Comparison between the observed Guzman et al. (2012) and calculated abundances of CF⁺ in both the Core and the PDR positions of the Horsehead nebula at 4.64 × 10⁴ yr and 1 × 10⁶ yr. Three different models are considered here concerning the fluorine chemistry: Model 1, corresponding to the one described in Sect. 2.3; Model 2, which contains a lower fluorine abundance (Goicoechea et al. 2006), and Model 3, with improved reaction rate coefficients discussed in the text. The model results are represented by the solid curves and the observations by the upper limit and hatched box represented in pink.