THE MID-INFRARED EXTINCTION LAW IN THE OPHIUCHUS, PERSEUS, AND SERPENS MOLECULAR CLOUDS

We compute the mid-IR extinction law from 3.6 to 24 mu m in three molecular clouds-Ophiuchus, Perseus, and Serpens-by combining data from the "Cores to Disks" Spitzer Legacy Science program with deep JHK(s) imaging. 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 chi(2) deviation of the data from two extinction law models. Because our chi(2) 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 mu m, we see evidence for grain growth. Below A(Ks) = 0.5, 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 A(Ks) >= 1, the data are more consistent with the Weingartner and Draine R(V) = 5.5 model that uses larger maximum dust grain sizes. At 24 mu m, 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 chi(2) 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.