The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit
Artikel i vetenskaplig tidskrift, 2017

Context. Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars. Aims. AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO(J = 3 -> 2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. Methods. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. Results. The estimated average mass-loss rate of W Aql is (M) over dot = 3.0 x 10(-6) M-circle dot yr(-1) and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10 0 0 of the CSE is asymmetric with arc-like structures at separations of 23" scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations. Conclusions. The CO(J = 3 -> 2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of similar to 10" and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yr) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.

binaries: general

circumstellar matter

stars: AGB and post-AGB

submillimeter: stars


S. Ramstedt

Uppsala universitet

S. Mohamed

South African Astronomical Observatory

National Institute for Theoretical Physics

University of Cape Town

Wouter Vlemmings

Astronomi och plasmafysik

Taissa Danilovich

KU Leuven

M. Brunner

Universität Wien

Elvire De Beck

Astronomi och plasmafysik

E. M. Humphreys

European Southern Observatory (ESO)

Michael Lindqvist

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

Matthias Maercker

Astronomi och plasmafysik

Hans Olofsson

Astronomi och plasmafysik

F. Kerschbaum

Universität Wien

G. Quintana-Lacaci

CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)

Astronomy and Astrophysics

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

Vol. 605 A126


Astronomi, astrofysik och kosmologi