Thermodynamics of dense hydrogen-helium fluids

The equilibrium thermodynamic properties of hydrogen-helium fluids are studied in the density-temperature regime appropriate to the interiors of evolving very low-mass stars and giant planets. The low-density molecular fluid is treated by a free energy minimization method utilizing several theoretical models and recent high-pressure experimental results for hydrogen. The high-density metallic fluid is treated by an extension of Thomas-Fermi theory. These approaches are combined to yield a detailed thermodynamic description for two fluids: pure hydrogen and a solar mixture. Equations of state are given for the density range from 0.0001 to 0.001 gm/cu cm and a temperature range from 200 to 60,000 K.