2018A&A...616A..34H -
Astronomy and Astrophysics, volume 616A, 34-34 (2018/8-1)
An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii.
HOMAN W., RICHARDS A., DECIN L., DE KOTER A. and KERVELLA P.
Abstract (from CDS):
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 0.1M☉, 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.
Abstract Copyright:
© ESO 2018
Journal keyword(s):
radiative transfer - stars: AGB and post-AGB - circumstellar matter - submillimeter: stars - molecular data
Errata:
erratum vol. 632, art. C1 (2019)
Simbad objects:
11
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