Letter to the Editor

Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: rrolffs@mpifr.de
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁴ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
⁵ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA

⁶ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
⁷ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
⁸ CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France

⁹ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
¹⁰ Centro de Astrobiología (CSIC/INTA), Laboratorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir km 4, 28850 Madrid, Spain

¹¹ LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 05, France
¹² LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
¹³ LUTH, UMR8102, Observatoire de Paris, Meudon, France

¹⁴ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA

¹⁵ Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA
¹⁶ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁷ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹⁸ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George St, Toronto, ON M5S 3H8, Canada
¹⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
²⁰ National University of Ireland Maynooth, Ireland

²¹ Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
²² Observatorio Astronómico Nacional (IGN), Centro Astronómico de Yebes, Apartado 148, 19080 Guadalajara, Spain
²³ Departamento de Astrofìsica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Calle Serrano 121, 28006 Madrid, Spain
²⁴ Microwave Laboratory, ETH Zurich, 8092 Zurich, Switzerland
²⁵ Chalmers University of Technology, 412 96 Göteborg, Sweden, Sweden
²⁶ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden

Received: 31 May 2010
Accepted: 7 July 2010

Abstract

Aims. To investigate the accretion and feedback processes in massive star formation, we analyze the shapes of emission lines from hot molecular cores, whose asymmetries trace infall and expansion motions.

Methods. The high-mass star forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in various lines of HCN and its isotopologues, complemented by APEX data. The observations are compared to spherically symmetric, centrally heated models with density power-law gradient and different velocity fields (infall or infall+expansion), using the radiative transfer code RATRAN.

Results. The HCN line profiles are asymmetric, with the emission peak shifting from blue to red with increasing J and decreasing line opacity (HCN to H¹³CN). This is most evident in the HCN 12–11 line at 1062 GHz. These line shapes are reproduced by a model whose velocity field changes from infall in the outer part to expansion in the inner part.

Conclusions. The qualitative reproduction of the HCN lines suggests that infall dominates in the colder, outer regions, but expansion dominates in the warmer, inner regions. We are thus witnessing the onset of feedback in massive star formation, starting to reverse the infall and finally disrupting the whole molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI were critically important.