Space balls

It is with a marked sense of irony, that spectroscopyNOW reports the publication of infrared spectroscopic data from the planetary nebula Tc 1 in the southern constellation Ara that have revealed convincing evidence that the fullerenes, C₆₀ and C₇₀, are present in large quantities in cosmic dust. Ironic because the fullerenes were discovered in sooty deposits here on Earth by Sir Harry Kroto back in the early 1990s while he and his colleagues were looking for an explanation for the diffuse interstellar bands.

Diffuse interstellar bands, or DIBs, are spectroscopic absorption features of astronomical objects. They are produced through the absorption of specific wavelengths of light by the interstellar medium and more than 600 bands have been observed in the near-ultraviolet, through the visible region and into the near-infrared portion of the electromagnetic spectrum. The origin of DIBs remained unclear for many years, with various suggestions that they arose because of the presence of polycyclic aromatic hydrocarbons and other large carbon-bearing molecules in the space between the stars. Agreement between laboratory measurements, theoretical calculations, and astronomical evidence was not obvious.

Back in the 1970s, Kroto had launched a research programme at Sussex University, UK, to look for carbon chains in the interstellar medium. Earlier work had revealed organic molecules such as cyanoacetylene, to be present and Kroto's group hoped to find evidence of longer chain molecules such as cyanobutadiyne and cyanohexatriyne, which they found between 1975 and 1978. They then turned to laboratory work to simulate the atmosphere of carbon-rich Red Giant stars, which they hoped would provide new clues. The work done in conjunction with the late Rick Smalley and Robert Curl at Rice University in Texas, USA, led to the discovery of, not a complex long carbon chain, but the spherical, soccerball-shaped allotrope of carbon, buckminsterfullerene, C₆₀ and, subsequently, the rugbyball-shaped C₇₀ and other related species. Ultimately, this work inspired research that led to the carbon nanotubes, which have proved themselves of repeated worth in the emerging field of nanotechnology.

The notion that the fullerene was the molecule that fell to Earth, as Kroto described it in his lectures at the time, was sealed and it seemed as if the molecule was unlikely to be an extraterrestrial. However, during the last few decades many other molecules and diverse dust features have been observed in space. Jan Cami of the Department of Physics and Astronomy, at the University of Western Ontario, London, Canada and the SETI Institute, in Mountain View, California, USA, and colleagues explain that most of the dust that determines the physical and chemical characteristics of the interstellar medium is formed in the outflows of asymptotic giant branch (AGB) stars and is further processed when these objects become planetary nebulae (PNe).

"Chemical reactions and nucleation in the AGB outflows transform the atomic gas into molecules and dust grains. For carbon-rich AGB stars (or carbon stars), this results in a large variety of carbonaceous compounds - to date, more than 60 individual molecular species and a handful of dust minerals have been identified in these outflows, including benzene, polyynes and cyanopolyynes up to about 13 atoms in size," the team says. "These environments are also thought to be the birthplace for large aromatic species such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes."

Now, working with colleagues Els Peeters and Sarah Elizabeth Malek, and Jeronimo Bernard-Salas of Cornell University, in Ithaca, New York, Cami and colleagues have investigated the environment of Tc 1, a peculiar planetary nebula in the Southern constellation of Ara. The nebula itself is some 6500 light years from Earth. The infrared spectrum of this nebula shows emissions that are coincident with the known spectral features of cold and neutral fullerenes, C₆₀ and C₇₀.

"The two molecules amount to a few percent of the available cosmic carbon in this region, showing that if the conditions are right, fullerenes can and do form efficiently in space," the researchers report.