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The CO(J=3-2) emission from the CSE of the binary S-type AGB star W Aql hasbeen observed at subarcsecond resolution using ALMA. The aim of this paper isto investigate the wind properties of the AGB star and to analyse how the knowncompanion has shaped the CSE. The average mass-loss rate during the creation ofthe detected CSE is estimated through modelling, using the ALMA brightnessdistribution and previously published single-dish measurements as observationalconstraints. The ALMA observations are presented and compared to the resultsfrom a 3D smoothed particle hydrodynamics (SPH) binary interaction model withthe same properties as the W Aql system and with two different orbitaleccentricities. Three-dimensional radiative transfer modelling is performed andthe response of the interferometer is modelled and discussed. The estimatedaverage mass-loss rate of W~Aql agrees with previous results. The size of theemitting region is consistent with photodissociation models. The CO(J=3-2)emission is dominated by a smooth component overlayed with two weak arcpatterns with different separations. The larger pattern is predicted by thebinary interaction model with separations of 10" and therefore likely due tothe known companion. It is consistent with a binary orbit with loweccentricity. The smaller separation pattern is asymmetric and coincides withthe dust distribution, but the separation timescale (200 yrs) is not consistentwith any known process of the system. The separation of the known companions ofthe system is large enough to not have a very strong effect on thecircumstellar morphology. The density contrast across the envelope of a binarywith 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 largermass-loss rate.

The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit