Heavenly Chemistry: Astronomers announce astrophysical anion

The discovery of a negatively charged organic molecule in space may provide new insight into the formation of amino acids, sugars, and other prebiologic compounds in interstellar gas clouds, the regions that spawn stars and planets.

The compound, known as octatetraynyl, is one of only three negatively charged ions identified in the heavens, and it’s the longest molecule of the three. Researchers over the past few decades have discovered about 130 neutral molecular species and about 12 types of positively charged ions in space. Not until late last year did astronomers find the first example of a single negatively charged ion, or anion.

Some researchers had suggested that anions, which have one or more extra electrons in comparison with a neutral molecule, might be too fragile to exist for long periods. By contrast, it seemed reasonable that positively charged ions, which can easily be created by cosmic rays striking neutral compounds, should be more abundant.

“Anions have so far been neglected in most of the current astrochemical models, but our findings suggest that they might play an important role in these chemical networks,” says Sandra Brünken of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., a coauthor of one of two studies on octatetraynyl in the July 20 Astrophysical Journal Letters.

“This opens up another avenue in the chemistry of interstellar space,” says Jan Hollis of NASA’s Goddard Space Flight Center in Greenbelt, Md., a coauthor of the other new study. By identifying molecules in regions of space that may collapse to form new generations of stars and planets, “we’re really looking for our molecular origins,” says Hollis. “The record of early Earth is lost, but [the record of prebiotic chemistry] may be preserved in these regions.”

Hollis and his colleagues identified octatetraynyl, which consists of eight carbon atoms and a single hydrogen atom, in the gaseous envelope around the elderly, giant star IRC +10 216, some 550 light-years from Earth. Brünken’s team detected the same anion in a cold cloud of molecular gas called TMC-1, a likely breeding ground for stars that lies 450 light-years from Earth. Both teams recorded the molecule using the 100-meter Green Bank (W.Va.) radiotelescope.

Isolated octatetraynyl molecules are free to tumble. As they change their rate of rotation, they emit a specific set of radio waves. Previous laboratory experiments by the Harvard team had revealed exactly which radio frequencies the molecule emits.

“Until recently, no one was able to measure the spectra of these molecules, so we had no idea” whether these compounds existed in space, comments theorist Eric Herbst of Ohio State University in Columbus.

He suggests that compounds in extremely low-density regions of space acquire a negative charge because electrons striking them stick to the molecules rather than fly off. The discovery will ultimately “help us understand the physical conditions” of these environments, shedding light on how more-complex organic molecules arise, he adds.

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