The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA II. The spiral-outflow observed at high-angular resolution
Journal article, 2020

Context. This study is a follow up to the previous analysis of lower-angular resolution data in which the kinematics and structure of the circumstellar envelope (CSE) around the S-type asymptotic giant branch (AGB) star pi(1) Gruis were investigated. The AGB star has a known companion (at a separation of 400 AU) that cannot explain the strong deviations from spherical symmetry of the CSE. Recently, hydrodynamic simulations of mass transfer in closer binary systems have successfully reproduced the spiral-shaped CSEs found around a handful of sources. There is growing evidence for an even closer, undetected companion complicating the case of pi(1) Gruis further. Aims. The improved spatial resolution allows for the investigation of the complex circumstellar morphology and the search for imprints on the CSE of the third component. Methods. We have observed the (CO)-C-12 J = 3-2 line emission from pi(1) Gruis using both the compact and extended array of Atacama Large Millimeter/submillimeter Array (ALMA). The interferometric data have furthermore been combined with data from the ALMA total power array. The imaged brightness distribution has been used to constrain a non-local, non-local thermodynamic equilibrium 3D radiative transfer model of the CSE. Results. The high-angular resolution ALMA data have revealed the first example of a source on the AGB where both a faster bipolar outflow and a spiral pattern along the orbital plane can be seen in the gas envelope. The spiral can be traced in the low- to intermediate-velocity (13-25 km s(-1)) equatorial torus. The largest spiral-arm separation is approximate to 5.'' 5 and consistent with a companion with an orbital period of approximate to 330 yr and a separation of less than 70 AU. The kinematics of the bipolar outflow is consistent with it being created during a mass-loss eruption where the mass-loss rate from the system increased by at least a factor of five for 10-15 yr. Conclusions. The spiral pattern is the result of an undetected companion. The bipolar outflow is the result of a rather recent mass-loss eruption event.

binaries: general

radio lines: general

stars: individual: pi(1) Gru

stars: AGB and post-AGB

stars: low-mass


L. Doan

Uppsala University

S. Ramstedt

Uppsala University

Wouter Vlemmings

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

S. Mohamed

National Institute for Theoretical Physics

University of Cape Town

South African Astronomical Observatory

S. Hofner

Uppsala University

Elvire de Beck

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

F. Kerschbaum

University of Vienna

Michael Lindqvist

Chalmers, Space, Earth and Environment, Onsala Space Observatory, Observation Support

Matthias Maercker

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

C. Paladini

European Southern Observatory Santiago

M. Wittkowski

European Southern Observatory Santiago

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 633 A13

Subject Categories

Astronomy, Astrophysics and Cosmology


Geosciences, Multidisciplinary


Onsala Space Observatory



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