Optical depth of the 113.144 GHz CN line emission of the reference circumstellar disk model. The optical depth is calculated along the LOS through the whole disk by averaging the optical depth over the velocity channels weighted with the corresponding net flux. The high optical depth above and below the midplane does not only correspond to high gas densities, but is mainly caused by the low dispersion of the velocity along the LOS. The contour lines correspond to optical depths of 2, 4, and 8.