Infrared $K_s$-band Photometry of Field RR Lyrae Variable Stars
  We present multi-epoch infrared photometry in the $K_s$-band for 74 bright RR
Lyrae variable stars tied directly to the 2MASS photometric system. We
systematize additional $K$-band photometry from the literature to the 2MASS
system and combine it to obtain photometry for 146 RR Lyrae stars on a
consistent, modern system. A set of outlier stars in the literature photometry
is identified and discussed. Reddening estimates for each star were gathered
from the literature and combined to provide an estimate of the interstellar
absorption affecting each star, and we find excellent agreement with another
source in the literature. We utilize trigonometric parallaxes from the Second
Data Release of ESA's Gaia astrometric satellite to determine the absolute
magnitude, $M_{K_s}$ for each of these stars, and analyze them using the
astrometry based luminosity prescription to obtain a parallax-based calibration
of $M_K$(RR). Our period-luminosity-metallicity relationship is $ M_{K_s} =
(-2.8\pm 0.2) (\log P +0.27) + (0.12\pm 0.02) ( [\mathrm{Fe/H}] + 1.3) -
(0.41\pm 0.03)$ mag. A Gaia global zero-point error of $\pi_{zp} = -0.042\pm
0.013\,$mas is determined for this sample of RR Lyrae stars.
