ATOMIUM: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae

I. Morpho-kinematical interpretation of CO and SiO emission

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles (ULB), CP 226 – 1060 Brussels, Belgium
e-mail: ward.homan@ulb.be
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D B2401, 3001 Leuven, Belgium
³ JBCA, Department Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁴ LESIA (CNRS UMR 8109), Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, Paris, France
⁵ Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, PO Box 94249 1090 GE Amsterdam, The Netherlands
⁶ Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
⁷ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁸ Radio and Geoastronomy Division, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
⁹ Laboratoire d’astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹⁰ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
¹¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
¹² Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
¹³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁴ School of Physics & Astronomy, Monash University, Clayton, Vic 3800, Australia
¹⁵ Solar System Exploration Division, Code 690 NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁷ Center for Mathematical Plasma Astrophysics, Celestijnenlaan 200B, 3001 Leuven, Belgium
¹⁸ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹⁹ National Astronomical Research Institute of Thailand, 260 Moo 4, T. Donkaew, A. Maerim, Chiangmai 50180, Thailand
²⁰ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
²¹ School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
²² Centre for Exoplanet Science, University of St Andrews, North Haugh, St Andrews KY169SS, UK
²³ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, 38058 Grenoble Cedex 9, France

Received: 8 February 2021
Accepted: 26 March 2021

Abstract

Evolved low- to intermediate-mass stars are known to shed their gaseous envelope into a large, dusty, molecule-rich circumstellar nebula which typically develops a high degree of structural complexity. Most of the large-scale, spatially correlated structures in the nebula are thought to originate from the interaction of the stellar wind with a companion. As part of the ATOMIUM large programme, we observed the M-type asymptotic giant branch (AGB) star R Hydrae with the Atacama Large Millimeter/submillimeter Array. The morphology of the inner wind of R Hya, which has a known companion at ∼3500 au, was determined from maps of CO and SiO obtained at high angular resolution. A map of the CO emission reveals a multi-layered structure consisting of a large elliptical feature at an angular scale of ∼10″ that is oriented along the north–south axis. The wind morphology within the elliptical feature is dominated by two hollow bubbles. The bubbles are on opposite sides of the AGB star and lie along an axis with a position angle of ∼115°. Both bubbles are offset from the central star, and their appearance in the SiO channel maps indicates that they might be shock waves travelling through the AGB wind. An estimate of the dynamical age of the bubbles yields an age of the order of 100 yr, which is in agreement with the previously proposed elapsed time since the star last underwent a thermal pulse. When the CO and SiO emission is examined on subarcsecond angular scales, there is evidence for an inclined, differentially rotating equatorial density enhancement, strongly suggesting the presence of a second nearby companion. The position angle of the major axis of this disc is ∼70° in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is ∼0.65 M_⊙ on the basis of the highest measured speeds in the disc and the location of its inner rim at ∼6 au from the AGB star.