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The nebular circumstellar environments of cool evolved stars are known toharbour a rich morphological complexity of gaseous structures on differentlength scales. A large part of these density structures are thought to bebrought about by the interaction of the stellar wind with a close companion.The S-type asymptotic giant branch star Pi1 Gruis, which has a known companionat ~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 ofthe ATOMIUM Large programme. In this work, the brightest CO, SiO, and HCNmolecular line transitions are analysed. The continuum map shows two maxima,separated by 0.04'' (6 au). The CO data unambiguously reveal that Pi1 Gru'scircumstellar environment harbours an inclined, radially outflowing, equatorialdensity enhancement. It contains a spiral structure at an angle of 38+/-3 degwith the line-of-sight. The HCN emission in the inner wind reveals a clockwisespiral, with a dynamical crossing time of the spiral arms consistent with acompanion at a distance of 0.04'' from the asymptotic giant branch star, whichis in agreement with the position of the secondary continuum peak. The innerwind dynamics imply a large acceleration region, consistent with a beta-lawpower of ~6. The CO emission suggests that the spiral is approximatelyArchimedean within 5'', beyond which this trend breaks down as the successionof the spiral arms becomes less periodic. The SiO emission at scales smallerthan 0.5'' exhibits signatures of gas in rotation, which is found to fit theexpected behaviour of gas in the wind-companion interaction zone. Aninvestigation of SiO maser emission reveals what could be a stream of gasaccelerating from the surface of the AGB star to the companion. Using thesedynamics, we have tentatively derived an upper limit on the companion mass tobe ~1.1 Msol.

astronomy and astrophysics

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