Letter to the Editor

Herschel observations of EXtra-Ordinary Sources (HEXOS): detecting spiral arm clouds by CH absorption lines^*

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: qin@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
⁴ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁵ LERMA & UMR8112 du CNRS, Observatoire de Paris, 61, Av. de l'Observatoire, 75014 Paris, France
⁶ Centre d'étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
⁷ CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
⁸ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, Cedex 9, France
⁹ Centro de Astrobiología (CSIC/INTA), Laboratiorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
¹¹ LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
¹² LUTH, UMR8102, Observatoire de Paris, Meudon, France
¹³ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
¹⁴ Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA
¹⁵ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁶ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹⁷ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George St, Toronto, ON M5S 3H8, Canada
¹⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
¹⁹ National University of Ireland Maynooth, Ireland
²⁰ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimor e, MD 21218, USA
²¹ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²² Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
²³ MPI für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
²⁴ Observatorio Astronómico Nacional (IGN), Centro Astronómico de Yebes, Apartado 148, 19080 Guadalajara, Spain
²⁵ Microwave Laboratory, ETH Zurich, 8092 Zurich, Switzerland
²⁶ Chalmers University of Technology, 412 96 Gteborg, Sweden, Sweden/ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, the Netherlands
²⁷ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden

Received: 31 May 2010
Accepted: 1 July 2010

Abstract

We have observed CH absorption lines (J = 3/2, N = 1 J = 1/2, N = 1) against the continuum source Sgr B2(M) using the Herschel/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show “spiral arm” cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr B2 envelope. The observations show that each “spiral arm” harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H₂ column densities found at lower A_V by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds.