Skye: A Differentiable Equation of State

Abstract Stellar evolution and numerical hydrodynamics simulations depend critically on access to fast, accurate, thermodynamically consistent equations of state. We present Skye, a new equation state for fully ionized matter. Skye includes the effects positrons, relativity, electron degeneracy, Coulomb interactions, nonlinear mixing effects, quantum corrections. determines point crystallization in self-consistent manner, accounting composition automatically. A defining feature this is that it uses analytic free energy terms provides thermodynamic quantities using automatic differentiation machinery. Because this, easily extended include by simply writing energy. also introduce novel extrapolation scheme extending fits beyond range fitting data while preserving desirable properties like positive entropy sound speed. demonstrate action MESA stellar software instrument computing white dwarf cooling curves.