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

TL;DR

This study presents high-resolution ALMA observations revealing a rare face-on spiral structure in the wind of the O-rich AGB star EP Aquarii, suggesting complex wind morphology possibly influenced by a companion or hydrodynamical effects.

Contribution

First convincing detection of a spiral morphology in an O-rich AGB star's wind, providing insights into wind structure and potential companion influence.

Findings

Detection of a bi-conical wind with a face-on spiral feature

Estimated inclination angle between 4° and 18°

Possible presence of a low-mass companion affecting the environment

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 centers of the two bi-conical wind hemispheres, we deduce the position angle of the inclination axis to be ~150 deg measured counterclockwise from north. Based on the velocity width of the spiral signature we estimate the inclination angle of the system to be between 4 deg and 18 deg. 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 Msol, based on its non-detection in the continuum...