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

Abstract

We compute the mid-infrared extinction law from 3.6-24 microns 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 chi-squared deviation of the data from two extinction law models. Because our chi-squared 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 microns, we see evidence for grain growth. Below $A_{K_s}=0.5$, our extinction law is well-fit by the Weingartner & Draine (2001) $R_V=3.1$ diffuse interstellar medium dust model. As the extinction increases, our law gradually flattens, and for $A_{K_s}>=1$, the data are more consistent with the Weingartner & Draine $R_V=5.5$ model that uses larger maximum dust grain sizes. At 24 microns, 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. (2007). Lastly, from our chi-squared 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.

38 pages, 19 figures in pre-print format. Accepted for publication in ApJ. A version with full-resolution figures can be found here: http://peggysue.as.utexas.edu/SIRTF/

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• The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds: VII. Ophiuchus Observed with MIPS
• Hunting Galaxies to (and for) Extinction

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