[Paper Review] 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

K. Ohnaka, G. Weigelt+1 more|arXiv (Cornell University)|Nov 14, 2016

Stellar, planetary, and galactic studies1 references19 citations

TL;DR

This study uses VLT/SPHERE-ZIMPOL polarimetry and VLTI/AMBER interferometry to reveal time-variable clumpy dust clouds and an extended atmosphere around the AGB star W Hydrae at minimum and pre-maximum light. It finds that small dust grains (0.1 μm) dominate at minimum light, contrasting with larger grains (0.5 μm) at pre-maximum, suggesting grain growth is ongoing during pulsation cycles.

ABSTRACT

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.

Motivation & Objective

• To investigate time variations in the clumpy dust structures and extended atmosphere of the AGB star W Hydrae across different pulsation phases.
• To determine the grain size distribution and composition in the circumstellar environment using high-spatial-resolution polarimetric and interferometric observations.
• To understand the link between pulsation-driven shocks, dust formation, and grain growth in the inner circumstellar envelope.
• To compare the physical conditions and dust properties at minimum light (phase 0.54) and pre-maximum light (phase 0.92) to infer evolutionary trends in dust formation.

Proposed method

• Acquired high-spatial-resolution visible polarimetric images of W Hya with VLT/SPHERE-ZIMPOL at three continuum wavelengths (645, 748, 820 nm), Hα (656.3 nm), and TiO (717 nm) at minimum light (phase 0.54).
• Performed high-spectral-resolution long-baseline interferometry with VLTI/AMBER in the CO first overtone region near 2.3 μm with a spectral resolution of 12,000.
• Fitted the AMBER continuum visibilities with a power-law-type limb-darkened disk model to derive the effective stellar disk size and limb-darkening parameter.
• Conducted 2D Monte Carlo radiative transfer modeling to interpret the SPHERE-ZIMPOL polarized intensity maps, assuming a dust shell with specific grain sizes and optical depths.
• Compared the observed polarization and intensity structures with models using Al2O3, Mg2SiO4, and MgSiO3 grains to constrain composition and grain size distribution.
• Analyzed time variations in Hα and TiO emission morphology and polarization between the two epochs to assess dynamical changes in the extended atmosphere.

Experimental results

Research questions

• RQ1 How do the morphology and polarization of dust clouds around W Hya vary between minimum light and pre-maximum light?
• RQ2 What is the characteristic grain size of dust in the inner circumstellar environment at minimum light, and how does it compare to the pre-maximum phase?
• RQ3 To what extent does the extended atmosphere, traced by Hα and TiO emission, exhibit time variability between the two pulsation phases?
• RQ4 How does the limb-darkening of the stellar disk change between minimum and pre-maximum light, and what does this imply about the atmospheric structure?
• RQ5 What is the physical mechanism driving the observed time variations in dust cloud distribution and polarization?

Key findings

• SPHERE-ZIMPOL detected three clumpy dust clouds at 34–50 mas (1.4–2.0 R⋆) at minimum light, with clear time variations: one new cloud formed and one previously detected cloud disappeared compared to the pre-maximum epoch.
• The degree of linear polarization at minimum light ranged from 13% to 18%, higher than at pre-maximum light, indicating stronger scattering or more asymmetric dust distribution.
• Hα and TiO emission extended to ~150 mas (~6 R⋆), and their morphology showed time variations between the two epochs, suggesting dynamic changes in the extended atmosphere.
• VLTI/AMBER data fitted with a power-law limb-darkened disk model yielded a limb-darkened disk diameter of 49.1 ± 1.5 mas and a limb-darkening parameter of 1.16 ± 0.49, indicating a more extended and less limb-darkened atmosphere at minimum light.
• Monte Carlo radiative transfer modeling of the SPHERE-ZIMPOL data at minimum light best fits a dust shell with 0.1 μm grains of Al2O3, Mg2SiO4, or MgSiO3, with a 550 nm optical depth of 0.6 ± 0.2 and radii of 1.3 R⋆ and 10 ± 2 R⋆.
• The model results indicate a clear contrast in grain size: 0.1 μm grains dominate at minimum light, whereas 0.5 μm grains were dominant at pre-maximum light, suggesting grain growth occurs during the pulsation cycle.