Thermodynamically consistent equation of state tables for use in giant planet and brown dwarf interior modeling

Abstract: Equation of state (EOS) tables used in giant planet modeling exhibit regions of strong thermodynamic inconsistency, or disobedience to the first law of thermodynamics. These generally result from smooth interpolation over physical discontinuities or between thermodynamic sub-domains governed by different physical models. The effect of thermodynamic inconsistency on structure models of planets evolved with these EOS has never been studied in detail, although it is understood to introduce spurious sources or sinks of energy or entropy in the models, leading to inaccuracies. Here, we present a method for improving the thermodynamic consistency of an existing EOS table, based on constructing a Helmholtz free energy $F(ρ,T)$ covering the EOS domain and deriving all the other necessary EOS quantities from it. This method produces EOS tables with roughly thousandfold better thermodynamic consistency across the EOS domain of interest, as quantified by three thermodynamic consistency metrics derived from the first law. We also present a controlled experiment comparing giant planet models evolved with MESA using the original EOS and the $F$-derived, thermodynamically consistent versions, and find that the models can differ by up to $5\%$ in radius at an age of 5 Gyr, and by up to $25\%$ in luminosity at earlier stages of evolution; thermodynamic consistency is thus an especially important consideration for characterising young directly imaged planets. We present the $F$-derived, thermodynamically consistent versions of the tables for download at \href{https://doi.org/10.5281/zenodo.21103561}{https://doi.org/10.5281/zenodo.21103561}, along with example MESA inlists to reproduce our evolution models.