Comparison between the HCN vs. HNC intensity ratio (L(HCN)∕L(HNC)) as function of the abundance ratio (X(HCN)∕X(HNC)) for different gas kinetic temperatures (T_K). Left: luminosity variations for N(HCN)∕N(HNC) = X(HCN)∕X(HNC) = [0.1, 100] for gas temperatures between 10 and 70 K (coloured lines) calculated using RADEX assuming n(H₂) = 10⁴ cm⁻³ and N(HNC) = 10¹⁴ cm⁻² (see text). Linear variations are indicated by dotted (x = y) and dashed (y = x–0.3 dex) lines. The abundance and intensity ratios expected for our model 4 are indicated by solid circles coloured according to their corresponding temperature. Right: differences between the predicted X(HCN)∕X(HNC) (solid line) and L(HCN)∕L(HNC) (dashed line) values for model 4 as a function of 1/T_K (solid circles; similar to the left panel) in comparison to our Orion observations (grey crosses), similar to Fig. 5. We note the close correspondence between these X(HCN)∕X(HNC) and L(HCN)∕L(HNC) measurements under the physical conditions considered in our models.