Letter to the Editor

Herschel/HIFI detections of hydrides towards AFGL 2591^*,**

Envelope emission versus tenuous cloud absorption

¹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland e-mail: simonbr@astro.phys.ethz.ch
² 
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Institut für 4D-Technologien, FHNW, 5210 Windisch, Switzerland
⁵ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
⁶ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁸ LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁹ INAF - Osservatorio Astronomico di Roma, 00040 Monte Porzio catone, Italy
¹⁰ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain Centre d'Etude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
¹¹ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹² Laboratory for Electromagnetic Fields ad Microwave Electronics, ETH Zurich, 8092 Zurich, Switzerland
¹³ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII,3. 28014, Madrid, Spain
¹⁴ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
¹⁵ INAF - Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁶ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹⁷ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁸ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
¹⁹ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
²⁰ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
²¹ Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA. Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid, Spain.
²² Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
²³ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²⁴ University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
²⁵ Observatorio Astronómico Nacional, Apartado 112, 28803 Alcalá de Henares, Spain
²⁶ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
²⁷ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
²⁸ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K., Denmark
²⁹ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
³⁰ California Institute of Technology, Cahill Center for Astronomy and Astrophysics, MS 301-17, Pasadena, CA 91125, USA
³¹ The University of Western Ontario, Department of Physics and Astronomy, London, Ontario, N6A 3K7, Canada
³² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
³³ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³⁵ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³⁶ Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
³⁷ Instituto de Radioastronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Núcleo Central, 18012 Granada, Spain

³⁸ CNRS/INSU, UMR 5187, 9 Avenue du Colonel Roche, 31028 Toulouse Cedex 4, France

Received: 31 May 2010
Accepted: 20 July 2010

Abstract

The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory allows the first observations of light diatomic molecules at high spectral resolution and in multiple transitions. Here, we report deep integrations using HIFI in different lines of hydrides towards the high-mass star forming region AFGL 2591. Detected are CH, CH⁺, NH, OH⁺, H₂O⁺, while NH⁺ and SH⁺ have not been detected. All molecules except for CH and CH⁺ are seen in absorption with low excitation temperatures and at velocities different from the systemic velocity of the protostellar envelope. Surprisingly, the CH(J_F,P = 3/2_2,- - 1/2_1,+ ) and CH⁺(J = 1–0, J = 2–1) lines are detected in emission at the systemic velocity. We can assign the absorption features to a foreground cloud and an outflow lobe, while the CH and CH⁺ emission stems from the envelope. The observed abundance and excitation of CH and CH⁺ can be explained in the scenario of FUV irradiated outflow walls, where a cavity etched out by the outflow allows protostellar FUV photons to irradiate and heat the envelope at larger distances driving the chemical reactions that produce these molecules.