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 star π1 Gruis, which has a known companion at ∼440 au, and is thought to harbour a second, closer-by (<10 au) companion, was observed with ALMA 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 reveals that π1 Gru’s CSE 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, 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. 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⊙.
|Journal||Astronomy and Astrophysics|
|Publication status||Accepted - 09 Oct 2020|
Homan, W., Montarges, M., Pimpanuwat, B., Richards, A. M. S., Wallström, S. H. J., & Millar, T. J. (Accepted/In press). ATOMIUM: A high-resolution view on the highly asymmetric wind of the AGB star π1 Gruis. Astronomy and Astrophysics.