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Thesis Research

Dust Extinction Law

The interstellar medium is filled with dust, sub-micron to micron sized carbonaceous and silicate grains. Understanding the nature of this dust is important since it attenuates, or extincts, the light from objects seen through the clouds. The degree of extinction is a function of wavelength, this is known as the extinction law. Different dust models will predict different extinction laws; therefore by observing the extinction law we learn about the properties of the dust grains. For my thesis, I used the Spitzer Space Telescope combined with deep near-infrared observations to measure the extinction law from 3.6-24 microns. I observed regions within three molecular clouds: Ophiuchus, Perseus, and Serpens, and four dense cores: L204C-2, L1152, L1155C-2, L1228. With my data, I was able to create chi-squared maps of the changing extinction law within each region. For example, in Ophiuchus:

Note the correlation between regions of high column density (upper panel) and chi-squared RV = 3.1 (middle panel). RV = 3.1 is shorthand for an dust model that fits the diffuse interstellar medium. The bottom panel shows RV = 5.5, a dust model designed for denser regions. For most of our data, dense regions also have a high RV = 3.1 chi-squared, meaning they are not fit by the predicted extinction law for diffuse regions.

This is not always the case. We found that both L1152 and L1228 contain molecular outflows. These outflows sweeping out cavities and destroying the larger dust grains in shocks. The net result is the extinction law is altered in a noticeable way:

Here, the color background shows the extinction (column density) while the magenta contours are the RV = 5.5 chi-squared values. The black lines show the direction of each outflow. Notice that at the end of each outflow is a region of high chi-squared. This is the large dust grains being destroyed by the outflow.

Future Work

The 24 micron extinction law still remains to be explained. Below we plotted our average extinction law for all four cores (green dots). The 3.6-8 micron extinction law is consistent with other authors' results and other dust models. At 24 microns, our data point is similar to other authors' values, but significantly higher than the predicted value from any dust model.