The Mid-Infrared Extinction Law in the Ophiuchus, Perseus, and Serpens Molecular Clouds

We compute the mid-infrared extinction law from 3.6−24μm in three molecular clouds: Ophiuchus, Perseus, and Serpens, by combining data from the “Cores to Disks” Spitzer Legacy Science program with deep JHKs imaging. Using a new technique, we are able to calculate the line-of-sight extinction law towards each background star in our fields. With these line-of-sight measurements, we create, for the first time, maps of the χ deviation of the data from two extinction law models. Because our χ 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 IRAC bands, 3.6− 8 μm, we see evidence for grain growth. Below AKs = 0.5, our extinction law is well-fit by the Weingartner & Draine (2001) RV = 3.1 diffuse interstellar medium dust model. As the extinction increases, our law gradually flattens, and for AKs ≥ 1, the data are more consistent with the Weingartner & Draine RV = 5.5 model that uses larger maximum dust grain sizes. At 24 μm, our extinction law is 2− 4× higher than the values predicted by theoretical dust models, but is more consistent with the observational results of Flaherty et al. (2007). Lastly, from our χ maps we identify a region in Perseus where the IRAC extinction law is anomalously high considering its column density. A steeper near-infrared extinction law than the one we have assumed may partially explain the IRAC extinction law in this region. Subject headings: infrared: stars—ISM: clouds—stars: formation Department of Astronomy, University of Maryland, College Park, MD 20742 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 301-429, Pasadena, CA 91109; [email protected] Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu 30043, Taiwan Department of Astronomy, University of Texas at Austin, 1 University Station C1400, Austin, TX 78712