Variation of the 9.7 micron Silicate Absorption Feature with Extinction in the Dense Interstellar Medium

Dust in the interstellar medium is important to astronomers because it absorbs light from more distant objects and because it provides the raw material from which planets form. One of the main components of interstellar dust is silicates. Studies of the 9.7 micron silicate absorption feature have been limited in the past by the effects of atmospheric absorption and by limited telescope sensitivity. This project uses data from the Spitzer Space Telescope to make a systematic study of this silicate feature over a range of extinctions in a single dense cloud, the Pipe Nebula. Infrared spectra of 31 stars are fit with reddened stellar photosphere models to divide out the contribution from background stars and show only the silicate feature. An absorption profile of water ice is also fit to each source. We plot the resulting optical depth of the 9.7 micron silicate feature against extinction and find that there is less silicate absorption for a given amount of total extinction than is found in more diffuse regions of the interstellar medium. The densest regions also show more silicate absorption at shorter wavelengths. These differences are thought to be due to dust grain growth in the dense interstellar medium