Logarithm of the line flux ratios as a function of the logarithm of the column densities (in cm⁻²) for the line pairs with similar upper level energies. The ratios for the slab models with T =300 K are shown in blue, whereas those for T = 500 K and T = 900 are shown in green and red, respectively. The horizontal black line indicates the line ratio taken from the continuum-subtracted spectrum of DR Tau. The dashed, vertical lines indicate where the different model ratios equal the observed line ratio. In addition, the dotted lines and the column densities listed in parenthesis indicate the results for the top line pair after subtracting off the contribution of the component with the lowest temperature (see Sect. 3.3).