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[71.18] Ice and Dust Composition along the Line of Sight Toward the Galactic Center

J.E. Chiar (NASA/Ames), A.G.G.M. Tielens (Kapteyn Astronomical Institute), D. Lutz (Max-Planck Institute), A.C.A. Boogert (Kapteyn Astronomical Institute), W.A. Schutte (Leiden Observatory), D.C.B. Whittet (Rensselaer Polytechnic Institute), E.F. van Dishoeck (Leiden Observatory)

We discuss the composition of dust and ice along the line of sight to the Galactic Center (GC) based on analysis of mid-infrared spectra (2.4--13 \mum) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). Bright infrared sources combined with 30 magnitudes of visual extinction have made the GC an attractive line of sight for the study of the composition of the general interstellar medium. Detection of interstellar ices along the line of sight to the GC has shown that one or more molecular clouds obscure the GC and contribute to the total extinction (A&A Special Issue, 1996).

We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sagitarius A* and the Quintuplet sources, GCS3 and GCS4. It is evident from the depth of the 3.0 \mum H₂O and the 4.27 \mum CO₂ ice features that there is more molecular cloud material along the line of sight toward Sgr A* than GCS3 and 4. In fact, Sgr A* has a rich solid state infrared spectrum which also reveals strong evidence for the presence of solid NH₃, HCOOH and CH₄ in the molecular cloud ices.

Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4 \mum, 6.85 \mum, and 7.3 \mum. Ground-based studies have identified the 3.4 \mum feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that some form of hydrogenated amorphous carbon may be their carrier. We attribute an absorption feature centered at 3.28 \mum in the GCS3 spectrum to polycyclic aromatic hydrocarbons. Since we only detected this feature toward GCS3, but not toward SgrA*, one of the key questions which now arises is whether this aromatic material is a widespread component of the diffuse interstellar medium, as is the 3.4 \mum aliphatic hydrocarbon feature.

This work is supported by NASA's Long Term Space Astrophysics Program.

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