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European researchers detect l-C3H+ hydrocarbon molecule in interstellar space

Using the 30m-telescope of the Institute for Radio Astronomy (IRAM) for astronomical observations in the millimeter range of wavelengths, researchers from France, Spain and Germany have detected for the first time the l-C3H+ hydrocarbon molecule in interstellar space—specifically in the Horsehead Nebula.

IRAM-astronomer Jérôme Pety and his team for the first time undertook a systematic survey of the chemical content of the Horsehead’s mane. The international project, called “Whisper”, would not have been possible without the recent technical upgrades of the telescope instruments.

Earlier, such a comprehensive enterprise would have taken at least one year of observations. Now we could complete measurements after one week”, says Arnaud Belloche at the Max Planck Institute for Radio Astronomy. This opens new possibilities to classify the different kinds of gas in the universe, based on the molecules they contain.

“The nebula contains 200 times more hydrocarbons than the total amount of water on Earth!”
—IRAM-astronomer Viviana Guzman

In their current survey, the scientists were able to detect 30 molecules in the region, including many small hydrocarbons, the smallest molecules that compose petroleum and natural gas. The researchers were surprised by the unexpectedly high levels of hydrocarbons.

In addition, one of these small hydrocarbons, the propynylidyne ion (C3H+), was observed for the first time in space as part of this work—even though this positively charged ion is a key player in the chemical reactions which link the small hydrocarbons together.

PAHs in space
In 2009, Xander Tielens, Professor of Physics and Chemistry of the Interstellar Space at Leiden University was awarded an ERC Advanced Grant to study polycyclic aromatic hydrocarbons (PAHs) in space. PAHs constitute a group of hundreds of organic materials made up of two or more benzene rings.
On Earth, they are familiar as the carcinogenic by-products of incomplete combustion of carbon-containing substances such as fossil fuels, grilled meat and other foodstuffs.
PAHs also occur in interstellar space; aided by UV light particles, they emit fluorescent radiation in infrared. They are large molecules of some 50-100 carbon atoms, there are a lot of them, and they are everywhere, on almost all objects found in space, as observations with the Infrared Space Observatory (ISO) and the Spitzer have shown since the mid-nineties.
Earlier this year, Prof. Tielens was awarded a Spinoza prize, the highest scientific prize in the Netherlands, in recognition of his groundbreaking work into polycyclic aromatic hydrocarbon (PAH) particles and on photodissociation regions (PDRs).

In a paper in the journal Astronomy & Astrophysics, Pety and his colleagues propose that the hydrocarbons result from the fragmentation of polycyclic aromatic hydrocarbons (PAHs). These giant molecules could be eroded by ultra-violet light, giving a large amount of small hydrocarbons. This mechanism would be particularly efficient in regions like the Horsehead Nebula where the interstellar gas is directly exposed to the light of a nearby massive star.

We observe the operation of a natural refinery of petroleum of gigantic size.

—Jérôme Pety

The authors noted that laboratory spectroscopy is underway to confirm these results. Interferometric imaging is needed to firmly constrain the small hydrocarbon chemistry in the Horsehead.

IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).


  • J. Pety et al. The IRAM-30m line survey of the Horsehead PDR: First detection of the l-C3H+ hydrocarbon cation. Astronomy & Astrophysics doi: 10.1051/0004-6361/201220062


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