W Hya through the eye of Odin Satellite observations of circumstellar submillimetre H$_2$O line emission

TL;DR

This study uses Odin satellite observations to analyze water emission from the AGB star W Hya, revealing a high water abundance likely due to evaporating icy bodies, and estimates the star's mass-loss rate.

Contribution

First detailed measurement of circumstellar water emission from W Hya using Odin, linking high water abundance to icy body evaporation.

Findings

Mass-loss rate of (2.5 +/- 0.5)E-7 Msol/yr

Circumstellar H2O abundance of (2.0 +/- 1.0)E-3

High water abundance explained by icy body evaporation

Abstract

We present Odin observations of the AGB star W Hya in the ground-state transition of ortho-H2O, 1(10)-1(01), at 557GHz. 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 (2.5 +/- 0.5)E-7 Msol/yr, and a circumstellar H2O abundance of (2.0 +/- 1.0)E-3. 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 earth mass.