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

Abstract

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 ∼2 R . 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 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 12 000. 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 34-50 mas (1.4-2.0 R ) 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 ∼150 mas (∼6 R ), 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 ± 1.5 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 shows that the second-epoch SPHERE-ZIMPOL data can be explained by a shell of 0.1 µm grains of Al 2 O 3 , Mg 2 SiO 4 , and MgSiO 3 with a 550 nm optical depth of 0.6±0.2 and an inner and outer radii of 1.3 R and 10±2 R , respectively. Our modeling suggests the predominance of small (0.1 µm) grains at minimum light, in marked contrast to the predominance of large (0.5 µm) 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.

Mass loss of stars on the asymptotic giant branch

“…This scenario of a pulsation-enhanced, dust-driven outflow (see Fig. 2) is supported by various observations, ranging from correlations of mass-loss indicators with pulsation properties (e.g., Whitelock et al 1991Whitelock et al , 2003Glass et al 2009;McDonald and Zijlstra 2016) to spatially resolved structures of dynamical atmospheres and dust formation regions (e.g., Tej et al 2003;Wittkowski et al 2007;Zhao-Geisler et al 2012;Karovicova et al 2013;Stewart et al 2016b;Khouri et al 2016a;Ohnaka et al 2017). …”

“…This requires grain sizes of about 0.1-1 µm, in order to make the dust particles efficient at scattering radiation in the near-IR wavelength region where the stellar flux peaks. Recent detections of dust grains with sizes of 0.1-0.5 µm in the close vicinity of several AGB stars (e.g., Norris et al 2012;Ohnaka et al 2016Ohnaka et al , 2017 lend strong support to this scenario. Furthermore, the dynamical models based on this mechanism are in good agreement with observations regarding mass-loss rates and wind velocities, as well as visual and near-IR colours, and their variation with pulsation phase (Bladh et al , 2015.…”

“…An alternative explanation could be of a morphological nature: In the current PEDDRO models, the outflows consist of complete, spherical layers of gas and dust. Recent observations of some nearby stars show clumpy dust and gas clouds with variable morphology in the extended stellar atmospheres (e.g., Ohnaka et al 2017;Khouri et al 2016a), probably due to the effects of convection or non-radial pulsations (see Sect. 4.1).…”

“…Interferometric monitoring reveals temporal variations of measured stellar diameters, as well as a strong dependence of diameters on wavelength, indicating extended, dynamical atmospheres with complex molecular layers (e.g., Burns et al 1998;Ireland et al 2004a, b;Woodruff et al 2008;Lacour et al 2009). Recent advances in high-angular-resolution techniques have made it possible to resolve the surfaces and extended atmospheres of some nearby AGB stars (e.g., Stewart et al 2016b;Ohnaka et al 2016;Wittkowski et al 2017), showing temporal changes in atmospheric structures and dust properties (e.g., Haubois et al 2015;Khouri et al 2016a;Ohnaka et al 2017). For the large majority of cool giants, however, dynamical processes have to be explored with indirect methods, using photometric monitoring or high-resolution spectroscopy.…”

Tomography of silicate dust around M-type AGB stars

“…Norris et al 2012;Ohnaka et al 2016Ohnaka et al , 2017. Fe-free silicate particles are very transparent at near-IR wavelengths, around the stellar flux maximum, which leads to low radiative heating and, consequently, to low grain temperatures.…”