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

We present new equations of state (EOS) for hydrogen and helium, inten ded for applications to low-mass stars (M < 1 M..), brown dwarfs, and giant planets. They cover the range 2.10 < log T(K) < 7.06 and 4 < log P (dyn cm(-2)) < 19 and include new physical treatments of partial di ssociation and ionization caused by both pressure cm and temperature e ffects. The hydrogen EOS is based on a careful study of nonideal inter actions. In the case of helium, the principal features of the EOS phys ics are retained in a simplified model. The calculation is based on th e free energy minimization method, and a detailed account of the physi cal model has been published elsewhere. Mixtures of hydrogen and heliu m are obtained with the additive volume rule and an additional ideal e ntropy-of-mixing term. In this paper, we present extensive tabular res ults for both the H and He EOS, together with a critical analysis. Par t of this analysis is based on a comparison with other EOS commonly us ed in astrophysics. Both EOS everywhere satisfy the requirements of th ermal and mechanical stability and are thermodynamically consistent ov er most of the phase diagram. A complete set of the tables is availabl e in the AAS CD-ROM Series, Vol. 5, and by anonymous FTP.