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

¹ Universidad Católica del Norte, Instituto de Astronomía, Avenida Angamos 0610, Antofagasta, Chile
e-mail: k1.ohnaka@gmail.com
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 31 January 2016
Accepted: 1 March 2016

Abstract

Context. Dust formation is thought to play an important role in the mass loss from stars at the asymptotic giant branch (AGB); however, where and how dust forms is still open to debate.

Aims. 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 taken with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Our goal is to spatially resolve the dust and molecule formation region within a few stellar radii.

Methods. We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Hα 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 μm with a spectral resolution of 12000.

Results. Taking advantage of the polarimetric imaging capability of SPHERE-ZIMPOL combined with the superb adaptive optics performance, we succeeded in spatially resolving three clumpy dust clouds located at ~50 mas (~2 R_⋆) 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 R_⋆. Furthermore, the SPHERE-ZIMPOL image taken over the Hα line shows emission with a radius of up to ~160 mas (~7 R_⋆). We found that dust, molecular gas, and Hα-emitting hot gas coexist within 2–3 R_⋆. Our modeling suggests that the observed polarized intensity maps can reasonably be explained by large (0.4–0.5 μm) grains of Al₂O₃, Mg₂SiO₄, or MgSiO₃ in an optically thin shell (τ_550nm = 0.1 ± 0.02) with an inner and outer boundary radius of 1.9–2.0 R_⋆ and 3 ± 0.5R_⋆, respectively. The observed clumpy structure can be reproduced by a density enhancement of a factor of 4 ± 1.

Conclusions. The grain size derived from our modeling of the SPHERE-ZIMPOL 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 3D simulations for AGB stars.