The mass-loss mechanism in asymptotic giant branch (AGB) stars is not yet fully understood. It is of paramount importance to probe the region within several stellar radii for solving this long-standing problem. We present 20-milliarcsecond resolution ALMA imaging of the well-studied AGB star W Hya in molecular lines of SiO, H2O, SO2, SO, HCN, AlO, and AlOH at 250–269 GHz. The high angular resolution imaging has allowed us to spatially resolve the star’s atmosphere from the continuum stellar radius of 30 mas to several stellar radii (~100 mas). The ALMA images reveal the complex morphology of the atmosphere with arcs, plumes, and clumps. The spatially resolved line profiles are broad, ranging from -15 to +15 km/s, suggesting the vigorous turbulent motion and/or pulsation within ~2 stellar radii. The outward velocity is faster than the terminal wind velocity of 5 km/s and only marginally slower than the local escape velocity. On the other hand, the data also suggest the presence of an accelerating infall from ~0 at 3–4 stellar radii to ~13 km/s at ~2 stellar radii. These results imply that the mass loss may occur when some of the fast upwelling components reach the local escape velocity within several stellar radii. Furthermore, we detected prominent emission over the stellar disk–instead of pure absorption as expected–in some of the SiO, H2O, and SO2 lines. This indicates maser actions, and the maser conditions suggest the presence of cool and dense cells within 3–4 stellar radii, in which dust can form.
Horarios: May 16, 2023 15:30
Publicado por: Claudia Aguilera