  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 ~2 Rstar. We present second-epoch
SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) in the
continuum (645, 748, and 820 nm), in the Halpha line (656.3 nm), and in the TiO
band (717 nm) as well as high-spectral resolution long-baseline interferometric
observations in 2.3 micron CO lines with the AMBER instrument at the Very Large
Telescope Interferometer (VLTI). The high-spatial resolution polarimetric
images have allowed us to detect clear time variations in the clumpy dust
clouds as close as 34--50~mas (1.4--2.0 Rstar) to the star. We detected the
formation of a new dust cloud and the disappearance of one of the dust clouds
detected at the first epoch. The Halpha and TiO emission extends to ~150 mas
(~6 Rstar), and the Halpha images reveal time variations. The degree of linear
polarization is higher at minimum light (13--18%) than that 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+/-1.5 mas 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 suggests the predominance of small (0.1
micron) grains of Al2O3, Mg2SiO4, and MgSiO3 at minimum light, in marked
contrast to the predominance of large (0.5 micron) grains at pre-maximum light.
The variability phase dependence of the 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.
