HCN J =1−0 HFS lines toward the Horsehead PDR. The right axis shows the normalized line intensity to make clear that the observed HFS emission differs from the optically thin LTE line ratios 1:5:3 (green lines). Red curves show the results of a non-LTE radiative transfer model including line overlaps (for details, see Sect. 5.1 in Goicoechea et al. 2022). Blue curves show the best LTE-HFS fit using CLASSfootnote 2.