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Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

Nature volume 617, pages 696–700 (2023)Cite this article

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Abstract

During their thermally pulsing phase, asymptotic giant branch (AGB) stars eject material that forms extended dusty envelopes1. Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars2,3,4,5,6. Inhomogeneous molecular gas has also been observed in multiple emission lines within several stellar radii of different oxygen-rich stars, including W Hya and Mira7,8,9,10. At the stellar surface level, infrared images have shown intricate structures around the carbon semiregular variable R Scl and in the S-type star π1 Gru11,12. Infrared images have also shown clumpy dust structures within a few stellar radii of the prototypical carbon AGB star IRC+10°216 (refs. 13,14), and studies of molecular gas distribution beyond the dust formation zone have also shown complex circumstellar structures15. Because of the lack of sufficient spatial resolution, however, the distribution of molecular gas in the stellar atmosphere and the dust formation zone of AGB carbon stars is not known, nor is how it is subsequently expelled. Here we report observations with a resolution of one stellar radius of the recently formed dust and molecular gas in the atmosphere of IRC+10°216. Lines of HCN, SiS and SiC2 appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse16. The convective cells coalesce with pulsation, causing anisotropies that, together with companions17,18, shape its circumstellar envelope.

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Fig. 1: Continuum emission of IRC+10°216 at 1 mm.
Fig. 2: Velocity-integrated maps of different emission lines.
Fig. 3: Velocity-integrated emission maps of the circumstellar environment of IRC+10°216 for different molecules.
Fig. 4: Analysis of intensity ratio of emission lines HCN ν3 = 1, J = 3–2 and SiS v = 3, J = 14–13.

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Data availability

All the observational products used here are public and available through the ALMA, SMA and IRAM-30m archives:https://almascience.nrao.edu/aq/https://lweb.cfa.harvard.edu/cgi-bin/sma/smaarch.plhttps://iram-institute.org/science-portal/data-archive/.

Code availability

The 2012 version of the MADEX code, including spectroscopic information for approximately 3,500 species, is available as an executable in https://nanocosmos.iff.csic.es/madex/. The chemical equilibrium and evolving envelope models are available on request. Enquiries about codes can be addressed to the corresponding authors and to the developers of the codes: M.A. for the chemical code, J.C. for the MADEX code and J.C. and J.P.F. for models of the evolving envelope.

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Acknowledgements

This publication is part of project ‘I+D+i’ (research, development and innovation, nos. PID2020-117034RJ-I00, PID2019-107115GB-C21, PID2019-106110GB-I00 and PID2019-105203GB-C22) supported by the Spanish Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033). This work was also supported by the European Research Council under Synergy Grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). This work is based on observations carried out with the IRAM, SMA and ALMA telescopes. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). SMA is a joint project between the Smithsonian Astrophysical Observatory (USA) and the Academia Sinica Institute of Astronomy and Astrophysics (Taiwan) and is funded by the Smithsonian Institution and Academia Sinica. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.01215.S, ADS/JAO.ALMA#2013.1.00432.S and ADS/JAO.ALMA#2018.1.01485. ALMA is a partnership that includes ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We made use of CASA software (https://casa.nrao.edu/). The GILDAS package was used to reduce and analyse data (https://www.iram.fr/IRAMFR/GILDAS). We acknowledge the GILDAS software team, in particular S. Bardeau, for their help and assistance. We also thank IRAM staff, who provided relevant information for flux calibration from the Northern Extended Millimetre Array observatory database.

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Authors and Affiliations

  1. Department of Molecular Astrophysics, Instituto de Física Fundamental, Madrid, Spain

    L. Velilla-Prieto, M. Agúndez, G. Quintana-Lacaci & J. Cernicharo

  2. Centro de Astrobiología, CSIC-INTA, Villanueva de la Cañada, Spain

    J. P. Fonfría

  3. Institut de Radioastronomie Millimétrique, Saint Martin d’Hères, France

    A. Castro-Carrizo & M. Guélin

  4. Departement Physik, Universität Basel, Basel, Switzerland

    I. Cherchneff

  5. Institut de Recherche en Astrophysique et Planétologie, Université Toulouse 3 – Paul Sabatier, CNRS, CNES, Toulouse, France

    C. Joblin

  6. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA

    M. C. McCarthy

  7. Group of Structure of Nanoscopic Systems, Instituto de Ciencia de Materiales de Madrid, Cantoblanco, Spain

    J. A. Martín-Gago

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Contributions

L.V.-P. led this publication, analysed and interpreted data, wrote the article and created all the figures except for the Extended data Fig. 3 and Extended data Fig. 4. J.P.F. delivered the method used to produce input data for the radiative transfer models from shock model output, and developed and produced the models of the evolving envelope. M.A. created the chemical code and associated images (Extended data Fig. 3 and Extended data Fig. 4). A.C.-C. calibrated and reduced the ALMA data. M.G. contributed subtantially to revision of the literature and detailed revision of the text. I.C. provided the input abundances that were used to produce the shock models. J.C. developed the MADEX and evolving envelope codes, led the ALMA proposal and contributed to the overall analysis of this work. All authors contributed to the conception, design and writing of the ALMA proposal and to discussion and revisions of the article.

Corresponding authors

Correspondence to L. Velilla-Prieto or J. Cernicharo.

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Extended data figures and tables

Extended Data Fig. 1 Velocity-integrated emission maps of SiS isotopologues in different vibrationally excited states.

The velocity-integrated emission maps are shown in colour scale. Note that the intensity scale varies within boxes for improved visualisation. The grey contours correspond to 10, 30, 50, 100, 200, 300, 400, 500, and 600 σ of the r.m.s. of the noise of the continuum emission, which is equal to 92 μJy beam−1. The cyan dashed contour represents the radio photosphere22. The shape of the synthetic beam is shown in the bottom-left corner of each box. North is up, and East to he left.

Extended Data Fig. 2 Velocity-integrated emission maps of HCN isotopologues in different vibrationally excited states.

The colour scale, contours, beam, and orientation are defined as in Extended Data Fig. 1.

Extended Data Fig. 3 Results from the chemical equilibrium models for C2, SiC2, HCN, and SiS.

The colour scale map represents the logarithm of the mole fraction of the given molecule as a function of the kinetic temperature and the logarithm of the pressure of the gas.

Extended Data Fig. 4 Fractional abundance profiles.

The abundance profiles are shown as a function of temperature for a fixed pressure of 5  ×  10−5 bar, which should be close to the gas pressure at the photosphere of IRC+10°216 ref. 49.

Extended Data Fig. 5 Predicted line profiles of the J = 14–13 SiS line in different vibrational states as a function of the pulsation phase.

a–b: density and temperature profiles, where oscillating lines represent the phases ϕ = 0 (black), ϕ = 0.25 (red), ϕ = 0.5 (green), and ϕ = 0.75 (blue). An additional model with no stellar pulsation using density and temperature profiles that falls as r−2 and r−0.7, respectively, is shown as a pink (non-oscillating) solid line. c–f: Emergent line profiles where the coloured lines, which follow the colour code mentioned before, represent the predictions for an angular resolution of \(0\,.\,\,{\rm{{\prime\prime} }}\,2\). g–h: Velocity profile of the gas following also this colour code.

Extended Data Fig. 6 Line profiles of the ν = 2 J = 14–13 SiS line as observed at different locations in the plane of the sky.

The lines have been obtained by averaging the spectra in an area of 10 mas surrounding each selected position. The different numbered offset positions are shown and numbered in the total intensity map to the right. Each corresponding position number is shown in red at the bottom-left corner of the boxes displaying the plots of the spectra.

Extended Data Fig. 7 Line profiles of the HCN 3\({{\boldsymbol{\nu }}}_{{\bf{2}}}^{{\bf{e}}}\)J = 3–2 line as observed at different locations in the plane of the sky.

Details of this figure are defined as in Extended Data Fig. 6.

Extended Data Fig. 8 Spectra in velocity units at different offsets from the source of the SiS ν = 2 J = 14–13 line.

The velocity range shown for each spectrum is ±  15 km s−1 with respect to the systemic velocity of the source, from negative (left) to positive (right). The intensity scale, in Jy beam−1, ranges between -0.03 and 0.06. The red circle represents the stellar disk in the IR (radius equal to 19 mas)33, while the blue circle correspond to the radio photosphere (radius equal to 40.5 mas)22.

Extended Data Fig. 9 Spectra in velocity units at different offsets from the source of the HCN 3\({{\boldsymbol{\nu }}}_{{\bf{2}}}^{{\bf{e}}}\) line.

Details of this figure are defined as in Extended Data Fig. 8.

Extended Data Fig. 10 Results from the growing envelope model.

Three cases are presented: a) control case of an isolated AGB star without formation of blobs, b) AGB steady wind in a binary system with an inclination of 60° separated by 25 AU, and c) as in b but for a closer companion located at 10 AU and including the randomly generated blobs at the AGB stellar photosphere. The maps are in normalised intensity units to their respective maximum intensity.

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Velilla-Prieto, L., Fonfría, J.P., Agúndez, M. et al. Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars. Nature 617, 696–700 (2023). https://doi.org/10.1038/s41586-023-05917-9

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