Circumstellar water vapour in M-type AGB stars: constraints from H<font size=-1>₂</font>O(1<font size=-1>₁₀</font> –1<font size=-1>₀₁</font>) lines obtained with Odin

Abstract

Context. A detailed radiative transfer code has been previously used to model circumstellar ortho-H2O line emission towards six M-type asymptotic giant branch stars using Infrared Space Observatory Long Wavelength Spectrometer data. Collisional and radiative excitation, including the v 2 = 1 state, was considered. Aims. Spectrally resolved circumstellar H2O(110-101) lines have been obtained towards three M-type AGB stars using the Odin satellite. This provides additional strong constraints on the properties of circumstellar H2O, in particular on the chemistry in the stellar atmosphere, and the photodissociation in the outer envelope. Methods. Infrared Space Observatory and Odin satellite H2O line data are used as constraints for radiative transfer models. Special consideration is taken to the spectrally resolved. Odin line profiles, and the effect of excitation to the first excited vibrational states of the stretching modes (v1 = 1 and v3 = 1) on the derived abundances is estimated. A non-local, radiative transfer code based on the accelerated lambda iteration formalism is used. A statistical analysis is performed to determine the best-fit models. Results. The H2O abundance estimates are in agreement with previous estimates. The inclusion of the Odin data sets stronger constraints on the size of the H2O envelope. The H 2O(110-101) line profiles require a significant reduction in expansion velocity compared to the terminal gas expansion velocity determined in models of CO radio line emission, indicating that the H 2O emission lines probe a region where the wind is still being accelerated. Including the v3 = 1 state significantly lowers the estimated abundances for the low-mass-loss-rate objects. This shows the importance of detailed modelling, in particular the details of the infrared spectrum in the range 3 to 6 μm, to estimate accurate circumstellar H 2O abundances. Conclusions. Spectrally resolved circumstellar H 2O emission lines are important probes of the physics and chemistry in the inner regions of circumstellar envelopes around asymptotic giant branch stars. Predictions for H2O emission lines in the spectral range of the upcoming Herschel/HIFI mission indicate that these observations will be very important in this context. © ESO 2009.