2024A&A...691L..14O

Aims. We present contemporaneous high-angular-resolution millimeter imaging and visible polarimetric imaging of the nearby asymptotic giant branch (AGB) star W Hya to better understand the dynamics and dust formation within a few stellar radii.
Methods. The star W Hya was observed in two vibrationally excited H₂O lines at 268 and 251 GHz with Atacama Large Millimeter/submillimeter Array (ALMA) at a spatial resolution of 16 × 20 mas and at 748 and 820 nm at a resolution of 26 × 27 mas with the Very Large Telescope (VLT)/Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE)-Zurich Imaging Polarimeter (ZIMPOL).
Results. ALMA's high spatial resolution allowed us to image strong emission of the vibrationally excited H₂O line at 268 GHz (v₂ = 2, J_Ka, Kc_ = 6_5, 2_–74, 3_) over the stellar surface instead of absorption against the continuum, which is expected for thermal excitation. Strong, spotty emission was also detected along and just outside the stellar disk limb at an angular distance of ∼40 mas (∼1.9 R_⋆), extending to ∼60 mas (∼2.9 R_⋆). Another H₂O line (v₂ = 2, J_Ka, Kc_ = 9_2, 8_–83, 5_) at 251 GHz with a similar upper-level energy was tentatively identified, which shows absorption over the stellar surface. This suggests that the emission over the surface seen in the 268 GHz H₂O line is suprathermal or even maser emission. The estimated gas temperature and H₂O density are consistent with the radiatively pumped masers. The 268 GHz H₂O line reveals global infall at up to ∼15 km s^–1 within 2-3 R_⋆, but outflows at up to ∼8 km s^–1 are also present. The polarized intensity maps obtained in the visible reveal clumpy dust clouds forming within ∼40 mas (∼1.9 R_⋆) with a particularly prominent cloud in the SW quadrant and a weaker cloud in the east. The 268 GHz H₂O emission overlaps very well with the visible polarized intensity maps, which suggests that both the nonthermal and likely maser H₂O emission and the dust originate from dense, cool pockets in the inhomogeneous atmosphere within ∼2-3 R_⋆.