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

Understand

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.