def Trho_idrad(rho,mu):
T = <NUM_LIT> * (old_div(rho,mu))**(old_div(<NUM_LIT:1.>,<NUM_LIT>))<EOL>return T<EOL>
T(rho) that separates P_rad from P_gas dominated regions.

Kippenhahn & Weigert, Eq. 16.10

Parameters
----------
rho : float
    Density array [cgs].
mu : float
    Mean molecular weight.