Letter to the Editor

CH⁺(1–0) and ¹³CH⁺(1–0) absorption lines in the direction of massive star-forming regions ^*,**

¹ LERMA, CNRS, Observatoire de Paris and ENS, France e-mail: edith.falgarone@ens.fr
² Institut d'Astrophysique Spatiale (IAS), Orsay, France
³ Centro de Astrobiología, CSIC-INTA, Madrid, Spain
⁴ California Institute of Technology, Pasadena, CA 91125, USA
⁵ Chalmers University of Technology, Göteborg, Sweden
⁶ CESR, Université Toulouse 3 and CNRS, Toulouse, France
⁷ CNRS; UMR5187; 31028 Toulouse, France
⁸ I. Physikalisches Institut, University of Cologne, Germany
⁹ JPL, California Institute of Technology, Pasadena, USA
¹⁰ Laboratoire d'Astrophysique de Marseille (LAM), France
¹¹ Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
¹² Laboratoire d'Astrophysique de Grenoble, France
¹³ Nicolaus Copernicus University, Toruń, Poland
¹⁴ Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland
¹⁵ Tata Institute of Fundamental Research, Mumbai, India
¹⁶ The Johns Hopkins University, Baltimore, MD 21218, USA
¹⁷ Dept. of Physics & Astronomy, University of Calgary, Canada
¹⁸ European Space Astronomy Centre, ESA, Madrid, Spain
¹⁹ MPI für Radioastronomie, Bonn, Germany
²⁰ Gemini telescope, Hilo, Hawaii, USA
²¹ SRON Netherlands Institute for Space Research, Netherlands
²² Sterrewacht Leiden, Netherlands
²³ Deutsches Zentrum für Luft- und Raumfahrt e. V., Raumfahrt-Agentur, Bonn, Germany
²⁴ Department of Physics and Astronomy, University of Waterloo, Canada
²⁵ Osservatorio Astrofisico di Arcetri-INAF- Florence, Italy
²⁶ IRAM, 300 rue de la Piscine, St. Martin d'Hères, France
²⁷ Laboratoire d'Astrophysique de Bordeaux (LAB), France
²⁸ Infrared Processing Analysis Center, California Institute of Technology, Pasadena, USA
²⁹ Observatorio AstronŃacional (IGN), Spain
³⁰ Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile

Received: 31 May 2010
Accepted: 19 July 2010

Abstract

We report the detection of the ground-state rotational transition of the methylidyne cation CH⁺ and its isotopologue ¹³CH⁺ toward the remote massive star-forming regions W33A, W49N, and W51 with the HIFI instrument onboard the Herschel satellite. Both lines are seen only in absorption against the dust continuum emission of the star-forming regions. The CH⁺ absorption is saturated over almost the entire velocity ranges sampled by the lines-of-sight that include gas associated with the star-forming regions (SFR) and Galactic foreground material. The CH⁺ column densities are inferred from the optically thin components. A lower limit of the isotopic ratio [ ¹²CH⁺] /[ ¹³CH⁺]  > 35.5 is derived from the absorptions of foreground material toward W49N. The column density ratio, N(CH⁺)/N(HCO⁺), is found to vary by at least a factor 10, between 4 and >40, in the Galactic foreground material. Line-of-sight ¹²CH⁺ average abundances relative to total hydrogen are estimated. Their average value, N(CH⁺)/N_H > 2.6×10^-8, is higher than that observed in the solar neighborhood and confirms the high abundances of CH⁺ in the Galactic interstellar medium. We compare this result to the predictions of turbulent dissipation regions (TDR) models and find that these high abundances can be reproduced for the inner Galaxy conditions. It is remarkable that the range of predicted N(CH⁺)/N(HCO⁺) ratios, from 1 to ~50, is comparable to that observed.