Standard Galactic Field RR Lyrae II: A Gaia DR2 calibration of the period-Wesenheit-metallicity relation

TL;DR

This paper calibrates multi-wavelength period-luminosity-metallicity and Wesenheit-metallicity relations for Galactic RR Lyrae stars using Gaia DR2 parallaxes, improving distance measurements and confirming previous estimates.

Contribution

It provides new Gaia DR2-based calibrations of PLZ and PWZ relations across multiple wavelengths for Galactic RR Lyrae stars, including the measurement of metallicity slopes.

Findings

Scatter in PLZ relations is about 0.2 mag, mainly due to parallax uncertainties.

Calibrated relations agree with previous distance estimates for M4, LMC, and SMC.

Metallicity slope can now be measured in all passbands.

Abstract

RR Lyrae stars have long been popular standard candles, but significant advances in methodology and technology have been made in recent years to increase their precision as distance indicators. We present multi-wavelength (optical $UBVR_cI_c$ and Gaia $G, BP, RP$; near-infrared $JHK_s$; mid-infrared $[3.6], [4.5]$) period-luminosity-metallicity (PLZ), period-Wesenheit-metallicity (PWZ) relations, calibrated using photometry obtained from The Carnegie RR Lyrae Program and parallaxes from the Gaia second data release for 55 Galactic field RR Lyrae stars. The metallicity slope, which has long been predicted by theoretical relations, can now be measured in all passbands. The scatter in the PLZ relations is on the order of 0.2 mag, and is still dominated by uncertainties in the parallaxes. As a consistency check of our PLZ relations, we also measure the distance modulus to the globular cluster M4, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), and our results are in excellent agreement with estimates from previous studies.