Inner dusty envelope of the AGB stars W~Hydrae, SW\,Virginis, and R~Crateris using SPHERE/ZIMPOL

Abstract

Stars with initial masses between $\sim 0.8$ and 8~$M_{\odot }$ present copious mass loss during the asymptotic giant branch (AGB) at the end of their lives. The accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the extended AGB stellar atmospheres. The details of this process are not yet well understood, however. Using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL, we observed light polarised by grains around W\,Hya, SW\,Vir, and R\,Crt, which have mass-loss rates between 10${}^{-7}$ and 10${}^{-6}{M}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. We find the distribution of dust to be asymmetric around 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 directly inferred from the ZIMPOL data. The dust grains polarise light more efficiently at 0.65~$\mu$m for R\,Crt and SW\,Vir and at 0.82~$\mu$m for W\,Hya. This indicates that at the time of the observations, the grains around SW\,Vir and R\,Crt had sizes $<0.1\mu$m, while those around W\,Hya were larger, with sizes $\gtrsim 0.1\mu$m. 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. The radial profile of the polarised light observed around W\,Hya reveal a steep dust density profile. We find the wind-acceleration region of W\,Hya to extend to at least $\sim 7R_{\star }$, in agreement with theoretical predictions of acceleration up to $\sim 12R_{\star }$.

16 pages, 9 figures, 3 tables

References in corpus (16)

• Radiative transfer in very optically thick circumstellar disks
• Too little radiation pressure on dust in the winds of oxygen-rich AGB stars
• A close halo of large transparent grains around extreme red giant stars
• The impact of companions on stellar evolution
• Isolated vs. Common Envelope Dynamos in Planetary Nebula Progenitors
• ALMA observations of the nearby AGB star L2 Puppis - I. Mass of the central star and detection of a candidate planet
• Exploring wind-driving dust species in cool luminous giants I. Basic criteria and dynamical models of M-type AGB stars
• Exploring wind-driving dust species in cool luminous giants III. Wind models for M-type AGB stars: dynamic and photometric properties
• The wind of W Hya as seen by Herschel - I. The CO envelope
• 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
• Exploring the origin of clumpy dust clouds around cool giants. A global 3D RHD model of a dust-forming M-type AGB star
• The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA
• An observational study of dust nucleation in Mira ($o$ Ceti): II. Titanium oxides are negligible for nucleation at high temperatures
• The wind of W Hya as seen by Herschel. II. The molecular envelope of W Hya
• Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss
• Tomography of silicate dust around M-type AGB stars I. Diagnostics based on dynamical models

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