Thermal index of npeγ matter as a function of baryon density n_b for temperatures ranging from T = 5 to 50 MeV in steps of 5 MeV. For each temperature, a solid line represents the thermal index of asymmetric matter at β equilibrium, accounting for interactions through the virial approximation. The dotted line indicates the approximation of the thermal index as given in Equation (30). Red dots mark the inflection densities of Equation (32). Gray dotted lines represent the values of Γ_th in both the relativistic and nonrelativistic limits. All data shown are consistent with the condition z < 0.3.