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

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 H alpha 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 mu m 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 similar to 50 mas (similar to 2 R-star) 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 similar to 3 R-star. Furthermore, the SPHERE-ZIMPOL image taken over the H alpha line shows emission with a radius of up to similar to 160 mas (similar to 7 R-star). We found that dust, molecular gas, and H alpha-emitting hot gas coexist within 2-3 R-star. Our modeling suggests that the observed polarized intensity maps can reasonably be explained by large (0.4-0.5 mu m) grains of Al2O3, Mg2SiO4, or MgSiO3 in an optically thin shell (tau(550nm) = 0.1 +/- 0.02) with an inner and outer boundary radius of 1.9-2.0 R-star and 3 +/- 0.5 R-star, 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.