The extended molecular envelope of the asymptotic giant branch star ${\pi }^1$ Gruis as seen by ALMA I. Large-scale kinematic structure and CO excitation properties

Abstract

The S-type asymptotic giant branch (AGB) star ${\pi }^1$ Gruis has a known companion at a separation of $\approx$400 AU. 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. The new ALMA ${}^{12}$CO and ${}^{13}$CO $J$=3-2 data, together with previously published ${}^{12}$CO $J$=2-1 data from the Submillimeter Array (SMA), and the ${}^{12}$CO $J$=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. 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$\times$10${}^{-7}$ M${}_{\odot }$ yr${}^{-1}$. The total mass and linear momentum of the fast outflow are estimated at 7.3$\times$10${}^{-4}$ M${}_{\odot }$ and 9.6$\times$10${}^{37}$ 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 ${}^{12}$CO/${}^{13}$CO abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed

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