Optical depth at line centre (τ) as a function of the energy of the upper level (E_u). τ was computed for a range of radii; the characteristic radius adopted (r_CO, Table B.1) is indicated with a thick line. In each panel, the inset shows the values of T_rot and M_H2 after the C_τ correction has been applied for the corresponding radii, along with the opacity-uncorrected results (open triangle).