Far-ultraviolet molecular hydrogen fluorescence in photodissociation regions

Molecular hydrogen constitutes a large fraction of the baryonic matter in the universe and plays a role in regulating the physics and chemistry of many astrophysical environments. It is an important coolant during the formation of the first structures in the universe, is the dominant mass component in star-forming regions today, and is ubiquitous in the interstellar medium. As a homonuclear molecule, it has no permanent electric dipole moment, but the quadrupole rovibrational transitions provide diagnostic information about the physical state of interstellar clouds. Electronic transitions of the molecule are allowed and the absorption lines arising from these transitions are used to probe the physical conditions of the interstellar medium. However, the subsequent emission that is the dominant de-excitation route of the molecule is poorly constrained by both observations and theory. The purpose of this dissertation is to examine molecular hydrogen by using new and existing ultraviolet observations to test models of the fluorescent emission process. This dissertation will report on new observations made by sounding rocket experiments and the Far Ultraviolet Spectroscopic Explorer to expand the observational basis for molecular hydrogen studies. The observational survey includes a variety of photodissociation regions, including the emission nebula IC 63, the star-forming region near the Orion Nebula, the emission/reflection nebula IC 405, and the pure reflection nebulae NGC 2023 and NGC 7023. We detect the fluorescent signature in the first three objects, and we use these data to develop the first model of this emission that is consistent with observation. We do not detect hydrogen fluorescence as presently understood in the latter two objects, and explanations are discussed.