Clumpy dust clouds and extended atmosphere of the AGB star W Hya revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Abstract

We present visible polarimetric imaging observations of the well-studied AGB star W Hya taken with VLT/SPHERE-ZIMPOL as well as high spectral resolution long-baseline interferometric observations with the AMBER instrument of the Very Large Telescope Interferometer (VLTI). We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Halpha line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 micron with a spectral resolution of 12000. Taking advantage of the polarimetric imaging capability of SPHERE-ZIMPOL combined with the superb adaptive optics performance, we have succeeded in spatially resolving three clumpy dust clouds located at ~50 mas (~2 Rstar) from the central star, revealing dust formation very close to the star. The AMBER data in the individual CO lines suggest a molecular outer atmosphere extending to ~3 Rstar. Furthermore, the SPHERE-ZIMPOL image taken over the Halpha line shows emission with a radius of up to ~160 mas (~7 Rstar). We found that dust, molecular gas, and Halpha-emitting hot gas are coexisting within 2--3 Rstar. Our modeling suggests that the observed polarized intensity maps can reasonably be explained by large (0.4--0.5 micron) grains of Al2O3 or Mg2SiO4 or MgSiO3 in an optically thin shell with an inner boundary radius of 1.9--2.0 Rstar. The observed clumpy structure can be reproduced by a density enhancement by a factor of 4 +/- 1. The grain size derived from our polarimetric images is consistent with the prediction of the hydrodynamical models for the mass loss driven by the scattering due to micron-sized grains. The detection of the clumpy dust clouds close to the star lends support to the dust formation induced by pulsation and large convective cells as predicted by the 3-D simulations for AGB stars.

12 pages, 9 figures, accepted for publication in Astronomy and Astrophysics, short discussion of non-equilibrium gas-dust chemistry models added

References in corpus (6)

• Interferometric data reduction with AMBER/VLTI.Principle,estimators and illustration
• Too little radiation pressure on dust in the winds of oxygen-rich AGB stars
• A close halo of large transparent grains around extreme red giant stars
• Imaging the Radio Photospheres of Mira Variables
• The Keck Aperture Masking Experiment: Multi-Wavelength Observations of 6 Mira Variables
• The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

Cited by in corpus (52)

• SPHERE: the exoplanet imager for the Very Large Telescope
• Global 3D radiation-hydrodynamics models of AGB stars. Effects of convection and radial pulsations on atmospheric structures
• Dynamic atmospheres and winds of cool luminous giants, I. Al${}_2$O${}_3$ and silicate dust in the close vicinity of M-type AGB stars
• Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL
• Tracing the phase transition of Al-bearing species from molecules to dust in AGB winds. Constraining the presence of gas-phase (Al2O3)n clusters
• Luminosities and mass-loss rates of Local Group AGB stars and Red Supergiants
• An observational study of dust nucleation in Mira ($o$ Ceti): I. Variable features of AlO and other Al-bearing species
• 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
• 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
• 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
• Aperture synthesis imaging of the carbon AGB star R Sculptoris: Detection of a complex structure and a dominating spot on the stellar disk
• 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
• High-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL
• Stellar Winds and Coronae of Low-mass Pop. II/III Stars
• Developing a self-consistent AGB wind model: II. Non-classical, non-equilibrium polymer nucleation in a chemical mixture
• ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results
• An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii
• Atomium: A high-resolution view on the highly asymmetric wind of the AGB star Pi1 Gruis. I. First detection of a new companion and its effect on the inner wind
• Inner dusty envelope of the AGB stars W~Hydrae, SW\,Virginis, and R~Crateris using SPHERE/ZIMPOL
• The inner dust shell of Betelgeuse detected by polarimetric aperture-masking interferometry
• Atmospheres and wind properties of non-spherical AGB stars
• Exploring the innermost dust formation region of the oxygen-rich AGB star IK~Tau with VLT/SPHERE-ZIMPOL and VLTI/AMBER
• Rotational spectra of vibrationally excited AlO and TiO in oxygen rich stars
• HD101584: Circumstellar characteristics and evolutionary status
• Pulsation-induced atmospheric dynamics in M-type AGB stars. Effects on wind properties, photometric variations and near-IR CO line profiles
• Alfvén wave-driven wind from RGB and AGB stars
• Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars
• The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview: Sample characterization through polarization analysis
• Infrared interferometric three-dimensional diagnosis of the atmospheric dynamics of the AGB star R Dor with VLTI/AMBER
• Optical interferometry and Gaia measurement uncertainties reveal the physics of asymptotic giant branch stars
• Simultaneous 183 GHz H2O Maser and SiO Observations Towards Evolved Stars Using APEX SEPIA Band 5
• Evidence for localized onset of episodic mass loss in Mira
• On the detection of CO and mass loss of Bulge OH/IR stars
• A CO-multilayer outer atmosphere for eight evolved stars revealed with VLTI/AMBER
• Modelling polarized light from dust shells surrounding asymptotic giant branch stars
• The unusual 3D distribution of NaCl around the AGB star IK Tau
• Tomography of cool giant and supergiant star atmospheres III. Validation of the method on VLTI/AMBER observations of the Mira star S Ori
• Tomography of silicate dust around M-type AGB stars I. Diagnostics based on dynamical models
• How to disentangle geometry and mass-loss rate from AGB-star spectral energy distributions -- The case of EP Aqr
• ALMA monitoring of mm line variation in IRC+10216: I. Overview of millimeter variability
• ATOMIUM: Continuum emission and evidence of dust enhancement from binary motion
• 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
• Asymmetries in asymptotic giant branch stars and their winds. I. From 3D RHD models to synthetic observables
• {\sc precision}: A fast python pipeline for high-contrast imaging -- application to SPHERE observations of the red supergiant VX Sagitariae
• Infrared interferometric imaging of the compact dust disk around the AGB star HR3126 with the bipolar Toby Jug Nebula
• High-angular-resolution ALMA imaging of the inhomogeneous dynamical atmosphere of the asymptotic giant branch star W Hya
• Infrared colours and bolometric corrections of SiO masing stars in the inner Milky Way
• San Pedro Meeting on Wide Field Variability Surveys: Some Concluding Comments
• Resolution and calibration effects in high contrast polarimetric imaging of circumstellar scattering regions