W Hya through the eye of Odin

Satellite observations of circumstellar submillimetre HO line emission

¹ Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: kay@astro.su.se
² Onsala Space Observatory, 439 92 Onsala, Sweden
³ Swedish Space Corporation, PO Box 4207, 171 04 Solna, Sweden
⁴ Dept. of Physics and Astronomy, University of Calgary, 500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan, ROC
⁶ Gemini Observatory, Southern Operation Center, La Serena, Chile
⁷ Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA

Received: 2 March 2005
Accepted: 4 May 2005

Abstract

We present Odin observations of the AGB star W Hya in the ground-state transition of ortho-H₂O, 1, at 557 GHz. The line is clearly of circumstellar origin. Radiative transfer modelling of the water lines observed by Odin and ISO results in a mass-loss rate of ( yr^-1, and a circumstellar H₂O abundance of (. The inferred mass-loss rate is consistent with that obtained from modelling the circumstellar CO radio line emission, and also with that obtained from the dust emission modelling combined with a dynamical model for the outflow. The very high water abundance, higher than the cosmic oxygen abundance, can be explained by invoking an injection of excess water from evaporating icy bodies in the system. The required extra mass of water is quite small, on the order of ~0.1 .