National Radio Astronomy Observatory

A big ear.
This giant radio dish in Green Bank, West Virginia, helped detect the faint signal of C6H-.

Anion in the Interstellar Garden

The vast space between the stars of the Milky Way is not empty. It's boiling with cosmic raw material, either expelled from exploding stars or whirling black holes, some of which is in the process of condensing back into new generations of stars and planets. Now, astronomers have for the first time detected a molecule with a negative electrical charge. The discovery of this long-missing ingredient for life--called an anion--could force some serious rethinking about interstellar chemistry.

By analyzing the characteristics of their faint signals, radio astronomers have detected the presence of 130 types of molecules, such as carbon, hydrogen gas, and oxygen, that carry no electrical charge and 14 exhibiting a positive charge. For quite some time, astronomers have expected to find anions, because they are necessary to join with their positive counterparts to form the organic precursors of life. The problem has been that determining the spectrum of specific molecules in space has remained extremely difficult. Also, the highly energetic radiation that pervades interstellar space, such as ultraviolet light, x-rays, and gamma rays, tends to knock out the extra electron that gives anions their negative charge.

Researchers at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, kept hope alive, however. After their laboratory experiments predicted the existence of a cosmic anion, the team tuned the new Robert C. Byrd radio telescope in Green Bank, West Virginia, to the proper frequency, aimed the dish at two locations deemed tantalizing by previous research, and detected the molecule, designated C6H-, at those points in the sky. The astronomers found it in a shell of gas surrounding the red giant star IRC +10216 in the constellation Leo, and in a free-floating cloud called TMC-1 in Taurus.

Team leader Michael McCarthy says finding the anion in two locations means the processes that formed it must be common enough to produce it elsewhere as well. Based on the approach, more anions should be detectable in the laboratory and in space, says McCarthy, and "some of these may be considerably easier to detect than one might suppose." The team presents its findings in the 1 December Astrophysical Journal Letters.

"It's a big deal in interstellar chemistry" because it tells us a lot about how life's building blocks form in cold, deep space, says radio astronomer Glen Langston of Green Bank, who was not affiliated with the study. The new research is already paying dividends, he says: Astronomers have found at least two more sites generating signals from C6H-.

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