THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 99:713-741, 1995 August

           AN EQUATION OF STATE FOR LOW-MASS STARS AND GIANT PLANETS

                  D. SAUMON, G. CHABRIER, AND H. M. VAN HORN


                                   ABSTRACT

        We present new equations of state (EOS) for hydrogen and helium,
     intended for applications to low-mass stars (M < 1 Msun), brown
     dwarfs, and giant planets.  They cover the range 2.10 < log T(K) <
     7.06 and 4 < log P (dyn/cm2) < 19 and include new physical treatments
     of partial dissociation and ionization caused by both pressure and
     temperature effects.  The hydrogen EOS is based on a careful study of
     nonideal interactions.  In the case of helium, the principal features
     of the EOS physics are retained in a simplified model.  The
     calculation is based on the free energy minimization method, and a
     detailed account of the physical model has been published elsewhere.
     Mixtures of hydrogen and helium are obtained with the additive volume
     rule and an additional ideal entropy-of-mixing term.  In this paper,
     we present extensive tabular results for both the H and He EOS,
     together with a critical analysis.  Part of this analysis is based on
     a comparison with other EOS commonly used in astrophysics.  Both EOS
     everywhere satisfy the requirements of thermal and mechanical
     stability and are thermodynamically consistent over most of the phase
     diagram.  A complete set of tables is available in the AAS CD-ROM
     Series, Vol. 5, and by anonymous FTP.

     Subject headings: equation of state -- planetary systems -- stars:
     interiors -- stars: low-mass, brown dwarfs