Spitzer Space Telescope - General Observer Proposal #3616


Solving the Mysteries of Interstellar Dust Composition in the Milky
Way

Principal Investigator: Jean Chiar
           Institution: SETI Institute

Co-Investigators: 
Michael Egan, MDA/AS                                                  
Alexander Tielens, Kapteyn Institute/SRON                             
G.C. Sloan, Cornell University                                        

Science Category: ISM 
 Observing Modes: IrsStare 
  Hours Approved: 16.3 

Abstract:
Interstellar dust is ubiquitous, yet its precise composition is still
widely debated.  Spitzer's Infrared Spectrometer provides the
sensitivity to study previously unattainable lines of sight throughout
the plane of the Milky Way.  In this proposal, we seek to study the
hydrocarbon and silicate dust components of the interstellar medium
for 56 lines of sight spanning visual extinctions from 6.6 to 32.2
magnitudes across a range of Galactic longitudes and latitudes.
Specifically, we will measure the hydrocarbon absorption features at
6.9 and 7.3 micron, and the silicate absorption features at 9.7 and
18.5 micron.  The ratio of the hydrocarbon and silicate features
provides a direct handle on the hydrocarbon to silicate dust volume.
It has been previously been noted that the optical depth to visual
extinction ratio is distinct for lines of sight within the Solar
neighborhood compared to the Galactic Center.  Thus, these features
will also be related to their visual extinction and Galactic location
to test whether location has an effect on the optical depth to Av
ratio.  Finally, the silicate mineralogy can be assessed by studying
the ratio of the 9.7 to 18.5 micron absorption features, whose
relative strengths have been shown to be indicative of olivine-rich or
pyroxene-rich silicates.  In order to obtain sufficient sampling of
both Galactic location and visual extinction, we will observe 56 lines
of sight requiring a total of 16.3 hours.