Optical–Infrared Extinction Variation in Nearby Molecular Clouds

The dust extinction curve is an important probe of the properties of dust grains, particularly their size distribution. The properties of dust grains are expected to change significantly in dense molecular clouds due to shielding from interstellar radiation and an increased frequency of grain–grain and grain–gas collisions. We measure the dust extinction-curve shape parameter R ( V ) in nine nearby dense molecular clouds using data from the Apache Point Observatory (APO) Galactic Evolution Experiment (APOGEE) spectroscopic survey, in combination with 10-band photometry from the Panoramic Survey Telescope and Rapid Response System (PS1), the Two Micron All Sky Survey (2MASS), and the Wide-field Infrared Survey Explorer (WISE). We analyze the results to determine how the extinction curve varies in nearby molecular clouds. We find that most molecular clouds show a positive correlation between R ( V ) and E ( B − V ), but the strength of the correlation varies significantly between clouds. In addition, we find that the clouds with the strongest correlation tend to have very high dispersion in R ( V ) at fixed E ( B − V ), which tightens dramatically when the clouds are subdivided into separate regions. This suggests that local environmental conditions beyond density shape the R ( V )– E ( B − V ) relation.