On the distance of the globular cluster M4 (NGC 6121) using RR Lyrae stars: II. Mid-infrared period-luminosity relations

TL;DR

This paper establishes new mid-infrared period-luminosity relations for RR Lyrae stars in globular cluster M4, providing precise distance measurements and insights into their properties using Spitzer data and Gaia parallaxes.

Contribution

It presents the first mid-infrared PL relations for RR Lyrae in M4 and calibrates them with Gaia parallaxes, improving distance estimates.

Findings

PL relation dispersion is 0.056 mag

Distance modulus for M4 is 11.399 with low statistical error

No MIR period-color relation indicates minimal CO absorption effects

Abstract

New mid-infrared period-luminosity (PL) relations are presented for \rrl{} variables in the globular cluster M4 (NGC 6121). Accurate photometry was obtained for 37 \rrl{} variables using observations from the Infrared Array Camera onboard the Spitzer Space Telescope. The dispersion of M4's PL relations is 0.056, and the uncertainty in the slope is 0.11 mag. Additionally, weestablished calibrated PL relations at 3.6 and 4.5~\micron{} using published Hubble Space Telescopegeometric parallaxes of five Galactic \rrl{} stars. The resulting band-averaged distance modulus for M4 is $ \mu = 11.399 \pm 0.007 \textrm{(stat)} \pm 0.080 \textrm{(syst)} \pm 0.015 \textrm{(cal)} \pm 0.020 \textrm{(ext)}$. The systematic uncertainty will be greatly reduced when parallaxes of more stars become available from the GAIA mission. Optical and infrared period-color (PC) relations are also presented, and the lack of a MIR PC relation suggests that \rrl{} stars are not affected by CO absorptionin the 4.5~\micron{} band.