The Identification of Aliphatic Organic Compounds in the Diffuse Interstellar Medium

The 'empty' space between the stars and between dense molecular clouds in the interstellar medium is known as the Diffuse Interstellar Medium (DISM). However, this space is not truly empty; it does contain some material, albeit at very low densities. Most of this material is in the form of atomic gas and dust grains (only a few molecules, like PAHs, can survive the rigors of the high radiation fields in this environment). This gas and dust scatters and absorbs starlight as it passes through the diffuse ISM. The result is a reddening of the starlight, an effect that increases as the distance between us and the star increases. At infrared frequencies, the chemical compounds in the dust selectively absorb specific wavelengths of light that depend on their molecular bonding and composition (for a more detailed explanation of this, click here). The positions of the resulting infrared absorption features can be measured using special infrared telescopes. Comparisons between the infrared absorption bands produced by the dust in the diffuse ISM and laboratory standards can then be used to constrain the composition of the dust. An example of how this is done is provided below.

The figure below shows one such absorption feature as measured along the lines-of-sight to two different stars in our galaxy, GC IRS 6E, which is a star located in the center of our galaxy, and VI Cygni #12, a star that lies in another direction and is closer to the Earth. The fact that both of these lines-of-sight produce the same absorption feature even though light from different types of stars is being measured suggests that the absorption feature is due to material in the diffuse ISM rather than in the stars themselves. This interpretation is further strengthened by the observation that the absorption feature grows in strength as you look at stars that are farther and farther away, just as you'd expect if the feature was due to material in the diffuse ISM (the spectrum of VI Cyg 12 has been scaled up by a factor of 5 in the figure below to make the profile comparison clearer).

The overall shape and position of the feature is highly diagnostic of the chemical functional groups present. It's position(s) indicates that the absorption is being produced by the C-H stretching vibrations of aliphatic organic materials, i.e., organic materials dominated by -CH2- and -CH3 groups. The overall profile indicates that the ratio of -CH2- to -CH3 groups is about 2.5 to 1 and that the aliphatics responsible also contain other perturbing chemical groups (possibly aromatic domains, -OH groups, etc.). The strengths of the observed bands and their detection along many lines-of-sight in our galaxy indicate that this material is ubiquitous and contains on the order of 10% of all the carbon in the diffuse ISM!

Indeed, this material appears to be ubiquitous not just in a galactic sense, but in an extragalactic sense as well. The figure below shows a comparison of the spectrum of this material in our galaxy compared with the spectrum of material in another galaxy, IRAS 0857+3915. We know the material responsible for the feature towards IRAS 0857+3915 resides in that galaxy rather than ours because the redshift due to its Hubble recessional velocity has to be accounted for to get the features to line up in the figure below!

Also of great interest is the observation that the absorption feature can be well matched by the bulk of the organics found in primitive meteorites like Murchison. Some of the organic molecules in these meteorites are known to have an interstellar origin because they contain large deuterium enrichments and the spectral match suggests that the organics in the diffuse ISM and in meteorites may be related.

The ultimate source(s) of this interstellar material is currently not well understood, although some of it probably comes from the outflow of carbon-rich stars near the end of their lives and some of it may be produced by photochemistry of ices in the dense molecular clouds from which stars form (for more about the organics that are produced when interstellar ices are irradiated, click here).

For more detailed information on the infrared spectral comparisons done in our laboratory for the purposes of determining the composition, abundance, and distribution of the organic materials in the diffuse interstellar medium, see:

Pendleton, Y. J. (1999). Organics in the ISM: Where Do They Come From? In Solid Interstellar Matter: The ISO Revolution, eds. L. d'Hendecourt, C. Joblin, & A. Jones, (EDP Sciences: Les Ulis), pp. 120-128.

Sandford, S. A., Pendleton, Y. J., & Allamandola, L. J. (1995). The Galactic Distribution of Aliphatic Hydrocarbons in the Diffuse Interstellar Medium. Astrophys. J. 440, 697-705.

Pendleton, Y. J., Sandford, S. A., Allamandola, L. J., Tielens, A. G. G. M., & Sellgren, K. (1994). Near Infrared Absorption Spectroscopy of Interstellar Hydrocarbon Grains. Astrophys. J. 437, 683-696.

Sandford, S. A., Allamandola, L. J., Tielens, A. G. G. M., Sellgren, K., Tapia, M., & Pendleton, Y. (1991). The Interstellar C-H Stretching Band near 3.4 µm: Constraints on the Composition of Organic Material in the Diffuse Interstellar Medium. Astrophys. J. 371, 607-620.

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