Most of our knowledge of the universe's composition comes from the fact that molecules emit and absorb radiation at distinctive wavelengths. However, the molecules associated with certain wavelengths have yet to be identified – despite decades of work, in some cases. New research ties some of these mystery wavelengths to fullerene ions, once thought to be purely artificial molecules. More speculative research associates other unexplained spectral lines with fullerene-metal complexes. If so, these would represent the largest molecules yet found in space.
After spectroscopy (the science of interpreting electromagnetic spectra) was invented, it was natural to apply it to the light of the Sun. During the solar eclipse of 1868, several astronomers independently noticed spectral lines at wavelengths not associated with any known element. The search for an explanation led chemists to discover helium.
We're now familiar with the spectral lines of all the elements likely to occur in nature, plus quite a few that only exist in labs. Nevertheless, molecules can produce spectral lines different from the unattached atoms that make them up. The Astrophysical Journal contains a possible explanation for lines astronomers have failed to match to a molecule since they were first observed in the 1970s. Particularly puzzling have been those at 11.21, 16.40 and 20–21 micrometers, all in the mid-infrared and in the heart of JWST's range. Most speculation has associated these lines with unspecified polycyclic aromatic hydrocarbons (PAHs).
Buckminsterfullerene C60 (usually shortened to buckyball or fullerene) is an almost spherical molecule of 60 carbon atoms joined in hexagonal rings. When exposed to ultraviolet light, it can shed a remarkable number of electrons, with resulting ions staying stable.
Theoretical modeling previously published by Dr SeyedAbdolreza Sadjadi and Professor Quentin Parker of the University of Hong Kong suggests fullerenes should be stable in space up to 26+ (ie the loss of 26 electrons), although highly charged versions react with hydrogen where it is abundant. Each ion has slightly different spectral lines, and the new paper by Sadjadi and Parker (and co-authors) predicts those produced by these highly positive fullerene ions.
“This work shows the infrared emission signatures from such species are an excellent match for some of the most prominent unidentified infrared emission features known,” Parker said in a statement.
Fullerenes are often said to resemble a (soccer) football's shape, and Sadjadi compared his previous work to; “Asking how much air you can push out of a football ball and the ball still maintains its shape.”
Spectral wavelengths are sometimes explained through an analogy with notes on a piano keyboard. The new paper, Sajadi said, sought to; “Determine the molecular vibrational notes of a celestial symphony, i.e., the spectral features that these ionized buckyballs would play/produce.”
The authors then matched the wavelengths they had identified to unexplained spectral lines found coming from planetary nebulae.
The fullerene-associated wavelengths 17.4 and 18.9μm have been identified previously in planetary nebulae, so we know they are produced by stars as they die. In the preprint of an unpublished paper Dr Gao-Lei Hou of KU Leuven, Belgium goes further.
Hou and co-authors produced complexes of fullerenes and common metals such as lithium, sodium, and iron. They measured their spectral lines, matching them to others observed in planetary nebulae and previously not matched to known molecules. Unlike fullerenes themselves, these buckyball-metal complexes have not previously been known to exist in space. If [C60-metal]+ ions are responsible for these lines, they would be the largest molecules we have ever observed in gas clouds.