Is there finally a plausible theoretical basis for the molecular origins and carriers of at least some of the most important so-called “UIE” (unidentified infrared emission) bands that have mystified astronomers for decades?
Theoretical astrophysicists and astrochemists at the Laboratory for Space Research (LSR) and the Department of Physics at the University of Hong Kong (HKU) seem to think so (at least in theory) in a peer-reviewed paper just published. in The Astrophysical Journal.
A team led by LSR Fellow Dr Seyed Abdolreza Sadjadi and LSR Director Professor Quentin Parker in the Department of Physics have now added some interesting theoretical work. It identifies highly ionized species of the famous football-shaped ‘Buckminsterfullerene’60 molecule as plausible carriers of at least some of the most important and enigmatic UIE bands that have challenged astronomers since they were first discovered and studied more than 30 years ago.
First, Dr. Sadjadi and Prof. Parker theoretically proved that C60 could survive, in steady states, being ionized up to +26 (i.e. 26 of the buckyball’s 60 electrons are knocked out) before the buckyball decays (Sadjadi & Parker 2021). Now they have shown, by applying the first principles of quantum chemical calculations, what theoretical mid-infrared signatures of these ionized forms of fullerene can be expected. The results are extremely interesting and provocative and could finally point the way to at least a partial resolution of this lingering astrophysical mystery.
Professor Parker said: “I am extremely honored to have played a part in the surprisingly complex quantum chemical research undertaken by Dr Sadjadi which led to these very exciting results. They first relate to the theoretical proof that fullerene – carbon 60 – can survive very high levels of ionization and now this work shows that the infrared emission signatures of these species perfectly match some of the emission characteristics. most significant unidentified infrared known. This should help reinvigorate this area of research.
The HKU core team found that some of these positively charged fullerenes exhibit strong emission bands that match extremely well the position of major astronomical UIE emission features at 11.21, 16.40 and 20-21 micrometers. (μm). This makes them key target species for the identification of currently unidentified UIE features and provides strong motivation for future astronomical observations in the mid-infrared wavelength range to test these theoretical findings. They also found that the group IR signatures of these C60 cations with q = 1–6 are well separated from the 6.2 μm bands, which are associated with free/isolated aromatic hydrocarbon molecules (called PAH, another potential UIE vector). This greatly facilitates their identification compared to other potential carriers. This discovery is particularly important for the discrimination and exploration of the coexistence of complex organic hydrocarbons and fullerenes in astronomical sources.
Dr Sadjadi said: “In our first paper, we theoretically showed that highly ionized fullerenes can exist and survive the harsh and chaotic environment of space. It’s like asking how much air you can push out of a soccer ball and the ball still holds its shape. In this article, we worked with two other eminent astrophysicists and planetary scientists, Prof. Yong Zhang and Dr. Chih-Hao Hsia, both former HKU staff but still affiliated with LSR, to determine the molecular vibrational ratings of a celestial symphony, i.e. the characteristics that these ionized buckyballs would play/produce. We then searched for them in space, showing that their notes/signatures are easily distinguishable from PAHs.
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