χ² minimisation for the Orion Bar. Left: in the 2D space (a_min,a–C and α). Right: as a function of M_a__c/M_H for five different couple a_min,a–C/α that are represented by the five symbols on the left figure. 1 (black dot): 0.35 nm/−5.96, 2 (blue triangle): 0.51 nm/−5.96. 3 (orange square): 0.63 nm/−7.44, 4 (green star): 0.71 nm/−8.92, 5 (pink diamond): 0.77 nm/−12.25. We also indicate with black arrows the M_a__c/M_H obtained for the Horsehead (see Paper I), IC63 (see Sect. 5.2), and the diffuse ISM (Jones et al. 2013, 2017). The vertical grey stripe corresponds to the range of M_a__c/M_H that minimise in the Orion Bar.