Abstract

We present a free energy model for fluid hydrogen at high-density and high-temperature. This model aims at describing pressure dissociation and ionization, which occur in partially ionized plasmas encountered in the interiors of giant planets and low-mass stars. The model describes an interacting mixture of H2,H,H+ and e− in chemical equilibrium. The concentrations of H2+ and H− ions are found to be negligible for equation of state purposes. Our model relies on the so-called chemical picture approach, based on the factorization of the partition function into translational, internal and configurational degrees of freedom. The present model is found to be unstable in the pressure-ionization regime and predicts the existence of a first-order plasma phase transition (PPT) which ends up at a critical point given by Tc = 15300 K, Pc = 0.614 Mbar, and ρc = 0.35 gcm−3. The transition occurs between a weakly ionized phase and a partially ionized (∼ 50%) phase.

Nous présentons un modèle d'énergie libre pour l'hydrogène fluide à haute densité et haute température. Le but de ce modèle est de décrire la dissociation et l'ionisation en pression, telles qu'elles se produisent dans les plasmas partiellement ionisés rencontrés à l'intérieur des planètes géantes et des étoiles de faible masse. Le modèle décrit un fluide en interaction composé de H2,H,H+ et e− en équilibre chimique.