Final Report Summary - PAHS (The Role of Large Polycyclic Aromatic Hydrocarbon molecules in the Universe)
Strong emission features dominate the IR spectra of the interstellar medium of the Milky Way, galaxies in the local Universe and out to redshifts of ~3. These features are generally attributed to IR fluorescence of large Polycyclic Aromatic Hydrocarbon (PAH) molecules pumped by UV photons. As a class, Polycyclic Aromatic Hydrocarbon molecules consist of fused benzene rings in molecular structures resembling chicken wire. On Earth, these molecules are familiar as carcinogenic by products in the combustion of hydrocarbons in, for example, car engines, over-barbeques meat, and forest fires. Polycyclic Aromatic Hydrocarbon molecules are also abundant, ubiquitous, and an important component of the interstellar medium of galaxies.
This program centers on determining the composition and evolution of Polycyclic Aromatic Hydrocarbon in space and their role in the formation of stars and planets and the evolution of galaxies. This highly interdisciplinary project combines molecular physics, quantum chemistry, infrared spectroscopy and astronomy. One key result of this program is the realization that photochemistry holds the key to the evolution of such species in space. Specifically, by combining data obtained with the Spitzer Space Telescope, the Stratospheric Observatory For Infrared Astronomy, and the Herschel Space Observatory on the emission characteristics of Polycyclic Aromatic Hydrocarbon molecules in regions where massive stars set their direct environment aglow, we have been able to show that PAHs are rapidly destroyed when approaching these stars. This program is supported by dedicated experiments where trapped PAH cations are irradiated by strong lasers. These experiments revealed that the buckyball, C60, is a key fragmentation product of large PAH molecules under UV irradiation. The C60 molecule – in the shape of a molecular-sized soccerball – is very stable and can much better survive in the harsh environments near stars. While previously, it was thought that such large molecules in space are only produced through the slow build up of species, one atom at a time, this study revealed for the first time the importance of top-down chemistry where large molecules – injected by stars into space – are broken down to produce smaller - but very stable – species.
This program centers on determining the composition and evolution of Polycyclic Aromatic Hydrocarbon in space and their role in the formation of stars and planets and the evolution of galaxies. This highly interdisciplinary project combines molecular physics, quantum chemistry, infrared spectroscopy and astronomy. One key result of this program is the realization that photochemistry holds the key to the evolution of such species in space. Specifically, by combining data obtained with the Spitzer Space Telescope, the Stratospheric Observatory For Infrared Astronomy, and the Herschel Space Observatory on the emission characteristics of Polycyclic Aromatic Hydrocarbon molecules in regions where massive stars set their direct environment aglow, we have been able to show that PAHs are rapidly destroyed when approaching these stars. This program is supported by dedicated experiments where trapped PAH cations are irradiated by strong lasers. These experiments revealed that the buckyball, C60, is a key fragmentation product of large PAH molecules under UV irradiation. The C60 molecule – in the shape of a molecular-sized soccerball – is very stable and can much better survive in the harsh environments near stars. While previously, it was thought that such large molecules in space are only produced through the slow build up of species, one atom at a time, this study revealed for the first time the importance of top-down chemistry where large molecules – injected by stars into space – are broken down to produce smaller - but very stable – species.