Large Magellanic Cloud Globular Clusters in the Near-infrared. I. RR Lyrae in Reticulum

Abstract

Reticulum is an old, metal-poor, and sparsely populated globular cluster in the outer regions of the Large Magellanic Cloud (LMC) and hosts a rich population of RR Lyrae stars. Being as close as possible to a single stellar population with negligible metallicity spread and low reddening, Reticulum is an ideal laboratory for testing stellar pulsation models and calibrating population II distance indicators. We present homogeneous multi-epoch near-infrared (NIR, JHKs) observations of RR Lyrae variables in Reticulum obtained with the Flamingos-2 imager on the 8.1-m Gemini South Telescope. Using NIR light-curve templates, we derive accurate intensity-averaged magnitudes and peak-to-peak amplitudes for 32 RR Lyrae stars, including 22 fundamental-mode (RRab), 4 first-overtone (RRc), and 6 mixed-mode (RRd) pulsators. The empirical JHKs period-luminosity (PL) relations of Reticulum RR Lyrae are very tight, exhibiting dispersions (~0.05 mag) comparable to those observed in Galactic globular clusters. The derived PL slopes are shallower than those reported for Galactic cluster variables. Adopting recent empirical and theoretical period-luminosity-metallicity (PLZ) calibrations based on Galactic globular clusters and pulsation models, we derive a true distance modulus of $\hat{I}{¼}_0=18.472\pm 0.035$ mag to Reticulum. This cluster distance is in excellent agreement with the precise geometric distance to the LMC and places Reticulum close to the LMC barycentric distance. The well-characterized RR Lyrae population and a precise distance make Reticulum a potential anchor for calibrating Population II distance ladder.

Accepted for publication in The Astronomical Journal