High-angular-resolution ALMA imaging of the inhomogeneous dynamical atmosphere of the asymptotic giant branch star W Hya

SiO, H₂O, SO₂, SO, HCN, AlO, AlOH, TiO, TiO₂, and OH lines

¹ Instituto de Astrofísica, Departamento de Física y Astronomía, Facultad de Ciencias Exactas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
² Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

^★ Corresponding author: k1.ohnaka@gmail.com

Received: 31 March 2025
Accepted: 9 September 2025

Abstract

Aims. We present high-angular-resolution imaging of the asymptotic giant branch star W Hya with the Atacama Large Millimeter/submillimeter Array (ALMA) to probe the dynamics and chemistry in the atmosphere and inner wind.

Methods. W Hya was observed with the longest baselines of ALMA at 250–268 GHz with an angular resolution of ~17×20 mas.

Results. ALMA’s high angular resolution allowed us to resolve the stellar disk of W Hya along with clumpy, irregularly shaped emission extending to ~100 mas. This emission includes a plume in the north-northwest, a tail in the south-southwest, and the extended atmosphere elongated in the east-northeast-west-southwest direction, with semimajor and semiminor axes of ~70 and 40 mas (~3.4 and 1.9 R_⋆), respectively. We identified 57 lines, which include SiO, H₂O, SO₂, SO, HCN, AlO, AlOH, TiO, TiO₂, OH, and some of their isotopologues, with about half of them being in vibrationally excited states. The molecular line images show spatially inhomogeneous molecular formation. Our ALMA data taken at phase 0.53 (minimum light) indicate global, accelerating infall within ~75 mas (3.6 R_⋆) but also outflow at up to ~10 km s⁻¹ in deeper layers. While 38 of the detected lines appear in absorption against the continuum stellar disk as expected, we detect nonthermal emission on top of the continuum over the stellar disk in 19 lines, including SiO, H₂O, SO₂, and AlO. The emission of the SiO, AlO, TiO, TiO₂, SO, and SO₂ lines coincides well with the clumpy dust cloud distribution obtained from contemporaneous visible polarimetric imaging in addition to H₂O reported in our previous work. This lends support to the idea that SiO, H₂O, and AlO are directly involved in grain nucleation. The overlap of SO/SO₂ (possibly also TiO/TiO₂) with the dust clouds suggests the formation of these molecules and dust behind shocks induced by pulsation and/or convection. We detect HCN emission close to the star, down to ~30 mas (~1.4 R_⋆), which is consistent with shock-induced chemistry.