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
e-mail: lam.doan@physics.uu.se
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

Abstract

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.