Mid-infrared PL relations for Globular Cluster RR Lyrae
  The period - metallicity - WISE W1- 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> = gamma(W1) - (2.381 +/- 0.097) log PF + (0.096 +/- 0.021)[Fe/H]
and <MW2> = gamma(W2)-(2.269 +/- 0.127)log PF + (0.108 +/- 0.021)[Fe/H], where
[Fe/H] values are on the scale of Carretta et al. (2009). 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 HST 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 \citet{sollima} 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).
