Letter to the Editor

HIFI observations of water in the atmosphere of comet C/2008 Q3 (Garradd) ^*,**

¹ Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany e-mail: hartogh@mps.mpg.de
² LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
³ California Institute of Technology, Pasadena, CA 91125, USA
⁴ Space Research Centre, Polish Academy of Sciences, Warsaw, Poland
⁵ DLR, German Aerospace Centre, Bonn-Oberkassel, Germany
⁶ Institute d'Astrophysique et de Geophysique, Université de Liège, Belgium
⁷ Rosetta Science Operations Centre, European Space Astronomy Centre, European Space Agency, Spain
⁸ Instituto de Astrofísica de Andalucía (CSIC), Spain
⁹ Space Science & Technology Department, Rutherford Appleton Laboratory, UK
¹⁰ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Belgium
¹¹ Astronomy Department, University of Michigan, USA
¹² Laboratory of Molecular Astrophysics, CAB-CSIC, INTA, Spain
¹³ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands

¹⁴ LERMA, Observatoire de Paris, France

¹⁵ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands

¹⁶ Leiden Observatory, University of Leiden, The Netherlands

¹⁷ Joint ALMA Observatory, Chile

¹⁸ Herschel Science Centre, European Space Astronomy Centre, European Space Agency, Spain

¹⁹ Department of Physics and Astronomy, University of Lethbridge, Canada

²⁰ 1st Physics Institute, University of Cologne, Germany

²¹ Physikalisches Institut, University of Bern, Switzerland

²² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

²³ Chalmers University of Technology, 412 96 Göteborg, Sweden

²⁴ Institute of Astronomy, ETH Zürich, 8093 Zürich, Switzerland


Received: 31 March 2010
Accepted: 13 May 2010

Abstract

High-resolution far-infrared and sub-millimetre spectroscopy of water lines is an important tool to understand the physical and chemical properties of cometary atmospheres. We present observations of several rotational ortho- and para-water transitions in comet C/2008 Q3 (Garradd) performed with HIFI on Herschel. These observations have provided the first detection of the 2₁₂–1₀₁ (1669 GHz) ortho and 1₁₁–0₀₀ (1113 GHz) para transitions of water in a cometary spectrum. In addition, the ground-state transition 1₁₀–1₀₁ at 557 GHz is detected and mapped. By detecting several water lines quasi-simultaneously and mapping their emission we can constrain the excitation parameters in the coma. Synthetic line profiles are computed using excitation models which include excitation by collisions, solar infrared radiation, and radiation trapping. We obtain the gas kinetic temperature, constrain the electron density profile, and estimate the coma expansion velocity by analyzing the map and line shapes. We derive water production rates of 1.7–2.8 × 10²⁸ s^-1 over the range r_h = 1.83–1.85 AU.