Letter to the Editor

HIFI observations of warm gas in DR21: Shock versus radiative heating^*

¹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
² SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
³ Laboratoire AIM, CEA/DSM – INSU/CNRS – Université Paris Diderot, IRFU/SAp CEA-Saclay, 91191 Gif-sur-Yvette, France
⁴ LERMA & UMR 8112 du CNRS, Observatoire de Paris and École Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 05, France
⁵ Institute for Astronomy, ETH Zürich, 8093 Zürich, Switzerland
⁶ Leiden Observatory, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
⁸ Jet Propulsion Laboratory, 4800 Oak Grove Drive, MC 302-231, Pasadena, CA 91109, USA
⁹ Université de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
¹⁰ CNRS, UMR 5187, 31028 Toulouse, France
¹¹ Observatorio Astronómico Nacional (IGN), Centro Astronómico de Yebes, Apartado 148, 19080 Guadalajara, Spain
¹² Department of Astronomy and Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹³ Observatorio Astronómico Nacional (OAN), Apdo. 112, 28803 Alcalá de Henares (Madrid), Spain
¹⁴ Centro de Astrobiología, CSIC-INTA, 28850 Madrid, Spain
¹⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁶ Astronomy Department, University of Maryland, College Park, MD 20742, USA
¹⁷ Experimental Physics Dept., National University of Ireland: Maynooth, Co. Kildare, Ireland
¹⁸ MPI für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
¹⁹ Instituto de Radio Astronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
²⁰ IPAC/Caltech, MS 100-22, Pasadena, CA 91125, USA
²¹ Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Mumbai 400005, India
²² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
²³ California Institute of Technology, 320-47, Pasadena, CA 91125-4700, USA
²⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
²⁵ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, Madrid 28080, Spain

Received: 30 March 2010
Accepted: 20 April 2010

Abstract

Context. The molecular gas in the DR21 massive star formation region is known to be affected by the strong UV field from the central star cluster and by a fast outflow creating a bright shock. The relative contribution of both heating mechanisms is the matter of a long debate.

Aims. By better sampling the excitation ladder of various tracers we provide a quantitative distinction between the different heating mechanisms.

Methods. HIFI observations of mid-J transitions of CO and HCO⁺ isotopes allow us to bridge the gap in excitation energies between observations from the ground, characterizing the cooler gas, and existing ISO LWS spectra, constraining the properties of the hot gas. Comparing the detailed line profiles allows to identify the physical structure of the different components.

Results. In spite of the known shock-excitation of H₂ and the clearly visible strong outflow, we find that the emission of all lines up to 2 THz can be explained by purely radiative heating of the material. However, the new Herschel/HIFI observations reveal two types of excitation conditions. We find hot and dense clumps close to the central cluster, probably dynamically affected by the outflow, and a more widespread distribution of cooler, but nevertheless dense, molecular clumps.