Three-component modeling of C-rich AGB star winds

Sandín, C.; Höfner, Susanne

doi:10.1051/0004-6361:20030515

Cited by 28 publications

(29 citation statements)

References 16 publications

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“…(iii) The approximation of a complete momentum and position coupling of gas and dust (i.e., absence of drift) and the approximation of the small-particle limit (or Rayleigh limit) in the determination of the dust grain opacity in the dynamic computations: gas and dust decouple when the gas of the wind is diluted as is the case for low mass-loss rate stars like R Scl. Sandin & Höfner (2003 find that decoupling the dust and gas phase increases the dust formation, which could reconcile the dynamic model with the midinfrared spectro-interferometric measurements. Similar arguments, regarding the difficulty of producing low massloss rate models, have also been pointed out by Mattsson et al (2010).…”

Section: Comparison Of the Dynamic Modeling Results To The Time-depensupporting

confidence: 66%

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Sacuto

Aringer

Hron

et al. 2010

A&A

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Context. We study the circumstellar environment of the carbon-rich star R Sculptoris using the near-and mid-infrared high spatial resolution observations from the ESO-VLTI focal instruments VINCI and MIDI, respectively. Aims. These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Methods. We first compare the spectro-interferometric measurements of the star at different epochs to detect the dynamic signatures of the circumstellar structures at different spatial and spectral scales. We then interpret these data using a self-consistent dynamic model atmosphere to discuss the dynamic picture deduced from the observations. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort is first applied to determining these parameters. Results. Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 μm. The model agrees well with the time-dependent flux data at 8.5 μm, whereas it is too faint at 11.3 and 12.5 μm. The VINCI visibility measurements are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C 2 H 2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Conclusions. Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The transition from windless atmospheres to models with considerable mass-loss rates occurs in a very narrow range of stellar parameters, especially for the effective temperature, the C/O ratio, and the pulsation amplitude. A denser sampling of such critical regions of the parameter space with additional models might lead to a better representation of the extended structures of low mass-loss carbon stars like R Sculptoris. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data.

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Section: Comparison Of the Dynamic Modeling Results To The Time-depensupporting

confidence: 66%

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Sacuto

Aringer

Hron

et al. 2010

A&A

Self Cite

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show abstract

“…However, this strong coupling between the dust and the gas phase is not obvious. In a previous attempt to relax this phase coupling approximation, Sandin & Höfner (2003 found that the effects of decoupling the phases might be quite significant. The most striking feature is that the dust formation may increase significantly, but this does not necessarily increase the predicted mass loss rates for a given set of stellar parameters.…”

Section: Dust Formationmentioning

confidence: 99%

“…Grain size appears to be a critical issue for M-type stars (Höfner 2008) and it may be of some importance also for the winds of carbon stars. It is known that drift does have an effect on the wind properties (see, e.g., Sandin & Höfner 2003, but it has not been studied for the frequency-dependent case together with a detailed description of dust condensation in time-dependent wind models.…”

Section: Uncertaintiesmentioning

confidence: 99%

Dust driven mass loss from carbon stars as a function of stellar parameters

Mattsson¹,

Wahlin²,

Höfner³

2010

A&A

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121

182

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Context. Knowing how the mass loss of carbon-rich AGB stars depends on stellar parameters is crucial for stellar evolution modelling, as well as for the understanding of when and how circumstellar structures emerge around these stars, e.g., dust shells and so-called detached shells of expelled gas. Aims. The purpose of this paper is to explore the stellar parameter space using a numerical radiation hydrodynamic (RHD) model of carbon-star atmospheres, including a detailed description of dust formation and frequency-dependent radiative transfer, in order to determine how the mass loss of carbon stars changes with stellar parameters. Methods. We have computed a grid of 900 numeric dynamic model atmospheres (DMAs) using a well-tested computer code. This grid of models covers most of the expected combinations of stellar parameters, which are made up of the stellar temperature, the stellar luminosity, the stellar mass, the abundance of condensible carbon, and the velocity amplitude of the pulsation. Results. The resultant mass-loss rates and wind speeds are clearly affected by the choice of stellar temperature, mass, luminosity and the abundance of available carbon. In certain parts of the parameter space there is also an inevitable mass-loss threshold, below which a dust-driven wind is not possible. Contrary to some previous studies, we find a strong dependence on the abundance of free carbon, which turns out to be a critical parameter. Furthermore, we have found that the dust grains that form in the atmosphere may grow too large for the commonly used small particle approximation of the dust opacity to be strictly valid. This may have some bearing on the wind properties, although further study of this problem is needed before quantitative conclusions can be drawn. Conclusions. The wind properties show relatively simple dependences on stellar parameters above the mass-loss threshold, while the threshold itself is of a more complicated nature. Hence, we chose not to derive any simplistic mass-loss formula, but rather provide a mass-loss prescription in the form of an easy-to-use FORTRAN routine (available at http://coolstars.astro.uu.se). Since this mass-loss routine is based on data coming from an essentially self-consistent model of mass loss, it may therefore serve as a better mass-loss prescription for stellar evolution calculations than empirical formulae. Furthermore, we conclude that there are still some issues that need to be investigated, such as the rôle of grain-sizes.

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“…Hence, radiative pressure acting on dust particles could accelerate the grain, and, if the gas and the dust were dynamically coupled, grains could drag the gas and provide the mass loss. However, this mechanism fails when gas and dust are not well coupled (Sandin & Höfner 2003). Another failing aspect of this mechanism is that there are observations attesting that the wind forms before the grain formation point (Carpenter et al 1995).…”

Section: Introductionmentioning

confidence: 99%

The Effects of Alfven Waves and Radiation Pressure in Dusty Winds of Late‐Type Stars. II. Dust‐Cyclotron Damping

Vidotto¹,

Jatenco‐Pereira²

2006

ApJ

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There are in the literature several theories to explain the mass loss in stellar winds. In particular, for late-type stars, some authors have proposed a wind model driven by an outward-directed flux of damped Alfvén waves. The winds of these stars present great amounts of dust particles that, if charged, can give rise to new wave modes or modify the preexisting ones. In this work, we study how dust can affect the propagation of Alfvén waves in these winds, taking into account a specific damping mechanism, dust-cyclotron damping. This damping affects the Alfvén wave propagation near the dust-cyclotron frequency. Hence, if we assume a dust size distribution, the damping occurs over a broad band of wave frequencies. In this work, we present a model of Alfvén wave-driven winds using the dust-cyclotron damping mechanism. On the basis of coronal holes in the Sun, which present a superradial expansion, our model also assumes a diverging geometry for the magnetic field. Thus, the mass, momentum, and energy equations are obtained and then solved in a self-consistent approach. Our results of wind velocity and temperature profiles for a typical K5 supergiant star show compatibility with observations. We also show that, considering the presence of charged dust particles, the wave flux is less damped due to dust-cyclotron damping than it would be if we were to consider some other damping mechanisms studied in the literature, such as nonlinear damping, resonant surface damping, and turbulent damping.

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Three-component modeling of C-rich AGB star winds

Cited by 28 publications

References 16 publications

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Dust driven mass loss from carbon stars as a function of stellar parameters

The Effects of Alfven Waves and Radiation Pressure in Dusty Winds of Late‐Type Stars. II. Dust‐Cyclotron Damping

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