This grant supports Dr. Draine's research on several topics within the area of theoretical astrophysics of diffuse interstellar matter. The topics include: (A) excitation of interstellar molecular hydrogen, with the aim of being able to calculate accurately the non-local thermodynamic equilibrium excitation of and emission from molecular hydrogen under various conditions; (B) structure of non-stationary photodissociation fronts, including both stationary and propagating fronts, with the aim of being able to predict detailed emission spectra; and (C) interstellar magneto- hydrodynamic shock waves in both diffuse and dense clouds, including study of the non-linear development of instabilities in magneto-hydrodynamic shocks. (A) As the most abundant molecule in the interstellar medium, molecular hydrogen is a critical diagnostic for regions of warm molecular gas such as photodissociation regions and shock fronts. It is essential to have accurate theoretical calculations of the spectrum of collisionally excited molecular hydrogen. (B) It has recently become clear that time-dependent photodissociation fronts are of major importance. The neglect of gas flow through such a front is a major limitation of current models, and Dr. Draine will attempt to rectify this defect. (C) One of the highlights of interstellar astrophysics of the last decade, the determination of the spectra of shocks in molecular gas, is in gross disagreement with observation. Dr. Draine's course of study should help to eliminate this discrepancy.
