SIMBAD references

C⁺ is a key species in the interstellar medium, but its 158 µm fine structure line cannot be observed from ground-based telescopes. Current models of fluorine chemistry predict that CF⁺ is the second-most important fluorine reservoir in regions where C⁺ is abundant. We detected the J=1-0 and J=2-1 rotational lines of CF⁺ with high signal-to-noise ratio toward the photo-dissociation region and dense core positions in the Horsehead. Using a rotational diagram analysis, we derive a column density of N(CF⁺)=(1.5-2.0)x10¹²cm^–2. Because of the simple fluorine chemistry, the CF⁺ column density is proportional to the fluorine abundance. We thus infer the fluorine gas-phase abundance to be F/H=(0.6-1.5)x10^–8. Photochemical models indicate that CF⁺ is found in the layers where C⁺ is abundant. The emission arises in the UV-illuminated skin of the nebula, tracing the outermost cloud layers. Indeed, CF⁺ and C⁺ are the only species observed to date in the Horsehead with a double-peaked line profile caused by kinematics. We therefore propose that CF⁺, which is detectable from the ground, can be used as a proxy of the C⁺ layers.