Letter to the Editor

A HIFI preview of warm molecular gas around χ Cygni: first detection of H₂O emission toward an S-type AGB star ^*

¹ Onsala Space Observatory, Chalmers University of Technology, Dept. Radio & Spece Science, 439 92 Onsala, Sweden e-mail: kay.justtanont@chalmers.se
² Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
⁴ University of Bonn, Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ European Southern Observatory, Karl Schwarzschild Str. 2, Garching bei München, Germany
⁶ Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
⁷ Observatorio Astronómico Nacional. Ap 112, 28803 Alcalá de Henares, Spain
⁸ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁹ Observatorio Astronómico Nacional (IGN), Alfonso XII N°3, 28014 Madrid, Spain
¹⁰ CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
¹² Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
¹³ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
¹⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁵ Johns Hopkins University, Baltimore, MD 21218, USA
¹⁶ Joint ALMA Observatory, El Golf 40, Las Condes, Santiago, Chile
¹⁷ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
¹⁸ Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile
¹⁹ Experimental Physics Dept., National University of Ireland Maynooth, Co. Kildare, Ireland
²⁰ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
²¹ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
²² Northrop Grumman Aerospace Systems, 1 Space Park, Redondo Beach, CA 90278, USA

Received: 31 May 2010
Accepted: 9 June 2010

Abstract

Aims. A set of new, sensitive, and spectrally resolved, sub-millimeter line observations are used to probe the warm circumstellar gas around the S-type AGB star χ Cyg. The observed lines involve high rotational quantum numbers, which, combined with previously obtained lower-frequency data, make it possible to study in detail the chemical and physical properties of, essentially, the entire circumstellar envelope of χ Cyg.

Methods. The data were obtained using the HIFI instrument aboard Herschel, whose high spectral resolution provides valuable information about the line profiles. Detailed, non-LTE, radiative transfer modelling, including dust radiative transfer coupled with a dynamical model, has been performed to derive the temperature, density, and velocity structure of the circumstellar envelope.

Results. We report the first detection of circumstellar H₂O rotational emission lines in an S-star. Using the high-J CO lines to derive the parameters for the circumstellar envelope, we modelled both the ortho- and para-H₂O lines. Our modelling results are consistent with the velocity structure expected for a dust-driven wind. The derived total H₂O abundance (relative to H₂) is (1.1±0.2) × 10^-5, much lower than that in O-rich stars. The derived ortho-to-para ratio of 2.1±0.6 is close to the high-temperature equilibrium limit, consistent with H₂O being formed in the photosphere.