The Identification of Ices and Other Materials in Dense Interstellar Molecular Clouds

The 'empty' space between the stars is not actually completely empty, although much of it comes pretty darn close. The bulk of the volume of our galaxy consists of nearly empty space, an environment refered to as the Diffuse Interstellar Medium (DISM). However, there are locations in the interstellar medium, known as Dense Interstellar Molecular Clouds, where the density of interstellar material is considerably higher, at least in a relative sense (although denser than the diffuse ISM, these dense clouds still have densities that rival good laboratory vacuums!).

Dense molecular clouds represent very special environments becuase they are the birth sites of new stars and planetary systems. Because these clouds typically contain enough material in them that they are "optically thick," this is, they absorb or scatter most of the starlight that falls on them from outside they look dark (i.e. the horsehead nebula), thus they are also called dark clouds. This has some very important implications for the chemistry of the interstellar medium. Unlike the diffuse medium where intense radiation rapidly destroys all but the most stable molecules, in dense clouds the general galactic radiation field is screened out and a much wider variety of chemical compounds can survive.

For this reason the temperatures in the dense clouds are extremely cold. Typical cloud temperatures are only 10-50 K (-263 to -223 C, about -440 to -370 oF)! As a result, most molecules in dense clouds are frozen into icy grain mantles rather than free floating gas phase molecules. These interstellar ices are often comprised primarily of water ice, although they also contain simple molecules like carbon monoxide (CO),carbon dioxide (CO2), methanol (CH3OH), and ammonia (NH3).

While these clouds look dark to the human eye, they are not in the infrared (IR). At IR frequencies, the chemical compounds in the dust selectively absorb specific wavelengths of light that depend on their molecular bonding and composition. The positions of the resulting infrared absorption features can be measured using special infrared telescopes. Comparisons between the infrared absorption bands seen through the telescope and measured in the laboratory can then be used to constrain the composition of the cloud. As a result IR astronomers now know the composition of clouds many light years away. Please follow these links for a representation how this is done, and for more technical details) about IR spectroscopy.

By making such comparisons, we have identified or assisted in the identification of a number of materials in dense clouds. These include:

Mixed-molecular Ices

Microdiamonds (!)

Evidence for Organic Photoproducts

For more detailed information and reviews on our laboratory work on interstellar and cometary ice analogs, see:

Sandford, S. A. (1998). Organic Chemistry: From the Interstellar Medium to the Solar System. In ORIGINS, Astron. Soc. Pacific Conf. Series, Vol. 148, Proceedings of the International Conference, Estes Park, Colorado, 19-23 May, 1997, C. E. Woodward, J. M. Shull, & H. A. Thronson, Jr. (eds.), (ASP: San Francisco), pp. 392-414.

Sandford, S. A. (1996). The Inventory of Interstellar Materials Available for the Formation of the Solar System. Meteoritics and Planetary Science 31, 449-476.

Sandford, S. A. (1996). The Composition of Interstellar Grains and Ices. In Polarimetry of the Interstellar Medium, eds. W. Roberge & D. C. B. Whittet, (Astron. Soc. Pac. Conf. Series, Vol. 97: San Francisco), pp. 29-47.

Sandford, S. A., & Allamandola, L. J. (1993). Condensation and Vaporization Studies of CH3OH and NH3Ices: Major Implications for Astrochemistry. Astrophys. J. 417, 815-825.

Tielens, A. G. G. M., Allamandola, L. J., & Sandford, S. A. (1991). Laboratory, Observational, and Theoretical Studies of Interstellar Ices. In Solid-State Astrophysics, (E. Bussoletti & G. Strazzula, eds.), Enrico Fermi International School of Physics, Course CXI on Solid State Astrophysics, Varenna, Italy, 27 June - 7 July, 1989, (N. Holland Pub. Co.: Amsterdam), pp.29-58.

Sandford, S. A., & Allamandola, L. J. (1990). The Volume and Surface Binding Energies of Ice Systems Containing CO, CO2, and H2O. Icarus 87, 188-192.

Sandford, S. A., & Allamandola, L. J. (1988). The condensation and vaporization behavior of H2O:CO ices and implications for interstellar grains and cometary behavior. Icarus 76, 201-224.

Allamandola, L. J., & Sandford, S. A. (1988). Laboratory simulation of dust spectra. In Dust in the Universe, (M.E. Bailey & D.A. Williams, eds.), Cambridge Univ. Press: Cambridge, pp. 229-263.

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