Silicate Composition Toward The Coalsack Dark Cloud Complex
Abstract
In the diffuse interstellar medium (ISM), a tight correlation exists between the optical depth of the 10-micron silicate feature and the near-infrared color excess, E(J-K). However, this relationship is not consistent for lines-of-sight toward dense molecular clouds, as the silicate optical depth is weaker per unit E(J-K). The dissimilarity in the observed correlation between dense molecular cloud lines-of-sight and that of the diffuse ISM has been attributed to differences in dust properties (e.g., chemical composition and grain growth), but the exact cause remains unclear. In an effort to better understand these underlying differences, we model 34 archival Spitzer-IRS spectra of background field stars toward the Coalsack dark cloud complex using a spectral decomposition analysis. To quantitatively assess the dust composition, our model includes Mg-rich amorphous silicates of olivine and pyroxene stoichiometry and amorphous water ice. Consistent with previous studies, we find that the silicate optical depth toward the Coalsack complex is shallower per unit E(J-K) than in the diffuse ISM. Furthermore, in contrast with the silicate composition of the diffuse ISM, silicates of pyroxene composition dominate the amorphous silicate mass fraction toward the Coalsack complex. In the context of composition, we re-examine whether differences in the measured dust properties between diffuse ISM and dense molecular cloud lines-of-sight can account for deviations from the invariable diffuse ISM correlation.
- Publication:
-
American Astronomical Society Meeting Abstracts #221
- Pub Date:
- January 2013
- Bibcode:
- 2013AAS...22144004H