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.