Ratio of the integrated intensities of couples of components of HCN(1−0) (upper panels), and H¹³CN(1−0) (lower panels), where R_ij represents the ratio between the integrated intensities W_i and W_j (see Table B.1 for component indices). Observational data: L1498 (yellow squares), L1521E (green circles), and TMC 2 (black triangles). One-layer model (black solid curve), two-layer model (magenta dashed curves).