Constraints obtained from ${\chi }_{min}^2/{\chi }_{tot}^2$ maps with ${\chi }_{min}^2=min\left({\chi }_{\text{tot}}^2\right)$ and ${\chi }_{min}^2/{\chi }_{tot}^2$ = 1 for the best fit. Top: ${\chi }_{min}^2/{\chi }_{tot}^2$ in the 2D space (α and a_min,a–C). Bottom: ${\chi }_{min}^2/{\chi }_{tot}^2$ in the 2D space (M_a–C/M_H, a_min,a–C). The white lines indicate the best-fitting l_PDR for each combination of M_a–C/M_H and a_min,a–C. A degeneracy is generally observed between solutions with high M_a–C/M_H and low l_PDR. The cross marker shows the parameters that minimize ${\chi }_{\text{tot}}^2$, representing the best fit. The dashed blue lines indicate the DISM values (M_a–C/M_H, = 0.17 × 10⁻², a_min,a–C = 0.4 nm, α = −5).