An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii^⋆

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D B2401, 3001 Leuven, Belgium
e-mail: ward.homan@ster.kuleuven.be
² JBCA, Department Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
³ School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
⁴ LESIA (CNRS UMR 8109), Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, France
⁵ Sterrenkundig Instituut “Anton Pannekoek”, Science Park 904, 1098 XH, Amsterdam, The Netherlands

Received: 15 February 2018
Accepted: 19 April 2018

Abstract

High-resolution interferometric observations of the circumstellar environments of AGB stars show a variety of morphologies. Guided by the unusual carbon monoxide line profile of the AGB star EP Aquarii, we have observed its circumstellar environment with ALMA band 6 in cycle 4. We describe the morphological complexity of the CO, SiO, and SO₂ molecular emission. The CO emission exhibits the characteristics of a bi-conical wind with a bright nearly face-on spiral feature around the systemic velocity. This is the first convincing detection of a spiral morphology in an O-rich wind. Based on the offsets of the centres of the two bi-conical wind hemispheres, we deduce the position angle of the inclination axis to be ~150^° measured anticlockwise from north. Based on the velocity width of the spiral signature, we estimate the inclination angle of the system to be between 4^° and 18^°. The central emission zone exhibits a morphology that resembles simulations modelling the spiral-inducing wind Roche-lobe overflow mechanism. Though the spiral may be a companion-induced density enhancement in the stellar outflow, the extremely narrow width of the spiral signature in velocity space suggests that it may be a hydrodynamical perturbation in a face-on differentially rotating disk. The SiO emission does not show the spiral, but exhibits a local emission void approximately 0.5″ west of the continuum brightness peak. We hypothesise that this may be a local environment caused by the presence of a stellar companion with a mass of at most 0.1 M_⊙, based on its non-detection in the continuum. Finally, the SO₂ emission remains confined to a 0.5″ radius, and does not show any obvious substructure, but it exhibits a clear rotation signature. Combined, the properties of the molecular emission favour the face-on rotating disk scenario. We observe unexpectedly large red- and blue-shifted wings in the spectral line of SiO, which could be explained by the potential non-local thermodynamic equilibrium (NLTE) nature of driven, mixed, partly granular fluids.