Too little radiation pressure on dust in the winds of oxygen-rich AGB stars

By Peter Woitke. 2006

164 citations recorded by the source corpus.

Aims. It is commonly assumed that the massive winds of AGB stars are dust-driven and pulsation-enhanced. However, detailed frequency-dependent dynamical models that can explain the observed magnitudes of mass loss rates and outflow velocities have been published so far only for C-stars. This letter reports on first results of similar models for oxygen-rich AGB stars. The aim is to provide a better understanding of the wind driving mechanism, the dust condensation sequence, and the role of pulsations. Methods. New dynamical models for dust-driven winds of oxygen-rich AGB stars are presented which include frequency-dependent Monte Carlo radiative transfer by means of a sparse opacity distribution technique and a time-dependent treatment of the nucleation, growth and evaporation of inhomogeneous dust grains composed of a mixture of Mg 2 SiO 4 , SiO 2 , Al 2 O 3 , TiO 2 , and solid Fe. Results. The frequency-dependent treatment of radiative transfer reveals that the gas is cold close to the star ($700{-}900\,$K at $1.5{-}2\,R_\star$) which facilitates the nucleation process. The dust temperatures are strongly material-dependent, with differences as large as 1000 K for different pure materials, which has an important influence on the dust formation sequence. Two dust layers are formed in the dynamical models: almost pure glassy Al 2 O 3 close to the star ($r \ga 1.5\,R_\star$) and the more opaque Fe-poor Mg-Fe-silicates further out. Solid Fe and Fe-rich silicates are found to be the only condensates that can efficiently absorb the stellar light in the near IR. Consequently, they play a key role in the wind driving mechanism and act as a thermostat. Only small amounts of Fe can be incorporated into the grains, because otherwise the grains become too hot. Thus, the models reveal almost no mass loss, and no dust shells. Conclusions. The observed dust sequence Al 2 O 3 $\to$ Fe-poor Mg-Fe-silicates for oxygen-rich AGB stars having low $\to$ high mass loss rates is in agreement with the presented model and can be understood as follows: Al 2 O 3 is present in the extended atmosphere of the star below the wind acceleration region, also without mass loss. The Mg-Fe-silicates form further out and, therefore, their amount depends on the mass loss rate. The driving mechanism of oxygen-rich AGB stars is still an unsolved puzzle.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/55ac50c2d5840948932977dd578579ad673c5e8a

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In Astronomy and Astrophysics

1. C. Conroy
2. L. Girardi
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4. P. Marigo
5. H. Olofsson
6. Dennis Zaritsky
7. S. Höfner
8. Michela Mapelli

Citations in the corpus

• Radiation thermo-chemical models of protoplanetary disks I. Hydrostatic disk structure and inner rim (2009, background): “…will be present which have not only different and pressure-dependent sublimation temperatures, but the dust temperatures are strongly dependent on dust kind due to dust opacity effects (see Woitke, 2006), which can be expected to result in a highly complex chemical structure of the inner rim.”
• Dust formation in the oxygen-rich AGB star IK Tauri (2015, methodology): “…that the growth of dust grains proceeds through surface deposition of atoms and molecules, and the final size of grains is controlled by accretion and evaporation under equilibrium conditions (Gail & Sedlmayr 1999; Ferraroti & Gail 2002; 2006; Woitke 2006; Höfner 2008; Dell’Agli et al. 2014).”
• Evolution of asymptotic giant branch stars. II. Optical to far-infrared isochrones with improved TP- (2007, background): “Instead in O-rich stars the possible type of grains are either not stable or opaque enough near∼ 1µm to explain the outflows (e.g. Ireland & Scholz 2006; Woitke 2006).”
• SURVEYING THE AGENTS OF GALAXY EVOLUTION IN THE TIDALLY STRIPPED, LOW METALLICITY SMALL MAGELLANIC CLOUD (SAGE-SMC). II. COOL EVOLVED STARS (2011, background): “Pulsations can levitate material from the stellar surface and provide density enhancements and shocks, which can encourage dust formation and re-processing (e.g., Bowen 1988; Winters et al. 2000, 2003; Schirrmacher et al. 2003; Woitke 2006, 2007; Mattsson et al. 2008; van Loon et al. 2008b).”
• Mass loss of stars on the asymptotic giant branch (2018, background): “Throughout the grain growth region, the Mg/Fe ratio is controlled by a self-regulating feedback between Fe-inclusion and resulting radiative heating (Woitke 2006).”
• Production of dust by massive stars at high redshift (2011, background): “12 In the case of M-stars the oxygen environment leads to formation of preferentially Fefree silicates (Woitke, 2006; Höfner and Andersen, 2007),in particular olivine and pyroxene type grains.”
• Equilibrium chemistry down to 100 K (2017, background): “…of gaseous molecules are calculated in thermo-chemical equilibrium (e.g. Gail et al. 1984; Sedlmayr & Dominik 1995; Woitke & Helling 2004; Woitke 2006; Höfner & Andersen 2007; Höfner et al. 2016; Helling & Fomins 2013), following the idea that dust formation is by far the slowest…”