Contemporaneous high-angular-resolution imaging of the AGB star W Hya in vibrationally excited H2O lines and visible polarized light with ALMA and VLT/SPHERE-ZIMPOL

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. The star W Hya was observed in two vibrationally excited H2O lines at 268 and 251 GHz with ALMA at a spatial resolution of 16 x 20 mas and at 748 and 820 nm at a resolution of 26 x 27 mas with the VLT/SPHERE-ZIMPOL. ALMA's high spatial resolution allowed us to image strong emission of the vibrationally excited H2O line at 268 GHz (v2 = 2, J_K_a,K_c = 6_5,2 - 7_4,3) 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 stellar radii), extending to ~60 mas (~2.9 stellar radii). Another H2O line (v2 = 2, J_K_a,K_c = 9_2,8 - 8_3,5) 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 GHzH2O 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 within 2--3 stellar radii, but outflows at up to ~8 km/s are also present. The polarized intensity maps obtained in the visible reveal clumpy dust clouds forming within ~40 mas (~1.9 stellar radii) 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 stellar radii.
Comment: 10 pages, 7 figures, published in Astronomy and Astrophysics