# Polytropic Models¶

Polytropic models represent stars in hydrostatic equilibrium that follow the polytropic equation of state,

$p = K \rho^{\frac{\npoly+1}{\n_poly},$

for piecewise-constant $$K$$ and $$\npoly$$. The structure of polytropic models is obtained by solving the Lane-Emden equation; GYRE doesn’t do this itself, but can read structure data from external files created by the build_poly executable (see the Building Polytropic Structure Files appendix for more details).

To use a polytropic model with GYRE, set the model_type parameter in the :n ml_g:model namelist group to 'POLY', and the file parameter to the name of the polytropic structure file.