Dusty wind of W Hydrae

Abstract

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.

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

“…However, the angular displacement expected over the time interval of 8.5 months between our two-epoch observations from the expansion velocity exp (km s −1 ) and the distance of 78 pc is 1.9 × exp mas. Since exp is expected to be smaller than the terminal velocity of 7.5 km s −1 (Khouri et al 2015), the angular displacement is smaller than 14 mas. This means that we cannot explain the detection of polarization in a more extended region in 2016 by means of the expansion of the dust clouds detected in 2015.…”

“…In Paper I, we adopted an effective temperature of 2500 K based on the modeling of the spectral energy distribution (SED) of W Hya carried out by Khouri et al (2015) and derived a luminosity of 5130 K by combining the effective temperature with the angular diameter measured with VLTI/AMBER and the distance of 78 pc. However, now that we have SPHERE-ZIMPOL data taken at two distinct phases, it is necessary to use the effective temperature and luminosity appropriate for each epoch.…”

A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling

“…The momentum equation for IK Tau on the other hand gives β = 0.6 (Decin et al 2010c). Khouri et al (2014a) adopt a β = 1.5 to describe the velocity profile in W Hya, and find evidence that higher values may fit the high-J transitions of CO better. Note that using the upper-state energy E up as a measure of the distance in the CSE and the line width to constrain the velocity profile as done by Justtanont et al (2012) can be misleading.…”

“…Observations of molecular emission lines have been used to determine these properties. In particular, observations of CO emission lines have proven to be a useful tool to describe the mass loss (e.g., Schöier et al 2002;González Delgado et al 2003;Ramstedt et al 2008;De Beck et al 2010;Khouri et al 2014a). Observations of other molecules have led to important insights in the chemistry and structure of AGB CSEs (e.g., González Delgado et al 2003;Schöier et al 2007;Maercker et al 2008Maercker et al , 2009Decin et al 2010a;Cernicharo et al 2010Cernicharo et al , 2011De Beck et al 2012;Justtanont et al 2012;Khouri et al 2014b; Lombaert et al 2016).…”

Study of the aluminium content in AGB winds using ALMA

“…Each target has a very different dust emission spectrum (see Sect. 2.3); the spectrum of IK Tau is dominated by amorphous olivine dust (magnesium-iron silicates (Mg 2−x ,Fe x )SiO 4 , with 0 ≤ x ≤ 2), while the infrared spectrum of R Dor does not show the presence of olivine dust but species such as corundum (an aluminium oxide, Al 2 O 3 ), the aluminiumcalcium bearing silicate gehlenite, the magnesium-aluminium member spinel, and CO 2 gas have been detected (Heras & Hony 2005;Khouri 2014). This supports the hypothesis that the dust condensation sequence in the low mass-loss rate object R Dor has experienced a process called "freeze-out" (Tielens 1990), where the sequence is not completed and stops at an intermediate product when the wind density drops below the density required for the next step; in this particular case the freeze-out occurs after the formation of aluminium oxide with only a small amount of silicon condensing into dust grains.…”