The IRAM-30m line survey of the Horsehead PDR I. CF+ as a tracer of C+ and as a measure of the fluorine abundance

C+ is a key species in the interstellar medium, but its 158 mu m 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) x 10(12) cm(-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) x 10(-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.