Lyrae in Kepler Field

TL;DR

This paper derives a new metallicity-light curve relation for ab-type RR Lyrae stars in the PTF/iPTF R-band system using Kepler field data, enabling metallicity estimation from photometric light curves.

Contribution

The study provides the first metallicity-light curve relation tailored for the PTF/iPTF R-band system based on Kepler RR Lyrae data, facilitating future Galactic halo studies.

Findings

Derived a metallicity-light curve relation with 0.118 dex dispersion.

Validated the relation with spectroscopic data showing good agreement.

Enabled metallicity estimation from photometric light curves for RR Lyrae.

Abstract

The wide-field synoptic sky surveys, known as the Palomar Transient Factory (PTF) and the intermediate Palomar Transient Factory (iPTF), will accumulate a large number of known and new RR Lyrae. These RR Lyrae are good tracers to study the substructure of the Galactic halo if their distance, metallicity, and galactocentric velocity can be measured. Candidates of halo RR Lyrae can be identified from their distance and metallicity before requesting spectroscopic observations for confirmation. This is because both quantities can be obtained via their photometric light curves, because the absolute V-band magnitude for RR Lyrae is correlated with metallicity, and the metallicity can be estimated using a metallicity-light curve relation. To fully utilize the PTF and iPTF light-curve data in related future work, it is necessary to derive the metallicity-light curve relation in the native PTF/iPTF R-band photometric system. In this work, we derived such a relation using the known ab-type RR Lyrae located in the Kepler field, and it is found to be $[Fe/H]_{PTF} = -4.089 - 7.346 P + 1.280 \phi_{31}$ (where $P$ is pulsational period and $\phi_{31}$ is one of the Fourier parameters describing the shape of the light curve), with a dispersion of 0.118 dex. We tested our metallicity-light curve relation with new spectroscopic observations of a few RR Lyrae in the Kepler field, as well as several data sets available in the literature. Our tests demonstrated that the derived metallicity-light curve relation could be used to estimate metallicities for the majority of the RR Lyrae, which are in agreement with the published values.