Absorption of Radiation by Hot, Dense Mixtures of Hydrogen and Helium

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This project is motivated by the inadequacy of existing calculations of collision-induced absorption involving H2 molecules in very cool white dwarfs. These stars are the remains of the earliest generation of stars in our Galaxy and form the basis for a method to determine the age of this stellar population. Knowledge of collision-induced absorption spectra at high densities, high temperatures and frequencies is crucial for the determination of atmospheric compositions, effective temperatures, cooling rates, and cooling ages.Here, opacities arising from collision induced absorption in dense hydrogen gas and hydrogen-helium mixtures at temperatures from 1000 to 6000 K and frequencies up to 25,000 inverse centimeters will be calculated from first principles. These collision-induced absorption coefficients will be obtained by direct integration of the Schrödinger equation of the H2-X scattering process in the presence of photons, with X = H2, He, and/or H. Besides the binary absorption coefficients, estimates of the ternary absorption of both the pairwise-additive components and the strongest long-range, irreducible components will be obtained. The contribution of ternary complexes such as H2-H2-H2, H2-He-He, and He-He-He, presently unaccounted for but known to be important will also be evaluated and made available through this work. Through collaboration with Dr. Didier Saumon (Los Alamos National Laboratory) and the Stellar Atmospheres Group at the University of Texas, headed by Dr. Don Winget, these new collision-induced absorption calculations will find immediate applications in the modeling of the atmospheres of very cool white dwarf stars.Several peculiar white dwarfs currently defy interpretation with current models and it is hoped that the work to be carried out here will shed light on these problems. It is further expected that the analysis capabilities enabled by the new collision-induced opacities will have immediate application in a number of other fields, such as Galactic astronomy and stellar evolution. Two graduate students will be supported through this project and trained in implementing algorithms for numerical solutions, intensive numerical computations, programming, and testing.