New Interstellar Molecule

Click on Image to enlarge. An electron attaches itself to the C8H molecule, freeing a burst of radiation (overall glow seen around the molecule) and leaving the negatively-charged ion C8H-. Credit: Bill Saxton, NRAO/AUI/NSF

Astronomers using data from the Green Bank Telescope (GBT) have found the largest negatively-charged molecule yet seen in space. The discovery of the third negatively-charged molecule, called an anion, in less than a year and the size of the latest anion will force a drastic revision of theoretical models of interstellar chemistry.

A team of scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) found negatively-charged octatetraynyl in a cold, dark cloud of molecular gas. A second team headed by Remijan found octatetraynyl in the envelope of gas around an old, evolved star. In both cases the molecule, a chain of eight carbon atoms and one hydrogen atom, had an extra electron, giving it a negative charge.
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About 130 neutral and about a dozen positively-charged molecules have been discovered in space, but the first negatively-charged molecule was not discovered until late last year. The largest previously-discovered negative ion found in space has six carbon atoms and one hydrogen atom.

Ultraviolet light from stars can knock an electron off a molecule, creating a positively-charged ion. Astronomers had thought that molecules would not be able to retain an extra electron, and thus a negative charge, in interstellar space for a significant time. “That obviously is not the case,” said Mike McCarthy of the CfA. “Anions are surprisingly abundant in these regions.”

Until recently, many theoretical models of how chemical reactions evolve in interstellar space have largely neglected the presence of anions. This can no longer be the case, and this means that there are many more ways to build large organic molecules in cosmic environments than have been explored.

Remijan and his colleagues found the octatetraynyl anions in the envelope of the evolved giant star IRC +10 216, about 550 light-years from Earth in the constellation Leo. They found radio waves emitted at specific frequencies characteristic of the charged molecule by searching archival data from the GBT, the largest fully-steerable radio telescope in the world.

Another team from the Harvard-Smithsonian Center for Astrophysics found the same characteristic emission when they observed a cold cloud of molecular gas called TMC-1 in the constellation Taurus. These observations also were done with the GBT. In both cases, preceding laboratory experiments by the CfA team showed which radio frequencies actually are emitted by the molecule, and thus told the astronomers what to look for.

It is essential that likely interstellar molecule candidates are first studied in laboratory experiments so that the radio frequencies they can emit are known in advance of an astronomical observation.

Both teams announced their results in the July 20 edition of the Astrophysical Journal Letters.

Read more: Discovery of New negatively charged Interstellar molecule

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