Letter to the Editor

Interstellar CH absorption in the diffuse interstellar medium along the sight-lines to G10.6–0.4 (W31C), W49N, and W51 ^*,**

¹ LERMA, CNRS, Observatoire de Paris and ENS, France e-mail: maryvonne.gerin@ens.fr
² Centro de Astrobiología, CSIC-INTA, Madrid, Spain
³ Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
⁴ Institut d'Astrophysique Spatiale (IAS), Orsay, France
⁵ California Institute of Technology, Pasadena, CA 91125, USA
⁶ Université de Toulouse; UPS; CESR; 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
⁷ CNRS; UMR5187; 31028 Toulouse, France
⁸ Nicolaus Copernicus University, Torún, Poland
⁹ The Johns Hopkins University, Baltimore, MD 21218, USA
¹⁰ Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
¹¹ JPL, California Institute of Technology, Pasadena, USA
¹² I. Physikalisches Institut, University of Cologne, Germany
¹³ Laboratoire d'Astrophysique de Marseille (LAM), France
¹⁴ Laboratoire d'Astrophysique de Grenoble, France
¹⁵ Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland
¹⁶ Tata Institute of Fundamental Research, Mumbai, India
¹⁷ Dept. of Physics & Astronomy, University of Calgary, Canada
¹⁸ Nicolaus Copernicus Astronomical Center, Poland
¹⁹ European Space Astronomy Centre, ESA, Madrid, Spain
²⁰ MPI für Extraterrestrische Physik, Garching, Germany
²¹ MPI für Radioastronomie, Bonn, Germany
²² Gemini telescope, Hilo, Hawaii, USA
²³ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
²⁴ SRON Netherlands Institute for Space Research, Netherlands

Received: 31 May 2010
Accepted: 9 July 2010

Abstract

We report the detection of the ground state N,J = 1,3/2 1,1/2 doublet of the methylidyne radical CH at ~532 GHz and ~536 GHz with the Herschel/HIFI instrument along the sight-line to the massive star-forming regions G10.6–0.4 (W31C), W49N, and W51. While the molecular cores associated with these massive star-forming regions show emission lines, clouds in the diffuse interstellar medium are detected in absorption against the strong submillimeter background. The combination of hyperfine structure with emission and absorption results in complex profiles, with overlap of the different hyperfine components. The opacities of most of the CH absorption features are linearly correlated with those of CCH, CN, and HCO⁺ in the same velocity intervals. In specific narrow velocity intervals, the opacities of CN and HCO⁺ deviate from the mean trends, giving rise to more opaque absorption features. We propose that CCH can be used as another tracer of the molecular gas in the absence of better tracers, with [CCH]/[ H₂] ~ 3.2±1.1×10^-8. The observed [CN]/[CH], [CCH]/[CH] abundance ratios suggest that the bulk of the diffuse matter along the lines of sight has gas densities n_H = n(H) + 2n(H₂) ranging between 100 and 1000 cm^-3.