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标题：An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii ⋆
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作者：Ward Homan ; Anita Richards ; Leen Decin 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2018
卷号：616
DOI：10.1051/0004-6361/201832834
语种：English
出版社：EDP Sciences
摘要：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 SO2molecular 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 SO2emission 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.
关键词：enradiative transferstars: AGB and post-AGBcircumstellar mattersubmillimeter: starsmolecular data