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: guzman@iram.fr; pety@iram.fr; gratier@iram.fr
² LERMA – LRA, UMR 8112, Observatoire de Paris and École normale Supérieure, 24 rue Lhomond, 75231 Paris, France
e-mail: maryvonne.gerin@lra.ens.fr
³ Centro de Astrobiología, CSIC-INTA, Carretera de Ajalvir, Km 4, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: jr.goicoechea@cab.inta-csic.es
⁴ LUTH UMR 8102, CNRS and Observatoire de Paris, Place J. Janssen, 92195 Meudon Cedex, France
e-mail: evelyne.roueff@obspm.fr
⁵ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Caada, Madrid, Spain
e-mail: dteyssier@sciops.esa.int

Received: 20 April 2012
Accepted: 2 June 2012

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) × 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.