The mid-infrared diameter of W Hydrae

Zhao-Geisler, R.; Quirrenbach, A.; Köhler, Rainer H.; López, B.; Leinert, Ch.

doi:10.1051/0004-6361/201016310

Cited by 32 publications

(64 citation statements)

References 72 publications

(129 reference statements)

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“…-The lower limit of 500 mas on the inner radius of the silicate emission shell set by Zhao-Geisler et al (2011). -The limits set by the gas-phase model of Paper I and Khouri et al (2014b) on the amount of elements available for dust formation in the wind.…”

Section: Towards a Comprehensive Present-day Mass-loss Modelmentioning

confidence: 99%

“…Although the composition of the dust could not be determined, the authors suggested that they may be composed of iron-free silicates such as forsterite (Mg 2 SiO 4 ) or enstatite (MgSiO 3 ), while corundum (Al 2 O 3 ) is a possible candidate as well. -Geisler et al (2011) monitored W Hya using MIDI at the VLT. The authors found that the silicate emission seen at 10 μm in the ISO spectrum must come from a region with an inner radius larger than 500 mas, corresponding to 25 times the stellar radius or 12 times the scattering shell radius as estimated by Norris et al (2012).…”

Section: Interferometric Observationsmentioning

confidence: 99%

“…We start by modelling the innermost observed dust component, namely the scattered light fractions and the amorphous Al 2 O 3 emission. Then, we model the silicate emission envelope, whose inner radius is at ten times the distance from where the scattered light is seen (Zhao-Geisler et al 2011). Finally, we compare our best inner wind model with the PACS 70 μm image, which provides constraints on both the dust distribution close to the star and at scales up to a few arcminutes (Cox et al 2012).…”

Section: Towards a Comprehensive Present-day Mass-loss Modelmentioning

confidence: 99%

“…These data probe the wind structure of W Hya on spatial scales ranging from tens of mas to minutes of arc and reveal grains with radii of about 0.3 μm at 40 mas (or 2 R ) seen in scattered light (Norris et al 2012); an inner radius for the amorphous silicate emission of 500 mas (or 25 R ; Zhao-Geisler et al 2011), and a mass loss that is variable on dynamical timescales of thousands of years (Hawkins 1990;Cox et al 2012) that resembles the formation of detached-shells (exclusively) seen around carbon-rich AGB stars (e.g. Olofsson et al 1990Olofsson et al , 1996.…”

Section: Introductionmentioning

confidence: 99%

“…Woitke 2006) and observations (e.g. Verhoelst et al 2006;Zhao-Geisler et al 2011). MIDI visibilities of RT Vir, for instance, point to the presence of amorphous Al 2 O 3 close to the star (at ∼1.5 R ; Sacuto et al 2013).…”

Section: Introductionmentioning

confidence: 99%

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Dusty wind of W Hydrae

Khouri

Waters

Koter

et al. 2015

A&A

107

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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 W Hya to understand its composition and structure and shed light on the mass-loss mechanism. Methods. We modelled the dust envelope of W Hya 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 W Hya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al 2 O 3 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 W Hya 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 W Hya 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.

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Section: Towards a Comprehensive Present-day Mass-loss Modelmentioning

confidence: 99%

Section: Interferometric Observationsmentioning

confidence: 99%

Section: Towards a Comprehensive Present-day Mass-loss Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Dusty wind of W Hydrae

Khouri

Waters

Koter

et al. 2015

A&A

107

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Towards continuous viewing of circumstellar maser sources over decades

Imai

2017

Proc. IAU

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The brightness of maser features are fascinating and give valuable insight for circumstellar physics of oxygen-rich, intermediate-mass stars, in particular the final evolution of circumstellar envelopes (CSEs). The variety of accompanying masers such as SiO, H2O, and OH in the CSEs may provide unique probes into different stages of rapid CSE evolution. However, with only sparse monitoring of these masers one can sometimes find it difficult to accurately interpret their spatio-kinematics, origins and excitation mechanisms. Examples can be seen in the variety of proposed models for water masers associated with “water fountains” and for silicon-monoxide masers. In order to better understand these issues, one needs to consider continuous monitoring of the individual maser gas clumps over a few stellar cycles or episodic ejection events. Here I present our previous long-term monitoring observations, especially for the water fountain source W43A. Our current efforts involve programs of intensive monitoring observations of circumstellar maser sources over decadal time periods. These programs with the East Asia VLBI Network observe H2O and SiO maser lines simultaneously mapped at high cadence (2–8 weeks) with VLBI observations.

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Imaging the dust and the gas around Mira using ALMA and SPHERE/ZIMPOL

Khouri¹,

Vlemmings²,

Olofsson³

et al. 2018

Proc. IAU

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The mass-loss mechanism of asymptotic giant branch stars has long been thought to rely on two processes: stellar pulsations and dust formation. The details of the mass-loss mechanism have remained elusive, however, because of the overall complexity of the dust formation process in the very dynamical pulsation-enhanced atmosphere. Recently, our understanding of AGB stars and the associated mass loss has evolved significantly, thanks both to new instruments which allow sensitive and high-angular-resolution observations and the development of models for the convective AGB envelopes and the dust formation process. ALMA and SPHERE/ZIMPOL on the VLT have been very important instruments in driving this advance in the last few years by providing high-angular resolution images in the sub-mm and visible wavelengths, respectively. I will present observations obtained using these instruments at the same epoch (2.5 weeks apart) of the AGB star Mira that resolve even the stellar disk. The ALMA data reveals the distribution and dynamics of the gas around the star, while the polarised light imaged using SPHERE shows the distribution of the dust grains expected to drive the outflows. Moreover, the observations show a central source surrounded by asymmetric distributions of gas and dust, with complementary structures seen in the two components. We model the observed CO v = 1, J = 3−2 line to determine the density, temperature and velocity of gas close to the star. This model is then used to estimate the abundance of AlO. Our results show that only a very small fraction of aluminium (≲0.1%) is locked in AlO molecules. We also calculate models to fit the observed polarised light based on the gas densities we find. The low level of visible-light polarisation detected using ZIMPOL implies that, at the time of the observations, aluminium atoms are either not efficiently depleted into dust or the aluminium-oxide grains are relatively small (≲0.02μm).

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The mid-infrared diameter of W Hydrae

Cited by 32 publications

References 72 publications

Dusty wind of W Hydrae

Dusty wind of W Hydrae

Towards continuous viewing of circumstellar maser sources over decades

Imaging the dust and the gas around Mira using ALMA and SPHERE/ZIMPOL

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