Standard Galactic Field RR Lyrae II: A Gaia DR2 calibration of the
  period-Wesenheit-metallicity relation
  RR Lyrae stars have long been popular standard candles, but significant
advances in methodology and technology have been made in recent years to
increase their precision as distance indicators. We present multi-wavelength
(optical $UBVR_cI_c$ and Gaia $G, BP, RP$; near-infrared $JHK_s$; mid-infrared
$[3.6], [4.5]$) period-luminosity-metallicity (PLZ),
period-Wesenheit-metallicity (PWZ) relations, calibrated using photometry
obtained from The Carnegie RR Lyrae Program and parallaxes from the Gaia second
data release for 55 Galactic field RR Lyrae stars. The metallicity slope, which
has long been predicted by theoretical relations, can now be measured in all
passbands. The scatter in the PLZ relations is on the order of 0.2 mag, and is
still dominated by uncertainties in the parallaxes. As a consistency check of
our PLZ relations, we also measure the distance modulus to the globular cluster
M4, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), and
our results are in excellent agreement with estimates from previous studies.
