Dependence of the gas physical conditions with the projected distance from the star, as determined from the relative intensities of the CH₃CN and CHCN lines near 349 GHz. Filled and empty black circles mark measurements in the eastern and western disk sides, and red and blue diamonds mark measurements in the north-eastern ad south-western apparent spirals, respectively (Table 2). Error bars of (only) midplane measurements are drawn, which account for an additional 10% uncertainty due to the assumptions. Upper panel: temperature distribution along the disk midplane as a function of the projected distance from the star. The dotted bold line traces the χ² fit to the sample distribution and the grey shadow shows the dispersion about the best fit (1σ). The best fit parameters and the linear correlation coefficient are indicated in the upper right. For comparison, the dashed blue line draws the increase of temperature with radius for a ‘classical’ power-law dependence (R^−0.75). Middle panel: similar to the upper panel, but for the column density values. A logarithmic scale is used for the y-axis. Lower panel:similar to the upper panels, but for the H₂ volume density averaged along the line-of-sight; this was calculated by assuming an edge-on disk geometry. The right axis provides the conversion to g cm⁻³.