Dr. Donald Shemansky will carry out a theoretical examination of the role of activated hydrogen in comet comae. Highly developed non-LTE molecular hydrogen (H2) is expected in comet comae, and recent evidence shows the presence of significant abundance of this species. The role of activated hydrogen has not been previously investigated. The role of molecular hydrogen will be explored theoretically to determine the extent to which transitions in this system contribute to the large number of unidentified emission lines that have been found in FUSE Observatory observations, and furthermore examine the level of importance activated H2 may hold in the overall development of comet comae. High resolution measurements of comet comae using the FUSE facility have identified emission lines from H2 bands stimulated by the solar H Ly alpha line. The two comets investigated showing these features also contain of order 50 emission lines between 900 and 1100 Angstrom that have not been identified. Preliminary investigation for this project has indicated a large number of the features correspond to transitions in H2 electronic systems. This region, however, contains of order 50,000 lines of the H2 electronic systems, and therefore a high probability exists that many H2 transitions would correlate with the observed features without constituting identification of origin. The early investigation has identified possible H2 lines in the spectra of comet C/2001 A2(LINEAR) that arise from very large rotational levels, such as J = 11, and 12. This is plausible because rotational levels in the H2 ground state have extremely long radiative lifetimes, and H2 is expected to be produced in extreme non-LTE states in the physical chemistry of the coma. A physical chemistry architecture has been developed for hydrogen at the University of Southern California that establishes chemical and physical rate processes at the rotational structure level, so that no assumptions are necessary in the model calculations in regard to the thermal condition of the gas. That is, the state of the gas is established only on the basis of the forcing functions, diffusion properties, and gas density in the activated volume. A preliminary investigation will be conducted to assess the scope of complexity required to address the role of activated hydrogen through the examination of the observed spectra against the theoretical model, which would include consideration of physical chemistry involving the non hydrogen species in the comet coma as reactants and sources.
The detailed hydrogen physical chemistry to be investigated here has never previously been examined for the comet coma environment. The starting point for this project is a fully developed detailed architecture for non-LTE hydrogen reactions at the rotational quantum number level, allowing generation of detailed predicted emission spectra, and exploration of the overall effect of activated hydrogen on the chemistry of the coma.. The development of magnetohydrodynamic MHD codes in recent years has provided a platform for remarkable advancement of global modeling of comet processes. The structure to be refined in the present effort would allow the introduction of non-LTE detail into these developed MHD programs. ***
