ATOMIUM: Continuum emission and evidence of dust enhancement from binary motion

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
² School of Physics & Astronomy, Monash University, Wellington Road, Clayton, 3800 Victoria, Australia
³ JBCA, Department Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁴ Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, 33615 Pessac, France
⁵ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92195 Meudon CEDEX, France
⁶ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and Universidad de Chile, Casilla 36-D, Santiago, Chile
⁷ Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Department of Mathematics, Kiel University, Heinrich-Hecht-Platz 6, 24118 Kiel, Germany
¹⁰ School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
¹¹ University of Amsterdam, Anton Pannekoek Institute for Astronomy, 1090 GE Amsterdam, The Netherlands
¹² Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles (ULB), CP 226, 1060 Brussels, Belgium
¹³ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
¹⁴ Departamento de Física, Universidad de Santiago de Chile, Av. Victor Jara, 3659 Santiago, Chile
¹⁵ Center for Interdisciplinary Research in Astrophysics and Space Exploration (CIRAS), USACH, Santiago, Chile
¹⁶ National Astronomical Research Institute of Thailand, Chiangmai 50180, Thailand
¹⁷ Instituto de Física Fundamental, CSIC, C/Serrano123, 28006 Madrid, Spain
¹⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁹ Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Swede
²⁰ Université Côte d’Azur, Laboratoire Lagrange, Observatoire de la Côte d’Azur, F-06304 Nice Cedex 4, France
²¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
²² Universität zu Köln, Astrophysik/I. Physikalisches Institut, 50937 Köln, Germany
²³ California Institute of Technology, Jet Propulsion Laboratory, Pasadena, CA 91109, USA
²⁴ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
²⁵ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516 SE-751 20 Uppsala, Sweden
²⁶ University College London, Department of Physics and Astronomy, London WC1E 6BT, UK
²⁷ School of Mathematical and Physical Sciences, Macquarie University, Sydney, New South Wales, Australia

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Received: 31 March 2025
Accepted: 7 October 2025

Abstract

Context. Low- and intermediate-mass stars on the asymptotic giant branch (AGB) account for a significant portion of the dust and chemical enrichment in their host galaxy. Understanding the dust formation process of these stars and their more massive counterparts, the red supergiants, is essential for quantifying galactic chemical evolution.

Aims. To improve our understanding of the dust nucleation and growth process, we aim to better constrain stellar properties at millimetre wavelengths. To characterise how this process varies with the mass-loss rate and pulsation period, we studied a sample of oxygen-rich and S-type evolved stars.

Methods. Here we present ALMA observations of the continuum emission around a sample of 17 stars from the ATOMIUM survey. We analysed the stellar parameters at 1.24 mm and the dust distributions at high angular resolutions.

Results. From our analysis of the stellar contributions to the continuum flux, we find that the semi-regular variables all have smaller physical radii and fainter monochromatic luminosities than the Mira variables. Comparing these properties with pulsation periods, we find a positive trend between the stellar radius and period only for the Mira variables with periods of more than 300 days, and we find and a positive trend between the period and the monochromatic luminosity only for the red supergiants and the most extreme AGB stars with periods of more than 500 days. We find that the continuum emission at 1.24 mm can be classified into four groups; (i) ‘featureless’ continuum emission is confined to the (unresolved) regions close to the star for five stars in our sample, (ii) relatively uniform extended flux is seen for four stars, (iii) tentative elongated features are seen for three stars, and (iv) the remaining five stars have unique or unusual morphological features in their continuum maps. These features can be explained by the fact that 10 of the 14 AGB stars in our sample have binary companions.

Conclusions. Based on our results, we conclude that there are two modes of dust formation: well-established pulsation-enhanced dust formation and our newly proposed companion-enhanced dust formation. If the companion is located close to the AGB star, in the wind acceleration region, then additional dust formed in the wake of the companion can increase the amount of mass lost through the dust-driven wind. This explains the different dust morphologies seen around our stars and partly accounts for the large scatter in literature mass-loss rates, especially among semi-regular stars with small pulsation periods.