Volume 543, July 2012
|Number of page(s)||4|
|Published online||21 June 2012|
The IRAM-30m line survey of the Horsehead PDR
I. CF+ as a tracer of C+ and as a measure of the fluorine abundance
1 IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères France
e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org
2 LERMA – LRA, UMR 8112, Observatoire de Paris and École normale Supérieure, 24 rue Lhomond, 75231 Paris, France
3 Centro de Astrobiología, CSIC-INTA, Carretera de Ajalvir, Km 4, Torrejón de Ardoz, 28850 Madrid, Spain
4 LUTH UMR 8102, CNRS and Observatoire de Paris, Place J. Janssen, 92195 Meudon Cedex, France
5 European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Caada, Madrid, Spain
Received: 20 April 2012
Accepted: 2 June 2012
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) × 1012 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.
Key words: astrochemistry / ISM: clouds / ISM: molecules / ISM: individual objects: Horsehead nebula / radio lines: ISM
© ESO, 2012
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