Upper panel: observed dependence of the I(HCN)∕I(HNC) intensity ratio as function of 1/T_K in the ISF (grey crosses; this work) in comparison with classical estimates from dense core surveys (grey triangles; Hirota et al. 1998). The positions with good temperature estimates (i.e., δT_K ∕T_K ≤ 5%) are colour-coded according to their total H41α intensity (see Fig. 1). The temperature dependence of the I(HCN)∕I(HNC) expected for an energy barrier of ΔE₁ = 200 K for reaction (1) is indicated by the black dashed line (Schilke et al. 1992; Graninger et al. 2014). Similarly, the newly proposed low energy barrier for ΔE₂ = 10 K for reaction (2) is shown in red. Lower panel: X(HCN)∕X(HNC) abundance variations predicted by our models (1–4) with different energy barriers (see also text) compared to the observed intensity variations in cores (grey triangles) and throughout the ISF (grey crosses). Our model results illustrate the differentdominant roles of reactions (1) and (2) determining the temperature structure of the ONC (blue area) and the rest of the cloud (grey area). The detailed equivalence between the observed intensity ratios and the corresponding abundance ratios in our models are discussed in Appendix B.