High resolution ALMA imaging of H2O, and SiO, and SO2 masers in the atmosphere of the AGB star W Hya

The mass-loss mechanism in asymptotic giant branch (AGB) stars is not yet fully understood. We present 20-milliarcsecond resolution ALMA imaging of the well-studied AGB star W Hya in multiple molecular lines at 250-269 GHz, including masers from SiO, H₂O, and SO₂. The images show complex plumes, arcs, and clumps over the stellar disk and in the atmosphere extending to several stellar radii. We detected prominent emission components over the stellar disk—instead of pure absorption as expected—in some Si¹⁷O, ³⁰SiO, H₂O, and SO₂ lines. The surface emission seen in the Si¹⁷O and vibrationally excited H₂O lines is particularly strong, indicating maser actions. The masers seen over the stellar disk indicate radial amplification.