Dust Structure and Composition Within Molecular Clouds and Cores

Abstract

We observed three molecular clouds and four isolated cores at wavelengths from 3.6-24 microns. The clouds we observed were Ophiuchus, Perseus, and Serpens and the cores were L204C-2, L1152, L1155C-2, and L1228. Our goal was to use these deep infrared data to map changes in the extinction law and the dust properties throughout the observed regions.

In our clouds, we found the lowest density regions have an IRAC extinction law similar to the one observed in the diffuse ISM. At higher extinctions, there is evidence for grain growth because the extinction law flattens compared to the diffuse ISM law and becomes more consistent with a model utilizing larger dust grains. In the densest regions of Serpens and Perseus, Ak > 2, it appears icy mantles are forming on the dust grains. We detected one low extinction region in Perseus with an anomalous extinction law that is not explained by current ideas about grain growth or the formation of ices onto dust grains.

The extinction law in the cores shows only a slight flattening of the extinction law with increased extinction. Even at the lowest extinctions, the extinction law is more consistent with a dust model containing grain growth, rather than with the diffuse ISM. Two of the four cores have evidence for ices forming the densest regions. Molecular outflows appear to have an impact on the dust grains in two of our cores: L1152 and L1228.

In both our clouds and cores, the extinction law at 24 microns is almost always higher than the value predicted by current dust models, but is consistent with other observations. We find some evidence for the 24 micron extinction law decreasing as the extinction increases. Overall, there are relatively few stars with detections >3 sigma at 24 microns. More observations are needed to understand the nature of the extinction law at this wavelength.