2 - Equation of state of stellar plasmas

Summary

Abstract

The equation of state (EOS) of astrophysical plasmas is, for a wide range of stars, nearly ideal; with only small non-ideal Coulomb corrections. Calculating the EOS of an ionizing plasma from a ground state ion, ideal gas model is easy, whereas, fundamental methods to include the small Coulomb corrections are difficult. Attempts to include excited bound states are also complicated by plasma screening and microfield phenomena that weaken and broaden these states. Nevertheless, the high quality of current observational data, particularly seismic, dictates that the best possible models should be used. The present article discusses these issues and describes how they are resolved by fundamental many-body quantum statistical methods. Particular emphasis is placed on the activity expansion method that is the basis of the OPAL opacity code. Some comparisons with standard methods are given.

Abstract

L'equation d'etat des plasmas astrophysiques est, pour un large domaine d'etoiles, pratiquement ideale; avec de petites corrections coulombiennes. Calculer l'equation d'etat d'un plasma ionise a partir d'un modele de gaz ideal d'ions dans leur etat fondamental est facile, alors que les methodes fondamentales pour inclure les petites corrections coulombiennes sont difficiles. Des tentatives pour inclure des etats lies excites sont aussi rendues difficiles par les effets d'ecran et le phenomene de microchamp qui affaiblissent et elargissent ces etats. Neanmoins, la haute qualite des observations actuelles, en particulier en sismologie, impose l'utilisation des tous meilleurs modeles.