Cepheid Period-Luminosity Relations in the Near-Infrared and the
  Distance to M31 from the Hubble Space Telescope Wide Field Camera 3
  We present measurements of 68 classical Cepheids with periods from 10 to 78
days observed in the near-infrared by the PHAT Program using the Wide Field
Camera 3 (WFC3) on the Hubble Space Telescope (HST). The combination of HST's
resolution and the use of near-infrared measurements provides a dramatic
reduction in the dispersion of the Period--Luminosity relation over the present
optical, ground-based data. Even using random-phase magnitudes we measure a
dispersion of just 0.17 mag, implying a dispersion of just 0.12 mag for mean
magnitudes. The error in the mean for this relation is 1% in distance. Combined
with similar observations of Cepheids in other hosts and independent distance
determinations, we measure a distance to M31 of mu_0=24.42 +/- 0.05
(statistical) +/- 0.03 (systematic), 765 +/- 28 kpc, in good agreement with
past measurements though with a better, 3% precision here. The result is also
in good agreement with independent distance determinations from two detached
eclipsing binaries allowing for an independent calibration of the Cepheid
luminosities and a determination of the Hubble constant.
