Constraining the chemistry of carbon-chain molecules in space

February 27, 2017, Harvard-Smithsonian Center for Astrophysics
An image of the Taurus Molecular Cloud, about 450 light-years from Earth. Many carbon-chain molecules have been detected towards dark clouds like these, but astronomers have sought HC11N without success. They speculate that chains this large preferentially transform into carbon rings. Credit: ESO; Digitized Sky Survey; Davide De Martin

The interstellar medium of the Milky Way contains 5-10% of the total mass of the galaxy (excluding its dark matter) and consists primarily of hydrogen gas. There are small but important contributions from other gases as well, including carbon-bearing molecules both simple, like carbon monoxide and carbon dioxide, and complex like ethene, benzene, propynal, methanol and other alcohols, and cyanides. There are even some very large molecules like polycyclic aromatic hydrocarbons and buckyballs with fifty or more carbon atoms. Some species like the cyanides have relative abundances similar to what is seen in comets in our solar system, suggesting that local carbon chemistry is not unique.

Astronomers think complex interstellar are probably produced on dust grains, although some molecules might be produced in the gas phase. About one percent by mass of the interstellar material, these tiny grains are composed predominantly of silicates and provide the with surfaces on which to react with other molecules. Carbon chain molecules are particularly interesting because they are thought to be the starting point for a significant fraction of the known complex chemicals in the interstellar medium. It is even suspected that carbon-chain species are a key stage in the formation of . Carbon-chain molecular chemistry thus provides insight into a large subset of interstellar chemistry.

A particularly well-studied family of carbon chains is the cyanopolyynes: linear molecules of the form HCnN, where n = 3, 5, 7, 9, etc. They have been observed in high abundance towards older stars and in cold dark clouds. The presence of the largest known cyanopolyyne, HC11N, however, is in dispute. It was reportedly detected in 1982 towards one dark cloud in Taurus, but that detection has not been confirmed. CfA astronomers Ryan Loomis and Brett McGuire and their colleagues used the Green Bank Telescope to search the Taurus region for HC11N in six of its characteristic radio wavelength transitions, including the two in which it was first reported, but without success.

The astronomers argue that the previous detection was an error, and they offer an explanation for the otherwise curious absence of the n=11 species. Laboratory experiments have shown that when carbon-chain molecules get to be longer than about n=9 they begin to curl on themselves and preferentially transform into -ring molecules, which are more stable. A similar process could be occurring in the , siphoning away HC11N to form cyclic species. The non-detection of HC11N thus suggests the importance of this chemical pathway in producing cyclic molecules, although the authors note that further observations and laboratory experiments are needed to confirm the model.

Explore further: The formation of carbon-rich molecules in space

More information: Ryan A. Loomis et al. Non-detection of HCN towards TMC-1: constraining the chemistry of large carbon-chain molecules, Monthly Notices of the Royal Astronomical Society (2016). DOI: 10.1093/mnras/stw2302

Related Stories

The formation of carbon-rich molecules in space

January 11, 2016

The space between stars is not empty, but contains an abundance of diffuse material, about 5-10% of the total mass of our galaxy (excluding dark matter). Most of the material is gas, predominantly hydrogen, but with a small ...

The discovery of the molecule Si-C-Si in space

June 29, 2015

The space between stars is not empty—it contains a vast reservoir of diffuse material with about 5-10% of the total mass of our Milky Way galaxy. Most of the material is gas, but about 1% of this mass (quite a lot in astronomical ...

Space: The final frontier in silicon chemistry

November 11, 2014

Silicon, which is one of the most common elements in the Earth's crust, is also sprinkled abundantly throughout interstellar space. The only way to identify silicon-containing molecules in the far corners of the cosmos - ...

Interstellar dust and the sun

November 12, 2012

(—The space between stars is not empty. It contains copious but diffuse amounts of gas and dust; in fact about 5-10% of the total mass of our Milky Way galaxy is in interstellar gas. About 1% of the mass of this ...

Organic conundrum in Large Magellanic Cloud

June 23, 2014

( —A group of organic chemicals that are considered carcinogens and pollutants today on Earth, but are also thought to be the building blocks for the origins of life, may hold clues to how carbon-rich chemicals ...

Recommended for you

New research challenges existing models of black holes

January 19, 2018

Chris Packham, associate professor of physics and astronomy at The University of Texas at San Antonio (UTSA), has collaborated on a new study that expands the scientific community's understanding of black holes in our galaxy ...

Neutron-star merger yields new puzzle for astrophysicists

January 18, 2018

The afterglow from the distant neutron-star merger detected last August has continued to brighten - much to the surprise of astrophysicists studying the aftermath of the massive collision that took place about 138 million ...

New technique for finding life on Mars

January 18, 2018

Researchers demonstrate for the first time the potential of existing technology to directly detect and characterize life on Mars and other planets. The study, published in Frontiers in Microbiology, used miniaturized scientific ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Feb 27, 2017
"The interstellar medium of the Milky Way contains 5-10% of the total mass of the galaxy (excluding its dark matter) and consists primarily of hydrogen gas." from article

How do they are counting?
Center of the galaxy (when we exclude the stars and other bodies) contains over 98% of the total matter of the Milky Way (the diameter of the Milky Way center is 30,000 x 40,000 ly http://www.astrod...y.html).
"5-10% of the total mass" is three times more gas than the total mass of all the stars!
Error or article for us laymen?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.