Infrared studies of large carbonaceous molecules.

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Polycyclic Aromatic Hydrocarbons (PAHs) are among the largest organic molecules found in space. They play a crucial role in various physical and chemical processes and in the prebiotic evolution toward life. Their infrared signature, the so-called Unidentified Infrared (UIR) emission bands, is omnipresent in our Galaxy and beyond. However, specifics of the emitting population remain elusive. Indeed, since the UIR bands originate from chemical bonds involving only nearest-neighbor atoms, they have only a weak dependence on the size and structure of the molecule, and it is therefore not possible to identify the individual PAH molecules that cause the infrared emission. Similarly, although it is firmly established that the PAH molecules are modified under the influence of the physical conditions of their environment (such as temperature, density, radiation field), the origin of these variations is not entirely understood. These unknowns are major roadblocks in i) understanding their connection with other cosmic organic species, ii) accurately quantifying their contribution to physical and chemical processes, and iii) the development of diagnostic tools that would transform the ubiquitous UIR bands into powerful probes of the physical and chemical conditions in the Universe. Today, the world is eagerly awaiting the launch of the James Webb Space Telescope (JWST). With its incredible improvement in spatial resolution, combined with its complete spectral coverage of the PAH infrared emission bands and superb sensitivity, JWST will revolutionize PAH research. Previous observations could only present spectra averaged over regions with vastly different properties, thus greatly confusing their interpretation. The amazing spatial resolution of JWST will disentangle these different regions. This will allow us to quantify precisely how PAHs are modified by the physical conditions of their host environment and thus trace how PAHs evolve across space. Because of its breakthrough potential for PAH research, I am leading a JWST program that will be among the first science observations done by JWST. The research program I propose will take full advantage of this unique opportunity to tackle key questions in PAH research. I will study how the infrared signature of PAHs changes on these fine spatial scales and how it is related to infrared signatures of other carbonaceous species. The observational data will be complemented by both a model of the physical and chemical evolution of PAHs and the NASA Ames PAH Infrared spectral database, which contains the physical and spectral properties of a large number of PAHs and related molecules. This combination allows to interpret the observational properties in terms of the composition of the PAH population and the mechanisms that are driving these changes.