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

Abstract

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. W Hya was observed with the longest baselines of ALMA at 250-268 GHz with an angular resolution of ~17x20 mas. 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: 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 Rstar), respectively. We identified 57 lines, which include SiO, H2O, SO2, SO, HCN, AlO, AlOH, TiO, TiO2, OH, and some of their isotopologues. 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 Rstar) but also outflow at up to ~10 km/s 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 of SiO, H2O, SO2, and AlO. The emission of SiO, AlO, TiO, TiO2, SO, and SO2 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 Rstar), which is consistent with shock-induced chemistry.

30 pages, 19 figures, published in Astronomy and Astrophysics

References in corpus (30)

• CASA, the Common Astronomy Software Applications for Radio Astronomy
• Multi-Scale CLEAN deconvolution of radio synthesis images
• A close halo of large transparent grains around extreme red giant stars
• Unexpectedly large mass loss during the thermal pulse cycle of the red giant R Sculptoris!
• (Sub)stellar companions shape the winds of evolved stars
• Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
• Distance estimates for AGB stars from parallax measurements
• The wind of W Hya as seen by Herschel - I. The CO envelope
• The evolution of DARWIN: current status of wind models for AGB stars
• An extensive grid of DARWIN models for M-type AGB stars I. Mass-loss rates and other properties of dust-driven winds
• Imaging the Radio Photospheres of Mira Variables
• Clumpy dust clouds and extended atmosphere of the AGB star W Hya revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER II. Time variations between pre-maximum and minimum light
• Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Dor: Non-LTE abundance analysis of CO, SiO, and HCN
• Global 3D radiation-hydrodynamical models of AGB stars with dust-driven winds
• Exploring the origin of clumpy dust clouds around cool giants. A global 3D RHD model of a dust-forming M-type AGB star
• The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA
• Resolving the extended stellar atmospheres of Asymptotic Giant Branch stars at (sub-)millimetre wavelengths
• ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results
• An observational study of dust nucleation in Mira ($o$ Ceti): II. Titanium oxides are negligible for nucleation at high temperatures
• The Role of Internal Photons on the Chemistry of the Circumstellar Envelopes of AGB Stars
• Continuum emission around AGB stars at 1.2 mm
• Pulsation-induced atmospheric dynamics in M-type AGB stars. Effects on wind properties, photometric variations and near-IR CO line profiles
• Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars
• Distribution and kinematics of the HCN(3-2) emission down to the innermost region in the envelope of the O-rich star W Hya
• Mechanisms of SiO oxidation: Implications for dust formation
• Detection of highly excited OH towards AGB stars: A new probe of shocked gas in the extended atmospheres
• One month convection timescale on the surface of a giant evolved star
• Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae
• 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
• Locating dust and molecules in the inner circumstellar environment of R~Sculptoris with MATISSE