Astronomy and Astrophysics, volume 605A, 28-28 (2017/9-1)
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.
DOAN L., RAMSTEDT S., VLEMMINGS W.H.T., HOFNER S., DE BECK E., KERSCHBAUM F., LINDQVIST M., MAERCKER M., MOHAMED S., PALADINI C. and WITTKOWSKI M.
Abstract (from CDS):
Context. The S-type asymptotic giant branch (AGB) star π1 Gru has a known companion at a separation of 2.7 (~=400AU). 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 13km/s along the equator plane; a radially expanding component at the center with a constant velocity of 14km/s; and a fast, bipolar outflow with velocity proportionally increasing from 14km/s at the base up to 100km/s 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 7.7x10–7M☉/yr. The total mass and linear momentum of the fast outflow are estimated at 7.3x10–4M☉ and 9.6x1037g.cm/s, 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.
© ESO, 2017
stars: AGB and post-AGB - stars: mass-loss - stars: individual: π 1 Gru - stars: general - radio lines: stars - binaries: general - binaries: general
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