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

Bagley, Megan; Chiar, J. E.; Whittet, D.; Waters, L.; Tielens, A. G. G. M.; Román-Zuñiga, C.; Pendleton, Y.; Min, M.; Lada, C.; van Breemen, J.; Tappe, A.
American Astronomical Society, AAS Meeting #215, #431.13; Bulletin of the American Astronomical Society, Vol. 42, p.364


Studies of the 9.7 micron silicate absorption feature have been limited in the past by the effects of atmospheric absorption and by limited sensitivity that made it difficult to observe sources behind very dense, highly obscuring clouds. We present here, using data from the Spitzer Space Telescope, a systematic study of the 9.7 micron silicate absorption feature over a range of extinctions in a single dense cloud, the Pipe Nebula. The densely-spaced red giants in the Galactic Bulge behind the Pipe Nebula make excellent background sources with which to probe the intervening dust. We fit infrared spectra of 31 stars with reddened stellar photosphere models and divide out the stellar continuum. 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 compare these data to the tight correlation between optical depth and extinction that has been seen in the diffuse interstellar medium. At higher extinctions, the dense sight lines of the Pipe Nebula are more scattered and at lower silicate optical depth than points in the diffuse interstellar medium at similar extinctions. We also consider the changes in the shape of the silicate absorption profile with increasing extinction.