Is there now finally some plausible theoretical basis for the molecular origins and carriers of at least some of the best-known so-called “UIE” (unidentified infrared emission) bands that have baffled astronomers for decades?
Theoretical astrophysicists and astrochemists from 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 Astrophysical magazine.
A team led by Dr. Seyed Abdolreza Sajjadi, LSR Fellow, and Professor Quentin Parker, LSR Director of the Department of Physics, has now thrown some interesting theoretical work into the mix. It identifies highly ionized species of the famous football-shaped Buckminsterfullerene C60 molecules as the likely carriers of at least some of the more prominent and enigmatic UIE bands that have challenged astronomers since they were first discovered and studied more than 30 years ago.
First, Dr. Sajadi and Professor Parker theoretically proved that C60 can survive in stable states after ionization up to +26 (ie 26 of the 60 electrons in the buckyball are removed) before the buckyball decays (Sadjadi & Parker 2021). Now they have shown, by applying first principles quantum chemical calculations, what theoretical signatures of these ionized forms of fullerene can be expected in the mid-infrared range. The results are extremely interesting and provocative and may finally point the way to at least a partial solution to this enduring astrophysical mystery.
Professor Parker said: “I am very honored to have been involved in the extremely complex quantum chemistry research carried out by Dr Sajjadi which has led to these very exciting results. They are primarily about theoretical proof that fullerene – carbon 60 – can survive to very high ionization levels, and this work now shows that infrared signatures from such species are in excellent agreement with some of the best-known features of unidentified infrared emission. This should help revitalize this area of research.”
The leading HKU team found that some of these positively charged fullerenes show strong emission bands that correspond very well to the positions of the key astronomical features of the UIE emission at 11.21, 16.40, and 20-21 micrometers (μm). This makes them prime target species for identifying currently unidentified UIE features and provides a strong motivation for future mid-infrared astronomical observations 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 (so-called PAHs, another potential UIE carrier). This greatly helps in identifying them from other potential carriers. This discovery is particularly important for the identification and study of the coexistence of complex organic hydrocarbons and fullerenes in astronomical sources.
Dr Sajjadi said: “In our first work, we have shown theoretically that highly ionized fullerenes can exist and survive in the harsh and chaotic environment of space. It’s like asking how much air you can squeeze out of a soccer ball and still have the ball retain its shape In this paper, we worked with two other leading astrophysicists and planetary scientists, Prof. Yong Zhang and Dr. Chi-Hao Hsia, both former HKU staff but still associated with LSR, to identify the molecular vibrational notes of the celestial symphony, i.e., the features that these ionized buckyballs will reproduce/produce. We then hunt them down in space, showing that their notes/signatures are easily distinguishable from PAHs.”