NEAR-INFRARED ABSORPTION-SPECTROSCOPY OF INTERSTELLAR HYDROCARBON GRAINS

We present new 3600-2700 cm(-1) (2.8-3.7 mu m) spectra of objects whose extinction is dominated by dust in the diffuse interstellar medium. The observations presented here augment an ongoing study of the organic component of the diffuse interstellar medium. These spectra contain a broad feature centered near 3300 cm(-1) (3.0 mu m) and/or a feature with a more complex profile near 2950 cm(-1) (3.4 um), the latter of which is attributed to saturated aliphatic hydrocarbons in interstellar grains and is the primary interest of this paper. As in our earlier work, the similarity of the absorption bands near 2950 cm(-1) (3.4 mu m) along different lines of sight and the correlation of these features with interstellar extinction reveal that the carrier of this band lies in the dust in the diffuse interstellar medium (DISM). At least 2.5% of the cosmic carbon in the local interstellar medium and 4% toward the Galactic center is tied up in the carrier of the 2950 cm(-1) (3.4 mu m) band. The spectral structure of the diffuse dust hydrocarbon C-H stretch absorption features is reasonably similar to UV photolyzed laboratory ice residues and is quite similar to the carbonaceous component of the Murchison meteorite. The similarity between the DISM and the meteoritic spectrum suggests that some of the interstellar material originally incorporated into the solar nebula may have survived relatively untouched in primitive solar system bodies. Comparisons of the DISM spectrum to hydrogenated amorphous carbon and quenched carbonaceous composite are also presented. The A(v)/tau ratio for the 2950 cm(-1) (3.4 mu m) feature is lower toward the Galactic center than toward sources in the local solar neighborhood (similar to 150 for the Galactic center sources vs. similar to 250 for the local ISM sources). A similar trend has been observed previously for silicates in the diffuse medium by Roche and Aitken, suggesting that (1) the silicate and carbonaceous materials in the DISM may be physically correlated and (2) there is either dust compositional variation in the galaxy or galactic variation in the grain population density distribution. We also note a possible absorption feature near 3050 cm(-1) (3.28 mu m), a wavelength position that is characteristic of polycyclic aromatic hydrocarbons (PAHs).