The Evolving ISM in the Milky Way & Nearby Galaxies Infrared Extinction Toward Nearby Star-Forming Regions

We present an independent estimate of the interstellar extinction law for the Spitzer IRAC bands as well as a rst attempt at extending the law to the 24m MIPS band. The source data for these measurements are observations of ve nearby star-forming regions: the Orion A cloud, NGC 2068/71, NGC 2024/23, Serpens and Ophiuchus. Color excess ratios EHKs=EKs[] were measured for stars without infrared excess dust emission from circumstellar disks/envelopes. For four of these ve regions, the extinction laws are similar at all wavelengths and dier systematically from a previous determination of the extinction law, which was dominated by the diuse ISM, derived for the IRAC bands. This dierence could be due to the dierence in the dust properties of the dense molecular clouds observed here and those of the diuse ISM. The extinction law at longer wavelengths toward the Ophiuchus region lies between that to the other four regions studied here and that for the ISM. In addition, we extended our extinction law determination to 24m for Serpens and NGC 2068=71 using Spitzer MIPS data. Our work conrms a relatively atter extinction curve from 4- 8m than the previously assumed standard. The extinction law at 24m is consistent with previous measurements and models, although there are relatively large uncertainties. Subject headings: dust, extinction | infrared:genera