The PAH emission bands as a diagnostic tool

点击翻译按钮获取中文摘要

Polycyclic Aromatic Hydrocarbons (PAHs) are among the largest organic molecules found in space, where they play a crucial role in various physical and chemical processes, and in the prebiotic evolution toward life. Hence, studying these abundant interstellar molecules, their formation and evolution, and the astrophysical processes that control their abundance is of tremendous importance to understand our cosmic roots and the Universe around us.******The infrared signature of PAHs - 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 size and structure, and it is therefore not possible to identify the individual PAH molecules that cause the IR emission. Similarly, although it is firmly established that the PAH emission bands vary from object to object and even spatially within objects, the origin of these variations is not entirely understood. These unknowns are major roadblocks in the development of diagnostic tools that would transform the ubiquitous UIR bands into powerful probes of the physical and chemical conditions in the Universe. ******The research program I propose addresses these unknowns by carrying out several innovative studies of PAHs in space using astronomical observations in conjunction with the NASA Ames PAH IR spectral database, which contains the physical and spectral properties of a large number of PAHs and related molecules. In particular, I will characterize the variations of the UIR bands in a wide variety of astronomical objects and compare these to local physical conditions, such as distance from the exciting source, radiation field, electron density, and temperature. I will then use the NASA Ames database to determine how these variations can be understood in terms of PAH molecular properties (e.g. size, charge). This will reveal how PAHs interact with and reflect the physical conditions of their environment. This observational approach will thus provide an empirical quantitative calibration of the PAH characteristics with physical conditions. This, in turn, will allow using the UIR bands as an environmental diagnostic tool (e.g., like a weather station) in regions near and far. I will also use these studies to investigate the processing of carbonaceous species. In particular, I will investigate whether PAHs are processed into fullerenes and small hydrocarbons, as well as whether UV processing causes a varying degree of aromaticity and changes in the PAH band profiles.********