Atomium: A high-resolution view on the highly asymmetric wind of the AGB star Pi1 Gruis. I. First detection of a new companion and its effect on the inner wind

Ward Homan, Miguel Montarges, Bannawit Pimpanuwat, Anita M. S. Richards, Sofia H. J. Wallstrom, Pierre Kervella, Leen Decin, Albert Zijlstra, Taissa Danilovich, Alex de Koter, Karl Menten, Raghvendra Sahai, John Plane, Kelvin Lee, Rens Waters, Alain Baudry, Ka Tat Wong, Tom J. Millar, Marie Van de Sande, Eric LagadecDavid Gobrecht, Jeremy Yates, Daniel Price, Emily Cannon, Jan Bolte, Frederik De Ceuster, Fabrice Herpin, Joe Nuth, Jan Philip Sindel, Dylan Kee, Malcolm D. Grey, Sandra Etoka, Manali Jeste, Carl A. Gottlieb, Elaine Gottlieb, Iain McDonald, Ileyk El Mellah, Holger S. P. Muller

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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 0.0400 (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 0.0400 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 500, beyond which this trend breaks down as the succession of the spiral arms becomes less periodic. The SiO emission at scales smaller than 0.500 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⊙.

Original languageEnglish
Article numberA61
Publication statusPublished - 1 Dec 2020


  • Circumstellar matter
  • Line: Profiles
  • Stars: AGB and post-AGB
  • Submillimeter: Stars


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