[Paper Review] Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss

T. Khouri, L. B. F. M. Waters+6 more|arXiv (Cornell University)|Mar 16, 2015

Stellar, planetary, and galactic studies55 references19 citations

TL;DR

This study models the dusty wind of the oxygen-rich AGB star W Hydrae using multi-wavelength observations and radiative transfer to constrain dust composition, structure, and mass-loss history. It identifies a gravitationally bound dust shell dominated by amorphous Al2O3 explaining infrared excess and scattered light, with silicate emission originating beyond 40 AU due to enhanced near-infrared opacity, and reveals a recent mass-loss peak ~3,500 years ago lasting ~300 years.

ABSTRACT

Low- and intermediate-mass stars go through a period of intense mass-loss at the end of their lives in a phase known as the asymptotic giant branch (AGB). During the AGB a significant fraction of their initial mass is expelled in a stellar wind. This process controls the final stages of their evolution 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 and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. We modelled the dust envelope of W Hya using an advanced radiative transfer code. The dust model was analysed in the light of a previously calculated gas-phase wind model and compared to measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. 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 ($\sim$\,0.3\,$μ$m) 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 and we find that they 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. 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.

Motivation & Objective

• To understand the dust composition and structure in the wind of W Hydrae, an oxygen-rich AGB star.
• To constrain the mass-loss rate history of W Hya using multi-wavelength observations.
• To investigate the role of dust species such as Al2O3 and silicates in driving the stellar wind.
• To reconcile observed infrared spectra, scattered light fractions, and dust images with a physically consistent dust model.
• To assess the contribution of calcium-bearing dust species and their opacity properties in the wind.

Proposed method

• Employed an advanced radiative transfer code to model the dust envelope of W Hya across multiple wavelengths.
• Combined constraints from ISO infrared spectra, PACS 70 µm images, MIDI interferometric data, and scattered light fractions.
• Used a gravitationally bound dust shell (GBDS) model with a modified Rice-Nordheim (MRN) size distribution for amorphous Al2O3 particles.
• Incorporated gas-phase wind models from prior work (Paper I) to ensure consistency with observed CO line profiles and mass-loss rates.
• Explored dust opacity variations, particularly near-infrared opacity enhancements in silicates, to explain emission at large radii.
• Tested calcium-bearing dust species using available optical constants, though limited data prevented precise quantification.

Experimental results

Research questions

• RQ1 What is the dominant dust species responsible for the observed infrared excess and scattered light in W Hya’s wind?
• RQ2 How does the dust emission structure relate to the kinematics and mass-loss history inferred from CO lines?
• RQ3 Can the observed silicate emission at large distances (beyond 40 AU) be explained by dust opacity properties and grain growth?
• RQ4 What is the role of calcium in the dust composition, and can its abundance be constrained by the observed emission?
• RQ5 What is the temporal evolution of the mass-loss rate in W Hya, and when did the most recent peak occur?

Key findings

• The dust spectrum of W Hya is best explained by a gravitationally bound dust shell (GBDS) dominated by amorphous Al2O3, which accounts for the observed infrared excess and scattered light fractions.
• Silicate emission in the thermal infrared originates from beyond 40 AU, requiring substantial near-infrared opacity to be visible at such large distances.
• The increase in near-infrared opacity of silicates at large radii coincides with a sudden rise in expansion velocity inferred from CO line profiles.
• The model indicates that calcium is likely a component of the dust, though optical constants for calcium-bearing species are limited, making precise abundance quantification difficult.
• The recent mass-loss history of W Hya shows a peak approximately 3,500 years ago, lasting for about 300 years, with a rate up to 20 times higher than the current level.
• A prior phase of lower mass-loss rate (factor of two below present) lasted for about 2,500 years, producing dust now located between 500 and 4,000 AU.