RR Lyrae in 15 Galactic globular clusters

Our final relations have the form <M_W1_>={gamma}_W1-(2.381+/-0.097)logP_F_+(0.096+/-0.021)[Fe/H] and <M_W2_>={gamma}_W2_-(2.269+/-0.127)logP_F_+(0.108+/-0.021)[Fe/H], where [Fe/H] values are on the scale of Carretta et al. (2009A&A...508..695C).

We obtained two appreciably discrepant estimates for the zero-points {gamma}_W1_ and {gamma}_W2_ of both relations: one based on a statistical-parallax analysis - {gamma}_W1_=-0.829+/-0.093 and {gamma}_W2_=-0.776+/-0.093 and another, significantly brighter one, based on Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) trigonometric parallaxes - {gamma}_W1,HST_=-1.150+/-0.077 and {gamma}_W2,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 ({Delta}DM0 =0.04 and 0.06, respectively, with a scatter of only 0.06).