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The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA

Nature Astronomy volume 1, pages 848–853 (2017)Cite this article

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Abstract

Our current understanding of the chemistry and mass-loss processes in Sun-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere1. Three-dimensional hydrodynamical stellar atmosphere models provide observational predictions2, but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimetre continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hydrae. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time longer than commonly assumed. A shocked hot layer will be an important ingredient in current models of stellar convection, pulsation and chemistry at the late stages of stellar evolution.

Asymptotic giant branch (AGB) stars are among the most important sources of enrichment of the galactic interstellar medium (ISM). Molecules and dust formed in the warm extended atmospheres and the cool and dense circumstellar envelopes around AGB stars are injected into the ISM by a stellar wind that has overcome stellar gravity1. It is generally assumed that the stellar wind is driven by radiation pressure on dust that forms at a few stellar radii, where the temperature in the circumstellar envelope has dropped, so that dust condensation can occur3. For the gas in the extended stellar atmosphere to reach the dust formation region, the most recent AGB mass-loss models typically invoke stellar pulsations and convective motions2,4,5,6.

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Fig. 1: Brightness temperature map of the surface of the AGB star W Hya as observed with ALMA at 338 GHz.
Fig. 2: The residual map after subtracting the fitted uniform elliptical disk from the continuum image of W Hya.
Fig. 3: Spectra of the CO J = 3→2, v = 1 transition.

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Acknowledgements

Support for this work was provided by the European Research Council (ERC) through the ERC consolidator grant number 614264, and by the Swedish Research Council (VR). E.D.B. further acknowledges support from the Swedish National Space Board. We are also indebted to the staff of the Nordic ALMA regional centre node, and in particular I. Martí-Vidal, for developments of the tools used in the data analysis and plotting. ALMA is a partnership of the European Southern Observatory (ESO, representing its member states), the National Science Foundation (United States) and the National Institutes of Natural Sciences (NINS, Japan), together with the National Research Council (Canada), the National Science Council (Taiwan), the NINS in collaboration with the Academia Sinica (Taiwan) and the Korea Astronomy and Space Science Institute (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities Inc./National Radio Astronomy Observatory and the National Astronomical Observatory of Japan.

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Authors and Affiliations

  1. Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92, Onsala, Sweden

    Wouter Vlemmings, Theo Khouri, Elvire De Beck, Boy Lankhaar, Matthias Maercker, Hans Olofsson & Daniel Tafoya

  2. Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland

    Eamon O’Gorman

  3. ESO Karl-Schwarzschild-Strasse 2, 85748, Garching, Germany

    Elizabeth Humphreys

  4. Department of Physics and Astronomy, Uppsala University, Box 516, 751 20, Uppsala, Sweden

    Sofia Ramstedt

  5. The Hakubi Center for Advanced Research/Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo, Kyoto, 606-8502, Japan

    Aki Takigawa

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  1. Wouter Vlemmings
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Contributions

W.V. reduced and analysed the data and wrote most of the manuscript. T.K. performed the radiative-transfer modelling and wrote the modelling section of the Methods. E.D.B. and B.L. analysed the unidentified line and provided the relevant text. A.T. obtained the ALMA data. All authors contributed with comments on the manuscript and data interpretation.

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Correspondence to Wouter Vlemmings.

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Vlemmings, W., Khouri, T., O’Gorman, E. et al. The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA. Nat Astron 1, 848–853 (2017). https://doi.org/10.1038/s41550-017-0288-9

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