The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA
I. Large-scale kinematic structure and CO excitation properties
1 Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
2 Department of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
3 Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
4 South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
5 Astronomy Department, University of Cape Town, University of Cape Town, 7701 Rondebosch, South Africa
6 National Institute for Theoretical Physics, Private Bag X1, 7602 Matieland, South Africa
7 Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Campus Plaine CP 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
8 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
Received: 27 February 2017
Accepted: 19 May 2017
Context. The S-type asymptotic giant branch (AGB) star π1 Gru has a known companion at a separation of 2.̋7 (≈400 AU). Previous observations of the circumstellar envelope (CSE) show strong deviations from spherical symmetry. The envelope structure, including an equatorial torus and a fast bipolar outflow, is rarely seen in the AGB phase and is particularly unexpected in such a wide binary system. Therefore a second, closer companion has been suggested, but the evidence is not conclusive.
Aims. The aim is to make a 3D model of the CSE and to constrain the density and temperature distribution using new spatially resolved observations of the CO rotational lines.
Methods. We have observed the J = 3–2 line emission from 12CO and 13CO using the compact arrays of the Atacama Large Millimeter/submillimeter Array (ALMA). The new ALMA data, together with previously published 12CO J = 2–1 data from the Submillimeter Array (SMA), and the 12CO J = 5–4 and J = 9–8 lines observed with Herschel/Heterodyne Instrument for the Far-Infrared (HIFI), is modeled with the 3D non-LTE radiative transfer code SHAPEMOL.
Results. The data analysis clearly confirms the torus-bipolar structure. The 3D model of the CSE that satisfactorily reproduces the data consists of three kinematic components: a radially expanding torus with velocity slowly increasing from 8 to 13 km s-1 along the equator plane; a radially expanding component at the center with a constant velocity of 14 km s-1; and a fast, bipolar outflow with velocity proportionally increasing from 14 km s-1 at the base up to 100 km s-1 at the tip, following a linear radial dependence. The results are used to estimate an average mass-loss rate during the creation of the torus of 7.7 × 10-7 M⊙ yr-1. The total mass and linear momentum of the fast outflow are estimated at 7.3 × 10-4 M⊙ and 9.6 × 1037 g cm s-1, respectively. The momentum of the outflow is in excess (by a factor of about 20) of what could be generated by radiation pressure alone, in agreement with recent findings for more evolved sources. The best-fit model also suggests a 12CO/13CO abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed.
Key words: stars: AGB and post-AGB / stars: mass-loss / stars: individual:π1Gru / stars: general / radio lines: stars / binaries: general
© ESO, 2017