The role of large molecules in space

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A key role in the physics and chemistry of circumstellar and interstellar environments is played by large carbonaceous molecules such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes. PAHs leave their strong and clear spectral fingerprint in almost all galactic and extragalactic mid-infrared observations. Variations in their observational characteristics depend on the physical conditions of the environment in which they reside, and thus PAHs are potentially powerful tools to study the Universe. However, specifics about the astrophysical PAH population and the underlying causes for these variations remain unclear, limiting their diagnostic power. I developed a method that quantitatively relates physical input parameters (e.g. strength of the radiation field) to spectral features, based on properties of numerous species from the NASA Ames PAH infrared spectroscopic database. I propose to apply this method to the PAH spectra of objects whose environment is well characterized; to investigate the origin of the numerous weak PAH features; and to implement band profile asymmetries in the model in a consistent way. Recently, we detected the largest molecules known to exist in space -- the fullerene species C60 and C70. Just like PAHs, fullerenes turn out to be widespread, and to show subtle variations in their observational characteristics. I propose to carry out several observational projects to study the excitation mechanism of fullerenes, their formation mechanism, evolution, abundance and distribution, and their relation to other extinction features, and in particular the Diffuse Interstellar Bands (DIBs). The proposed program will lead to a better knowledge and understanding of the formation, evolution and distribution of large molecules in space and their specific role in the physics and chemistry of interstellar and circumstellar environments.