# Polytropic Models

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

$P = K \rho^{(n+1)/n}$

for piecewise-constant $$K$$ and polytropic index $$n$$. The formalism behind these models is described in the Composite Polytropes section. The build_poly executable can be used to create a polytropic model and write it to a file in POLY format.

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