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Title:
A Synoptic Comparison of the Mihalas-Hummer-Däppen and OPAL Equations of State
Authors:
Trampedach, R.; Däppen, W.; Baturin, V. A.
Affiliation:
AA(Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia; ), AB(Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-1342 ), AC(Sternberg Astronomical Institute, Universitetsky Prospect 13, Moscow 119899, Russia; )
Publication:
The Astrophysical Journal, Volume 646, Issue 1, pp. 560-578. (ApJ Homepage)
Publication Date:
07/2006
Origin:
UCP
ApJ Keywords:
Atomic Processes, Equation of State, Plasmas, Sun: Interior
DOI:
10.1086/504883
Bibliographic Code:
2006ApJ...646..560T

Abstract

A detailed comparison is carried out between two popular equations of state (EOSs), the Mihalas-Hummer-Däppen (MHD) and OPAL equations of state, which have found widespread use in solar and stellar modeling during the past two decades. They are parts of two independent efforts to recalculate stellar opacities: the international Opacity Project (OP) and the Livermore-based OPAL project. We examine the difference between the two EOSs in a broad sense, over the whole applicable ϱ-T range, and for three different chemical mixtures. Such a global comparison highlights both their differences and their similarities. We find that omitting a questionable hard-sphere correction, τ, to the Coulomb interaction in the MHD formulation, greatly improves the agreement between the MHD and OPAL EOSs. We also find signs of differences that could stem from quantum effects not yet included in the MHD EOS, and differences in the ionization zones that are probably caused by differences in the mechanisms for pressure ionization. Our analysis not only gives a clearer perception of the limitations of each EOS for astrophysical applications, but also serves as guidance for future work on the physical issues behind the differences. The outcome should be an improvement of both EOSs.
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