The galactic distribution of aliphatic hydrocarbons in the diffuse interstellar medium

Area of Science:

• Astrochemistry
• Interstellar Medium Physics
• Dust Grain Properties

Background:

• The 2950 cm-1 (3.4 µm) infrared absorption band, attributed to C-H stretching in aliphatic hydrocarbons within interstellar dust, is a key tracer of the diffuse interstellar medium (ISM).
• Previous studies assumed a uniform distribution of these carbonaceous materials relative to visual extinction across the galaxy.

Purpose of the Study:

• To investigate the spatial distribution of aliphatic hydrocarbon-rich dust grains in the diffuse ISM.
• To compare the distribution of these carbonaceous grains with that of silicate grains and visual extinction.
• To explore potential coupling mechanisms between carbonaceous and silicate dust components.

Main Methods:

• Analysis of infrared absorption spectra, specifically the 2950 cm-1 band and its subfeatures (2955 cm-1 for -CH3, 2925 cm-1 for -CH2-).
• Comparison of the band strength with visual extinction measurements for objects at varying galactic longitudes.
• Modeling of dust grain distribution and composition based on observational data.

Main Results:

• The strength of the aliphatic C-H stretching band does not scale linearly with visual extinction throughout the galaxy.
• A stronger increase in band strength is observed for objects closer to the galactic center, mirroring the behavior of silicate Si-O stretching bands.
• The carrier density of aliphatic material in the inner galaxy is estimated to be 5 to 35 times higher than in the local ISM.

Conclusions:

• The grains responsible for the diffuse ISM aliphatic C-H and silicate Si-O stretching bands are likely different from those dominating visual extinction.
• The distribution of carbonaceous dust is non-uniform, increasing in density towards the galactic center.
• The coupled behavior of C-H and Si-O bands suggests a possible silicate-core, organic-mantle grain structure.