The Carnegie-Chicago Hubble Program: Calibration of the Near-Infrared RR
  Lyrae Period-Luminosity Relation With HST
  We present photometry of 30 Galactic RR Lyrae variables taken with HST
WFC3/IR for the Carnegie-Chicago Hubble Program. These measurements form the
base of the distance ladder measurements that comprise a pure Population II
base to a measurement of Ho at an accuracy of 3%. These data are taken with the
same instrument and filter (F160W) as our observations of RR Lyrae stars in
external galaxies so as to to minimize sources of systematic error in our
calibration of the extragalactic distance scale. We calculate mean magnitudes
based on one to three measurements for each RR Lyrae star using star-by-star
templates generated from densely time-sampled data at optical and mid-infrared
wavelengths. We use four RR Lyrae stars from our sample with well-measured HST
parallaxes to determine a zero point. This zero point will soon be improved
with the large number of precise parallaxes to be provided by Gaia. We also
provide preliminary calibration with the TGAS & Gaia DR2 data, and all three
zero points are in agreement, to within their uncertainties.
