Letter to the Editor

Water production in comet 81P/Wild 2 as determined by Herschel/HIFI ^*,**

¹ Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany e-mail: deval@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 Institut 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 STFC Rutherford Appleton Laboratory, Harwell Innovation Campus, Didcot, OX11 0QX, 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 Joint ALMA Observatory, Chile
¹⁶ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands
¹⁷ Leiden Observatory, University of Leiden, The Netherlands
¹⁸ Science Centre, European Space Astronomy Centre, European Space Agency, Spain
¹⁹ Department of Physics and Astronomy, University of Lethbridge, Canada
²⁰ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
²¹ Physikalisches Institut, University of Bern, Switzerland
²² N. Copernicus Astronomical Center, Rabianska 8, 87-100, Torun, Poland
²³ Istituto Fisica Spazio Interplanetario-INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
²⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 5 June 2010
Accepted: 1 July 2010

Abstract

The high spectral resolution and sensitivity of Herschel/HIFI allows for the detection of multiple rotational water lines and accurate determinations of water production rates in comets. In this Letter we present HIFI observations of the fundamental 1₁₀–1₀₁ (557 GHz) ortho and 1₁₁–₀₀ (1113 GHz) para rotational transitions of water in comet 81P/Wild 2 acquired in February 2010. We mapped the extent of the water line emission with five point scans. Line profiles are computed using excitation models which include excitation by collisions with electrons and neutrals and solar infrared radiation. We derive a mean water production rate of 1.0 × 10²⁸ molecules s^-1 at a heliocentric distance of 1.61 AU about 20 days before perihelion, in agreement with production rates measured from the ground using observations of the 18-cm OH lines. Furthermore, we constrain the electron density profile and gas kinetic temperature, and estimate the coma expansion velocity by fitting the water line shapes.