2014A&A...570A..67K

The evolution of low- and intermediate-mass stars on the asymptotic giant branch (AGB) is mainly controlled by the rate at which these stars lose mass in a stellar wind. Understanding the driving mechanism and strength of the stellar winds of AGB stars and the processes enriching their surfaces with products of nucleosynthesis are paramount to constraining AGB evolution and predicting the chemical evolution of galaxies. In a previous paper we have constrained the structure of the outflowing envelope of W Hya using spectral lines of the ¹²CO molecule. Here we broaden this study by including an extensive set of H₂O and ²⁸SiO lines. It is the first time such a comprehensive study is performed for this source. The oxygen isotopic ratios and the ²⁸SiO abundance profile can be connected to the initial stellar mass and to crucial aspects of dust formation at the base of the stellar wind, respectively. We model the molecular emission observed by the three instruments on board Herschel Space Observatory using a state-of-the-art molecular excitation and radiative transfer code. We also account for the dust radiation field in our calculations. We find an H₂O ortho-to-para ratio of 2.5^+2.5_–1.0, consistent with what is expected for an AGB wind. The O¹⁶/O¹⁷ ratio indicates that W Hya has an initial mass of about 1.5M_☉. Although the ortho- and para-H₂O lines observed by HIFI appear to trace gas of slightly different physical properties, we find that a turbulence velocity of 0.7±0.1km/s fits the HIFI lines of both spin isomers and those of ²⁸SiO well. The modelling of H₂O and ²⁸SiO confirms the properties of the envelope model of W Hya, as derived from ¹²CO lines, and allows us to constrain the turbulence velocity. The ortho- and para-H₂¹⁶O and ²⁸SiO abundances relative to H₂ are (6⁺³_–2)x10^–4, (3⁺²_–1)x10^–4, and (3.3±0.8)x10^–5, respectively, in agreement with expectations for oxygen-rich AGB outflows. Assuming a solar silicon-to-carbon ratio, the ²⁸SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles.