White dwarfs are the end product of stellar evolution for stars of mass similar to the Sun. Old white dwarfs with hydrogen atmospheres have photospheres with high densities and low temperatures. The dominant sources of opacity determining their spectral appearance are the negative hydrogen ion, collision induced absorption of the hydrogen molecule, and Rayleigh scattering of hydrogen atoms and molecules. The greatest source of uncertainty in the theoretical prediction of the colors of these white dwarfs is the opacity of the negative hydrogen ion at high density. This opacity has not previously been calculated accurately for matter densities greater then 0.001 grams per cubic centimeter, a regime in which neighboring particles perturb the ion's bound state structure. This project is a study of how collisions modify the properties of the negative hydrogen bound state, including its binding energy, its partition function, its thermodynamic abundance, and its bound-free and free-free cross sections. The project director will calculate the opacity of this ion at high densities.
Broader Impact. The research program will educate and train individuals in the application of basic physics to complex problems. The participants include students just starting a career in science, as well as individuals with more experience who will extend their skills to new areas. The training will contribute to the development of a proficient, scientifically informed group of individuals of general utility to a technologically advanced society. In an outreach program for levels K-12, the project director will partner with two science museums (the Science Center and the Museum of the Earth) and present interactive discussions on the topic of our modern scientific understanding of the basic observable features (size, brightness, color) of the Sun and other stars. A direct connection will be made between relevant fundamental microphysical processes (nuclear, atomic and molecular, including those that are the subject of the research program) and the observable properties of the Sun.
