# Infrared $K_s$-band Photometry of Field RR Lyrae Variable Stars

## TL;DR

This study provides a precise calibration of the infrared period-luminosity-metallicity relation for RR Lyrae stars using Gaia parallaxes, improving distance measurements in astronomy.

## Contribution

It offers a new, Gaia-based calibration of the $K_s$-band period-luminosity-metallicity relation for RR Lyrae stars, integrating multi-epoch photometry and literature data.

## Key findings

- Calibrated the $M_{K_s}$ relation with Gaia parallaxes.
- Identified and corrected literature photometry outliers.
- Determined a Gaia zero-point error of -0.042 mas.

## Abstract

We present multi-epoch infrared photometry in the $K_s$-band for 74 bright RR Lyrae variable stars tied directly to the 2MASS photometric system. We systematize additional $K$-band photometry from the literature to the 2MASS system and combine it to obtain photometry for 146 RR Lyrae stars on a consistent, modern system. A set of outlier stars in the literature photometry is identified and discussed. Reddening estimates for each star were gathered from the literature and combined to provide an estimate of the interstellar absorption affecting each star, and we find excellent agreement with another source in the literature. We utilize trigonometric parallaxes from the Second Data Release of ESA's Gaia astrometric satellite to determine the absolute magnitude, $M_{K_s}$ for each of these stars, and analyze them using the astrometry based luminosity prescription to obtain a parallax-based calibration of $M_K$(RR). Our period-luminosity-metallicity relationship is $ M_{K_s} = (-2.8\pm 0.2) (\log P +0.27) + (0.12\pm 0.02) ( [\mathrm{Fe/H}] + 1.3) - (0.41\pm 0.03)$ mag. A Gaia global zero-point error of $\pi_{zp} = -0.042\pm 0.013\,$mas is determined for this sample of RR Lyrae stars.