A Color-locus Method for Mapping $R_V$ Using Ensembles of Stars
  We present a simple but effective technique for measuring angular variation
in $R_V$ across the sky. We divide stars from the Pan-STARRS1 catalog into
Healpix pixels and determine the posterior distribution of reddening and $R_V$
for each pixel using two independent Monte Carlo methods. We find the two
methods to be self-consistent in the limits where they are expected to perform
similarly. We also find some agreement with high-precision photometric studies
of $R_V$ in Perseus and Ophiuchus, as well as with a map of reddening near the
Galactic plane based on stellar spectra from APOGEE. While current studies of
$R_V$ are mostly limited to isolated clouds, we have developed a systematic
method for comparing $R_V$ values for the majority of observable dust. This is
a proof of concept for a more rigorous Galactic reddening map.x
