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The scent of the universe

Former PhD student Cameron Mackie will been awarded not one, but two dissertation prizes for his thesis on the aromatic universe. His work could provide us with a virtual sniff of space. ‘These molecules in space likely smell like a big charcoal grill!’

‘These two prizes exemplify the unique bridge that Mackie forged between astronomy and chemistry,’ says Scientific Director Huub Rottgering of Leiden Observatory. Both the American Astronomical Society as the American Chemical Society rewarded him for his work.

Pyrene is an example of a PAH. It consists of four carbon rings, resulting is a flat aromatic system

Molecules with a scent

During his PhD, Mackie investigated the infrared spectra of so-called polycyclic aromatic hydrocarbons (PAHs). ‘These large molecules are found on Earth as a natural by-product of combustion,’ he tells. ‘However, more interestingly, these PAHs are also observed in space. They are thought to form in the out-gassing of dying carbon-rich stars, which act much like giant sooting candles.’ PAHs are a class of molecules characterized by containing hexagonal rings of carbon atoms and their free edges capped by hydrogen atoms.  Though ‘aromatic’ refers to the type of chemical bond in PAHs, many fragrant substances contain aromatic bonds, hence the relation to the name.

In order to study these interstellar PAHs, scientists measure the infrared light they emit after being energized by absorbing light from a nearby star. Mackie combined computational chemistry software and quantum mechanics to predict the expected infrared lights the PAH would emit. ‘This allows other astronomers to better know what they are looking at when they come across infrared signatures of PAHs in space.’ If astronomers could accurately determine the types of PAHs present, it would be possible to recreate the smell of them on Earth, uncovering the scent of space. Mackie predicts: ‘The PAHs in space likely smell like a big charcoal grill!’

No perfect spring

Mackie took a different approach than previous studies, that made use of harmonic approximations to calculate the infrared signature of molecules. ‘In the harmonic approximation the forces between the atoms are modelled as ‘perfect’ springs. In my work, I apply a more physical model with anharmonicities, in which we assume that the forces between the atoms do not behave as perfect springs.’ This allowed Mackie and his colleagues to predict the infrared signatures of PAHs to a higher degree of accuracy, as well as to take temperature effects into account.

The pictures from its predecessor the Hubble Space Telescope brought wonder and amazement to many people. I believe the James Webb Space telescope will do the same, if not more!

PAHs and the origin of life

With his work, Mackie helped underline the shortcomings of current calculated PAH infrared signatures and showed where the current models need improvement. ‘Our work will greatly aid astronomers in understanding their observations’, he adds.

But according to Mackie, the work does not only concern astronomers. ‘The new James Webb Space Telescope will be launched in the coming years. The pictures from its predecessor the Hubble Space Telescope brought wonder and amazement to many people. I believe the James Webb Space telescope will do the same, if not more! Its unprecedented resolution will enable much better observation of the PAH signatures.’ According to Mackie, the release of new images will therefore stimulate much discussion. ‘Complex carbon chemistry in space will evoke questions such as “Where does life originate” and “Do PAHs play a role?” The work I performed will help scientists and the general public better understand what we are looking at when we see these PAHs.’

Cameron Mackie

Recognition by two communities

Mackie will receive both the Dissertation prize of the Laboratory Astrophysics division of the American Astronomical Society (in June) and the Dissertation prize of the Astrochemistry subdivision of the American Chemical Society (in August). The granting of the double prize did cause some confusion: ‘I first got an email about winning the Dissertation Prize from the Chemical Society. A couple of days later, I received a second mail about winning a prize from the Astronomical Society. At first, I thought that was just a follow-up. It wasn't until the next day that I realized the second email was congratulating me on a separate award!’ Of course, he was thrilled: ‘To be recognized by both the chemistry and astronomy community really makes all of my hard work seem worth it.’

Mackie believes he owes the prizes due to the breadth of his work. ‘This work was highly anticipated by the astronomical PAH community and will be of great value when the James Webb Space Telescope is launched.’

 

Text: Hilde Pracht

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