Mapping the Extinction Curve in 3D: Structure on Kiloparsec Scales

Near-infrared spectroscopy from APOGEE and wide-field optical photometry from Pan-STARRS1 have recently made possible precise measurements of the shape of the extinction curve for tens of thousands of stars, parameterized by R(V). These measurements revealed structures in R(V) with large angular scales, which are challenging to explain in existing dust paradigms. In this work, we combine three-dimensional maps of dust column density with R(V) measurements to constrain the three-dimensional distribution of R(V) in the Milky Way. We find that variations in R(V) are correlated on kiloparsec scales. In particular, most of the dust within one kiloparsec in the outer Galaxy, including many local molecular clouds (Orion, Taurus, Perseus, California, Cepheus), has a significantly lower R(V) than more distant dust in the Milky Way. These results provide new input to models of dust evolution and processing, and complicate application of locally derived extinction curves to more distant regions of the Milky Way and to other galaxies.