Mid-infrared period–luminosity relations for globular cluster RR Lyrae

The period–metallicity–WISEW1- and W2-band luminosity relations are derived for RR Lyrae stars based on WISE epoch photometry for 360 and 275 stars in 15 and 9 Galactic globular clusters, respectively. Our final relations have the form 〈MW1〉 = γW1− (2.381 ± 0.097) log PF + (0.096 ± 0.021)[Fe/H] and 〈MW2〉 = γW2 −(2.269 ± 0.127)log PF + (0.108 ± 0.021)[Fe/H], where [Fe/H] values are on the scale of Carretta et al. We obtained two appreciably discrepant estimates for the zero-points γW1 and γW2 of both relations: one based on a statistical-parallax analysis – γW1 = −0.829 ± 0.093 and γW2 = −0.776 ± 0.093 and another, significantly brighter one, based on Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) trigonometric parallaxes – γW1,HST = −1.150 ± 0.077 and γW2,HST = −1.105 ± 0.077. The period–metallicity–luminosity relations in the two bands yield highly consistent distance moduli for the calibrator clusters and the distance moduli computed using the W1- and W2-band relations with the HST zero-points agree well with those computed by Sollima, Cacciari & Valenti based on their derived period–metallicity–K-band luminosity relation whose zero-point is tied to the HST trigonometric parallax of RR Lyrae itself (ΔDM0 = 0.04 and 0.06, respectively, with a scatter of only 0.06).