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Context. The observed orbital characteristics of post-asymptotic giant branch and post-red giant branch (post-RGB) binaries are not understood. We suspect that the missing ingredients needed to explain them probably lie in the continuous interaction of the central binary with its circumstellar environment. Aims. We aim at studying the circumbinary material in these complex systems by investigating the connection between the innermost structures and large-scale structures. Methods. We perform high-angular resolution observations of HD 101584 in the near-infrared continuum. HD 101584 has a complex structure as seen at millimeter wavelengths, with a disk-like morphology and a bipolar outflow due to an episode of a strong binary interaction. To account for the complexity of the target, we first perform an image reconstruction and use this result to fit a geometrical model to extract the morphological and thermal features of the environment. Results. The image reveals an unexpected double ring structure. We interpret the inner ring as having been produced by emission from dust located in the plane of the disk, and the outer ring having been produced by emission from dust that is located 1.6 [D/1kpc] au above the disk plane. The inner ring diameter (3.94 [D/1kpc] au) and temperature (T = 1540 ± 10 K) are compatible with the dust sublimation front of the disk. The origin of the out-of-plane ring (with a diameter of 7.39 [D/1kpc] au and a temperature of 1014 ± 10 K) could be episodic ejection or a dust condensation front in the outflow. Conclusions. The observed outer ring is possibly linked with the blue-shifted side of the large-scale outflow seen by the Atacama Large Millimeter/submillimeter Array and may trace its launching location to the central star. Such observations place morphological constraints on the ejection mechanism. Additional observations are needed to constrain the origin of the out-of-plane structure. [ABSTRACT FROM AUTHOR]

Context. The asymptotic giant branch (AGB) marks the final evolutionary stage of stars with initial masses between ~0.8 and 8 M®. During this phase, stars undergo copious mass loss, which contributes significantly to the enrichment of the interstellar medium. The well-accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the density-enhanced and extended AGB stellar atmospheres. The details of the mass-loss process are not yet well understood, however. For oxygen-rich AGB stars, which are the focus of this study, the dust grains that drive the wind are expected to scatter visible light very efficiently because their sizes are relative large. Aims. We study the distribution of dust in the inner wind of oxygen-rich AGB stars to advance our understanding of the wind-driving process. Methods. We observed light scattered off dust grains that form around three oxygen-rich AGB stars (W Hya, SWVir, and RCrt) with mass-loss rates between 10-7 and 10-6 M® yr-1 using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL with three filters centred at 0.65, 0.75 and 0.82 μm. We compared the observed morphologies and the spectral dependence of the scattered light between the three sources and determined the radial profile, per image octant, of the dust density distribution around the closest target, WHya. Results. We find the distribution of dust to be asymmetric for the three targets. A biconical morphology is seen for R Crt, with a position angle that is very similar to those inferred from interferometric observations of maser emission and of mid-infrared continuum emission. The cause of the biconical outflow cannot be inferred from the ZIMPOL data, but we speculate that it might be the consequence of a circumstellar disc or of the action of strong magnetic fields. The dust grains polarise light more efficiently at 0.65 μm for R Crt and SWVir and at 0.82 μm for WHya. This indicates that at the time of the observations, the grains around SWVir and R Crt had sizes <0.1 μm, while those aroundWHya were larger, with sizes &0.1 μm. The asymmetric distribution of dust around R Crt makes the interpretation more uncertain for this star, however. We find that polarised light is produced already from within the visible photosphere of W Hya, which we reproduce using models with an inner dust shell that is optically thick to scattering. We fit radiative transfer models to the radial profile of the polarised light observed around WHya and find a steep dust density profile, with steepness varying considerably with direction. We find the wind-acceleration region of WHya to extend to at least ~7 R. This is in agreement with theoretical predictions of wind acceleration up to ~12 R, and highlights that ZIMPOL observations probe the crucial region around AGB stars where dust forms and is accelerated. [ABSTRACT FROM AUTHOR]

Context. Low- and intermediate-mass stars go through a period of intense mass-loss at the end of their lives, during the asymptotic giant branch (AGB) phase. While on the AGB a significant part, or even most, of their initial mass is expelled in a stellar wind. This process controls the final stages of the evolution of these stars and contributes to the chemical evolution of galaxies. However, the wind-driving mechanism of AGB stars is not yet well understood, especially so for oxygen-rich sources. Characterizing both the present-day mass-loss rate and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. Aims. We study the dusty wind of the oxygen-rich AGB star WHya to understand its composition and structure and shed light on the mass-loss mechanism. Methods. We modelled the dust envelope of WHya using an advanced radiative transfer code. We analysed our dust model in the light of a previously calculated gas-phase wind model and compared it with measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. Results. We find that the dust spectrum of WHya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al2O3 emission. The composition of the large (∼0.3 μm) grains needed to explain the scattered light cannot be constrained, but probably is dominated by silicates. Silicate emission in the thermal infrared was found to originate from beyond 40 AU from the star. In our model, the silicates need to have substantial near-infrared opacities to be visible at such large distances. The increase in near-infrared opacity of the dust at these distances roughly coincides with a sudden increase in expansion velocity as deduced from the gas-phase CO lines. The dust envelope of WHya probably contains an important amount of calcium but we were not able to obtain a dust model that reproduces the observed emission while respecting the limit set by the gas mass-loss rate. Finally, the recent mass loss of WHya is confirmed to be highly variable and we identify a strong peak in the mass-loss rate that occurred about 3500 years ago and lasted for a few hundred years. [ABSTRACT FROM AUTHOR]

Context. The evolution of low- and intermediate-mass stars on the asymptotic giant branch (AGB) is mainly controlled by the rate at which these stars lose mass in a stellar wind. Understanding the driving mechanism and strength of the stellar winds of AGB stars and the processes enriching their surfaces with products of nucleosynthesis are paramount to constraining AGB evolution and predicting the chemical evolution of galaxies. Aims. In a previous paper we have constrained the structure of the outflowing envelope of W Hya using spectral lines of the 12CO molecule. Here we broaden this study by including an extensive set of H2O and 28SiO lines. It is the first time such a comprehensive study is performed for this source. The oxygen isotopic ratios and the 28SiO abundance profile can be connected to the initial stellar mass and to crucial aspects of dust formation at the base of the stellar wind, respectively. Methods. We model the molecular emission observed by the three instruments on board Herschel Space Observatory using a stateof- the-art molecular excitation and radiative transfer code. We also account for the dust radiation field in our calculations. Results. We find an H2O ortho-to-para ratio of 2.5 -1.0+2.5, consistent with what is expected for an AGB wind. The O16/O17 ratio indicates that W Hya has an initial mass of about 1.5 M☉. Although the ortho- and para-H2O lines observed by HIFI appear to trace gas of slightly different physical properties, we find that a turbulence velocity of 0.7 ± 0.1 kms-1 fits the HIFI lines of both spin isomers and those of 28SiO well. Conclusions. The modelling of H2O and 28SiO confirms the properties of the envelope model of W Hya, as derived from 12CO lines, and allows us to constrain the turbulence velocity. The ortho- and para-H216 O and 28SiO abundances relative to H2 are (6-2+3) × 10-4, (3-1+2) × 10-4, and (3.3 ± 0.8) × 10-5, respectively, in agreement with expectations for oxygen-rich AGB outflows. Assuming a solar silicon-to-carbon ratio, the 28SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles. [ABSTRACT FROM AUTHOR]