Optical and near-infrared pulsation properties of RR Lyrae and Population II Cepheid variables in the Messier 15 globular cluster

TL;DR

This study uses optical and near-infrared observations of variable stars in Messier 15 to refine their properties, establish precise period-luminosity relations, and accurately determine the cluster's distance.

Contribution

It presents new multi-epoch optical and NIR data for variable stars in M15, deriving improved period-luminosity relations and a precise distance measurement using RR Lyrae and Population II Cepheids.

Findings

Established tight period-luminosity relation in $K_s$-band with 0.037 mag scatter.

Derived a highly accurate distance modulus of 15.196 mag for M15.

Confirmed consistency of distance estimates with Gaia parallaxes and other methods.

Abstract

Messier 15 (NGC 7078) is an old and metal-poor post core-collapse globular cluster which hosts a rich population of variable stars. We report new optical ($gi$) and near-infrared (NIR, $JK_s$) multi-epoch observations for 129 RR Lyrae, 4 Population II Cepheids (3 BL Herculis, 1 W Virginis), and 1 anomalous Cepheid variable candidate in M15 obtained using the MegaCam and the WIRCam instruments on the 3.6-m Canada-France-Hawaii Telescope. Multi-band data are used to improve the periods and classification of variable stars, and determine accurate mean magnitudes and pulsational amplitudes from the light curves fitted with optical and NIR templates. We derive optical and NIR period-luminosity relations for RR Lyrae stars which are best constrained in the $K_s$-band, $m_{K_s} = -2.333~(0.054) \log P + 13.948~(0.015)$ with a scatter of only $0.037$ mag. Theoretical and empirical calibrations of RR Lyrae period-luminosity-metallicity relations are used to derive a true distance modulus to M15: $15.196~\pm~0.026$~(statistical)~$\pm~ 0.039$~(systematic) mag. Our precise distance moduli based on RR Lyrae stars and Population II Cepheid variables are mutually consistent and agree with recent distance measurements in the literature based on {\it Gaia} parallaxes and other independent methods.