Astrophysical Equation of State and Opacity

Abstract

In stellar models, the equation of state and opacity are, together with nuclear reaction rates, the fundamental material properties. They have to be smooth, consistent, valid over a large range of temperatures and densities, and must incorporate the most important astrophysically relevant chemical elements. The equation of state appears as a necessary part of stellar modeling as well as of any opacity calculation. For the latter, it has to provide ionization equilibrium concentrations and level populations. However, the interest in the stellar equation of state is not merely motivated by astrophysics. It has turned out that one star - the Sun - is very special in two respects. First, the methods of helioseismology allow us to infer conditions in the solar interior very accurately (in particular, sound speed and density). Second, in the solar convection zone, helioseismology presents an opportunity to isolate the question of the equation of state from opacity and nuclear reaction rates, since the stratification is essentially adiabatic and thus determined by thermodynamics (Christensen-Dalsgaard & Dappen 1992).Thus the Sun has become an astrophysical laboratory to study thermodynamic properties of a Coulomb system under conditions that cannot be achieved on Earth. Indirectly thus, the solar experiment also addresses a broader range of plasmas. In astrophysics, results for denser plasmas will have impact on models of Jupiter, Saturn, and probably brown and white dwarfs (Cauble et al. 1998), as well as of low-mass stars.