Organic Material in the Interstellar Medium

Abstract

Spectra of objects which lie along several lines of sight through the diffuse interstellar medium (ISM) all contain an absorption feature near 3.4 μm (2950 cm-1) which has been attributed to saturated aliphatic hydrocarbons on interstellar grains. The similarity of the absorption bands near 3.4 μm along different lines of sight reveals that the carrier of this band lies in the diffuse dust. A remarkable similarity between the spectrum of the diffuse dust and an organic extract from the Murchison meteorite suggests that some of the interstellar organic material may be preserved in primitive solar system bodies. The recent discovery of the 3.4 μm absorption feature in other galaxies has led to comparisons between the extragalactic, galactic, and solar system organics. The comparisons show strong similarities in position peaks and profile structure between the three spectra. However, the absence, in our own galaxy, of the aliphatic hydrocarbon signature in the spectra of dense cloud objects is puzzling in the light of the widespread distribution of the aliphatic material throughout the diffuse medium and the short time scales thought to govern the transition of that material back into dense molecular clouds. The connection between the diffuse ISM dust and solar system objects is made more difficult to understand if the aliphatics are truly absent in the dense cloud phase where proto-solar nebulae must form. In an effort to further investigate the 3.4 μm absorption feature in the diffuse ISM, comparisons of the diffuse medium dust to several materials which have been proposed as “fits” to the 3.4 μm feature are presented. The optical depth/extinction τ/Av ratio for the 3.4 μm (2950 cm-1) band is higher toward the galactic center than toward sources which sample the interstellar medium in the local neighborhood. A similar trend has been observed previously for silicates, indicating that the two materials may be simultaneously enhanced in the galactic center. Such a trend is consistent with the presence of grains composed of silicate cores and organic refractory mantles.