Abstract

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) × 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) × 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. © 2012 ESO.