2017A&A...597A..20O

Aims. Our recent visible polarimetric images of the well-studied AGB star W Hya taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed clumpy dust clouds close to the star at ∼2R_*. We present second-epoch SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) as well as high-spectral resolution long-baseline interferometric observations with the AMBER instrument at the Very Large Telescope Interferometer (VLTI).
Methods. We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820nm), in the Hα line at 656.3nm, and in the TiO band at 717nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3µm with a spectral resolution of 12000.
Results. The high-spatial resolution polarimetric images obtained with SPHERE-ZIMPOL have allowed us to detect clear time variations in the clumpy dust clouds as close as 34-50mas (1.4-2.0R_*) to the star. We detected the formation of a new dust cloud as well as the disappearance of one of the dust clouds detected at the first epoch. The Hα and TiO emission extends to ∼150mas (∼6R_*), and the Hα images obtained at two epochs reveal time variations. The degree of linear polarization measured at minimum light, which ranges from 13 to 18%, is higher than that observed at pre-maximum light. The power-law-type limb-darkened disk fit to the AMBER data in the continuum results in a limb-darkened disk diameter of 49.1±1.5mas and a limb-darkening parameter of 1.16±0.49, indicating that the atmosphere is more extended with weaker limb-darkening compared to pre-maximum light. Our Monte Carlo radiative transfer modeling shows that the second-epoch SPHERE-ZIMPOL data can be explained by a shell of 0.1µm grains of Al₂O₃, Mg₂SiO₄, and MgSiO₃ with a 550 nm optical depth of 0.6±0.2 and an inner and outer radii of 1.3R_* and 10±2R_*, respectively. Our modeling suggests the predominance of small (0.1µm) grains at minimum light, in marked contrast to the predominance of large (0.5µm) grains at pre-maximum light.
Conclusions. The variability phase dependence of the characteristic grain size implies that small grains might just have started to form at minimum light in the wake of a shock, while the pre-maximum light phase might have corresponded to the phase of efficient grain growth.