Volume 644, December 2020
|Number of page(s)||18|
|Section||Stellar structure and evolution|
|Published online||01 December 2020|
ATOMIUM: A high-resolution view on the highly asymmetric wind of the AGB star π1Gruis
I. First detection of a new companion and its effect on the inner wind
Institute of Astronomy, KU Leuven, Celestijnenlaan 200D B2401, 3001 Leuven, Belgium
2 JBCA, Department Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
3 School of Chemistry, University of Leeds, Leeds LS2 9JT, 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’Héres, 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 Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, USA
Accepted: 9 October 2020
The nebular circumstellar environments of cool evolved stars are known to harbour a rich morphological complexity of gaseous structures on different length scales. A large part of these density structures are thought to be brought about by the interaction of the stellar wind with a close companion. The S-type asymptotic giant branch (AGB) star π1Gruis, which has a known companion at ∼440 au and is thought to harbour a second, closer-by (< 10 au) companion, was observed with the Atacama Large Millimeter/submillimeter Array as part of the ATOMIUM Large programme. In this work, the brightest CO, SiO, and HCN molecular line transitions are analysed. The continuum map shows two maxima, separated by 0.04″ (6 au). The CO data unambiguously reveal that π1Gru’s circumstellar environment harbours an inclined, radially outflowing, equatorial density enhancement. It contains a spiral structure at an angle of ∼38 ± 3° with the line-of-sight. The HCN emission in the inner wind reveals a clockwise spiral, with a dynamical crossing time of the spiral arms consistent with a companion at a distance of 0.04″ from the AGB star, which is in agreement with the position of the secondary continuum peak. The inner wind dynamics imply a large acceleration region, consistent with a beta-law power of ∼6. The CO emission suggests that the spiral is approximately Archimedean within 5″, beyond which this trend breaks down as the succession of the spiral arms becomes less periodic. The SiO emission at scales smaller than 0.5″ exhibits signatures of gas in rotation, which is found to fit the expected behaviour of gas in the wind-companion interaction zone. An investigation of SiO maser emission reveals what could be a stream of gas accelerating from the surface of the AGB star to the companion. Using these dynamics, we have tentatively derived an upper limit on the companion mass to be ∼1.1 M⊙.
Key words: line: profiles / stars: AGB and post-AGB / submillimeter: stars / circumstellar matter
© ESO 2020
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