Modeling pressure-ionization of hydrogen in the context of astrophysics

The recent development of techniques for laser-driven shock compression of hydrogen has opened the door to the experimental determination of its behav ior under conditions characteristic of stellar and planetary interiors. The new data probe the equation of state (EOS) of dense hydrogen in the comple x regime of pressure ionization. The structure and evolution of dense astro physical bodies depend on whether the pressure ionization of hydrogen occur s continuously or through a "plasma phase transition" (PPT) between a molec ular state and a plasma state. For the first time, the new experiments cons train predictions for the PPT. We show here that the EOS model developed by Saumon and Chabrier can successfully account for the data, and we propose an experiment that should provide a definitive test of the predicted PPT of hydrogen. The usefulness of the chemical picture for computing astrophysic al EOS and in modeling pressure ionization is discussed.