Letter to the Editor

Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI ^*,**

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: Leen.Decin@ster.kuleuven.be
² Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
³ Onsala Space observatory, Chalmers University of Technology, Dept. Radio & Spece Science, 439 92 Onsala, Sweden
⁴ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
⁵ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁶ Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden
⁷ Observatorio Astronómico Nacional. Ap 112, 28803 Alcalá de Henares, Spain e-mail: v.bujarrabal@oan.es
⁸ Observatorio Astronómico Nacional (IGN), Alfonso XII No 3, 28014 Madrid, Spain
⁹ Laboratory of Molecular Astrophysics, CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹¹ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ John Hopkins University, Baltimore, MD 21218, USA
¹⁴ Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
¹⁵ Joint ALMA Observatory, El Golf 40, Las Condes, Santiago, Chile
¹⁶ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
¹⁷ Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile
¹⁸ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁹ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
²⁰ Observatorio Astronómico Nacional (IGN), Centro Astronómico de Yebes, Apartado 148. 19080 Guadalajara, Spain
²¹ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
²² CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France

Received: 28 May 2010
Accepted: 16 June 2010

Abstract

During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI spectrometer onboard Herschel. We report on the first detection of HO and the rarer isotopologues HO and HO in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 × 10^-5, and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H₂O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of ¹²CO, ¹³CO, ²⁸SiO, ²⁹SiO, ³⁰SiO, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.