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

TL;DR

This study demonstrates that CF+ can serve as a ground-based observable proxy for C+ in interstellar regions, enabling better understanding of fluorine abundance and the structure of photon-dominated regions.

Contribution

The paper presents the first detection of CF+ lines in the Horsehead PDR, establishing CF+ as a ground-accessible tracer of C+ and providing a new method to measure fluorine abundance in space.

Findings

CF+ column density is (1.5 - 2.0) x 10^12 cm^-2.

Fluorine abundance in the gas phase is (0.6 - 1.5) x 10^-8.

CF+ emission traces the UV illuminated outer layers where C+ is abundant.

Abstract

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 towards the PDR and dense core positions in the Horsehead. Using a rotational diagram analysis, we derive a column density of N(CF+) = (1.5 - 2.0) \times 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) \times 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.