The Galactic Distribution of Aliphatic Hydrocarbons in the Diffuse Interstellar Medium

The infrared absorption feature near 2950 cm-1 (3.4 micrometers), characteristic of dust in the diffuse interstellar medium (ISM), is attributed to C-H stretching vibrations of aliphatic hydrocarbons. We show here that the strength of the band does not scale linearly with visual extinction everywhere, but instead increases more rapidly for objects near the center of the Galaxy, a behavior that parallels that of the Si-O stretching band due to silicate materials in the diffuse ISM. This implies that the grains responsible for the diffuse medium aliphatic C-H and silicate Si-O stretching bands are different from those responsible for much of the observed visual extinction. It also suggests that the distribution of the carbonaceous component of the diffuse ISM is not uniform throughout the Galaxy, but instead may increase in density toward the center of the Galaxy. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of silicate-core, organic-mantle grains. Several possible models of the distribution of this material are presented and it is demonstrated that the inner parts of the Galaxy has a carrier density that is 5 to 35 times higher than in the local ISM. Depending on the model used, the density of aliphatic material in the local ISM is found to be about 1 to 2 -CH3 groups m-3 and about 2 to 5 -CH2- groups m-3. These densities are consistent with the strengths of the 2955 and 2925 cm-1 (3.38 and 3.42 micrometers) subfeatures (due to -CH3 and -CH2- groups, respectively) within the overall 2950 cm-1 (3.4 micrometers) band being described by the relations A upsilon/tau(2955 cm-1) = 270 +/- 40 and A upsilon/tau(2925 cm-1) = 250 +/- 40 in the local diffuse ISM.