2009ApJ...690..496C

We compute the mid-IR extinction law from 3.6 to 24 µm in three molecular clouds–Ophiuchus, Perseus, and Serpens–by combining data from the "Cores to Disks" Spitzer Legacy Science program with deep JHK_simaging. Using a new technique, we are able to calculate the line-of-sight (LOS) extinction law toward each background star in our fields. With these LOS measurements, we create, for the first time, maps of the χ² deviation of the data from two extinction law models. Because our χ² maps have the same spatial resolution as our extinction maps, we can directly observe the changing extinction law as a function of the total column density. In the Spitzer Infrared Array Camera (IRAC) bands, 3.6-8 µm, we see evidence for grain growth. Below, our extinction law is well fitted by the Weingartner and Draine R_V = 3.1 diffuse interstellar-medium dust model. As the extinction increases, our law gradually flattens, and for, the data are more consistent with the Weingartner and Draine R_V= 5.5 model that uses larger maximum dust grain sizes. At 24 µm, our extinction law is 2-4 times higher than the values predicted by theoretical dust models, but is more consistent with the observational results of Flaherty et al. Finally, from our χ² maps we identify a region in Perseus where the IRAC extinction law is anomalously high considering its column density. A steeper near-IR extinction law than the one we have assumed may partially explain the IRAC extinction law in this region.