The wind of W Hya as seen by Herschel. II. The molecular envelope of W Hya

Abstract

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 ${}^{12}$CO molecule. Here we broaden this study by modelling an extensive set of H${}_2$O and ${}^{28}$SiO lines observed by the three instruments on board Herschel using a state-of-the-art molecular excitation and radiative transfer code. The oxygen isotopic ratios and the ${}^{28}$SiO 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. The modelling of H${}_2$O and ${}^{28}$SiO confirms the properties of the envelope model of W Hya derived from ${}^{12}$CO lines. We find an H${}_2$O ortho-to-para ratio of 2.5\,${}_{-1.0}^{+2.5}$, consistent with what is expected for an AGB wind. The O${}^{16}$/O${}^{17}$ ratio indicates that W Hya has an initial mass of about 1.5 M${}_{\odot }$. Although the ortho- and para-H${}_2$O lines observed by HIFI appear to trace gas of slightly different physical properties, a turbulence velocity of $0.7\pm 0.1$ km s${}^{-1}$ fits the HIFI lines of both spin isomers and those of ${}^{28}$SiO well. The ortho- and para-H${}_2^{16}$O and ${}^{28}$SiO abundances relative to H${}_2$ are $(6_{-2}^{+3})\times 10^{-4}$, $(3_{-1}^{+2})\times 10^{-4}$, and $(3.3\pm 0.8)\times 10^{-5}$, respectively. Assuming a solar silicon-to-carbon ratio, the ${}^{28}$SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles.

References in corpus (7)

• Radiative transfer in very optically thick circumstellar disks
• Too little radiation pressure on dust in the winds of oxygen-rich AGB stars
• A close halo of large transparent grains around extreme red giant stars
• Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2
• The wind of W Hya as seen by Herschel - I. The CO envelope
• Circumstellar water vapour in M-type AGB stars: Constraints from H2O(1_10 - 1_01) lines obtained with Odin
• H2O vapor excitation in dusty AGB envelopes. A PACS view of OH 127.8+0.0

Cited by in corpus (9)

• Carbon and oxygen isotopic ratios for nearby miras
• Resolving the extended stellar atmospheres of Asymptotic Giant Branch stars at (sub-)millimetre wavelengths
• An ALMA view of CS and SiS around oxygen-rich AGB stars
• AGB winds in interacting binary stars
• Dust-gas chemistry in AGB outflows: Chemical modelling of dust-gas chemistry within AGB outflows I. Effect on the gas-phase chemistry
• Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss
• The mass-loss rates of red supergiants at low metallicity: Detection of rotational CO emission from two red supergiants in the Large Magellanic Cloud
• Mapping circumstellar magnetic fields of late-type evolved stars with the Goldreich-Kylafis effect: CARMA observations at $\lambda 1.3$ mm of R Crt and R Leo
• Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae