Double-mode RR Lyrae star -- robust distance and metallicity indicators

TL;DR

This paper develops a new method using double-mode RR Lyrae stars to measure distances and metallicities with high precision, improving the accuracy of cosmic distance measurements and providing a potential independent distance ladder.

Contribution

It introduces a linear relation between metallicity and period for RRd stars, enabling metallicity estimates and a metallicity-independent period-luminosity relation for precise distance measurements.

Findings

Metallicity can be predicted from period ratios with high accuracy.

A new metallicity-independent PL relation for RRd stars is established.

Distances to globular clusters and dwarf galaxies are measured with 2-3% and 1-2% accuracy.

Abstract

RR Lyrae (RR Lyr) stars are a well-known and useful distance indicator for old stellar populations such as globular clusters and dwarf galaxies. Fundamental-mode RR Lyr (RRab) stars are commonly used to measure distances, and the accuracy of the determined distance is strongly constrained by metallicity. Here, we investigate the metallicity dependence in the period-luminosity (PL) relation of double-mode RR Lyr (RRd) stars. We find and establish a linear relation between metallicity and period or period ratio for RRd stars. This relation can predict the metallicity as accurately as the low-resolution spectra. Based on this relation, we establish a metallicity-independent PL relation for RRd stars. Combining the distance of the Large Magellanic Cloud and Gaia parallaxes, we calibrate the zero point of the derived PL relation to an error of 0.022 mag. Using RRd stars, we measure the distances of globular clusters and dwarf galaxies with an accuracy of 2-3% and 1-2%, respectively. In the future, RRd stars could anchor galaxy distances to an accuracy of 1.0% and become an independent distance ladder in the Local Group.