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Our recent visible polarimetric images of the well-studied AGB star W Hyataken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealedclumpy dust clouds close to the star at ~2 Rstar. We present second-epochSPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) in thecontinuum (645, 748, and 820 nm), in the Halpha line (656.3 nm), and in the TiOband (717 nm) as well as high-spectral resolution long-baseline interferometricobservations in 2.3 micron CO lines with the AMBER instrument at the Very LargeTelescope Interferometer (VLTI). The high-spatial resolution polarimetricimages have allowed us to detect clear time variations in the clumpy dustclouds as close as 34--50~mas (1.4--2.0 Rstar) to the star. We detected theformation of a new dust cloud and the disappearance of one of the dust cloudsdetected 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 linearpolarization is higher at minimum light (13--18%) than that at pre-maximumlight. The power-law-type limb-darkened disk fit to the AMBER data in thecontinuum results in a limb-darkened disk diameter of 49.1+/-1.5 mas and alimb-darkening parameter of 1.16+/-0.49, indicating that the atmosphere is moreextended with weaker limb-darkening compared to pre-maximum light. Our MonteCarlo radiative transfer modeling suggests the predominance of small (0.1micron) grains of Al2O3, Mg2SiO4, and MgSiO3 at minimum light, in markedcontrast to the predominance of large (0.5 micron) grains at pre-maximum light.The variability phase dependence of the grain size implies that small grainsmight just have started to form at minimum light in the wake of a shock, whilethe pre-maximum light phase might have corresponded to the phase of efficientgrain growth.

astronomy and astrophysics

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