Physical structure and excitation temperatures of CN(ν = 340.265 GHz) and C¹⁵N, within the CN-ring, at radii of R = 60 (top) and 85 AU (bottom). The total proton number density and kinetic temperature profiles are taken from (Qi et al. 2013). The corresponding cumulative column density, increasing towards the midplane, is also shown (right scale). The excitation temperatures T_ex of the CN and C¹⁵N hyperfine components (star triangles and open circles, resp.) considered in our uv-plane analysis (see Sects. A.2 and A.3) were obtained in the escape probability formalism using the RADEX code (van der Tak et al. 2007), and adopting a FWHM of 0.25 km s^-1. For each density and temperature, a range of CN total column densities were adopted, from 10¹² to 10¹⁵ cm^-2, with C¹⁵N being 300 times less abundant. Also shown is the CN/C¹⁵N isotopic ratio (open triangles, inset scale) obtained by use of Eq. (C.1) when adopting the CN and C¹⁵N excitation temperatures from the non-LTE calculations.