Dust formation in winds of long-period variables

Andersen, Anja C.; Höfner, Susanne; Gautschy-Loidl, R.

doi:10.1051/0004-6361:20030036

Cited by 38 publications

(44 citation statements)

References 28 publications

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“…4.3.3). The AmC dust grain optical constants from Rouleau & Martin (1991) are the same as adopted for the hydrodynamic calculation (Andersen et al 2003;Höfner et al 2003). Good agreement is also found with the works of Dehaes et al (2007) and Lorenz-Martins et al (2001), who determined a visual optical depth of 0.5 +0.2 −0.3 and a 1.0 μm optical depth of 0.1, respectively.…”

Section: Determination Of the Sic Dust Parameterssupporting

confidence: 77%

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: Determination Of the Sic Dust Parameterssupporting

confidence: 77%

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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“…However, as we shall see, this approximation may need to be relaxed, since the carbon grains tend to grow quite rapidly. The models in this paper are calculated using the refractive index data of Rouleau & Martin (1991) to obtain the dust extinction (see Andersen et al 2003;Höfner et al 2003, for further discussion). The intrinsic density of the grain material used in the model is set to ρ gr = 1.85 g cm −3 , which matches the material in Rouleau & Martin (1991).…”

Section: Dust Formationmentioning

confidence: 99%

“…We assume that dust grains can be considered to be spherical, consisting of amorphous carbon only, since we are modelling carbon stars (see Andersen et al 2003, for details about these assumptions). The nucleation, growth and evaporation of grains is assumed to proceed by reactions involving C, C 2 , C 2 H and C 2 H 2 .…”

Section: Dust Formationmentioning

confidence: 99%

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

Mattsson¹,

Wahlin²,

Höfner³

2010

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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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“…These stars, including Mira type stars, exhibit many complex processes such as shock fronts propagating through the stellar atmosphere, largeamplitude pulsation, and molecule and dust formation leading to strong mass loss via a dense and dusty outflow from an extended stellar atmosphere (Andersen et al 2003). The mass loss rates can reach up to 10 −4 M yr −1 (Matsuura et al 2009) with expansion velocities of 5-30 km s −1 (Höfner & Andersen 2007).…”

Section: Introductionmentioning

confidence: 99%

Mid-infrared interferometric monitoring of evolved stars

Karovicova

Wittkowski²,

Boboltz³

et al. 2011

A&A

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Aims. We present a unique multi-epoch infrared interferometric study of the oxygen-rich Mira variable RR Aql in comparison to radiative transfer models of the dust shell. We investigate flux and visibility spectra at 8-13 μm with the aim of better understanding the pulsation mechanism and its connection to the dust condensation sequence and mass-loss process. Methods. We obtained 13 epochs of mid-infrared interferometry with the MIDI instrument at the VLTI between April 2004 and July 2007, covering minimum to pre-maximum pulsation phases (0.45-0.85) within four cycles. The data are modeled with a radiative transfer model of the dust shell where the central stellar intensity profile is described by a series of dust-free dynamic model atmospheres based on self-excited pulsation models. We examined two dust species, silicate and Al 2 O 3 grains. We performed model simulations using variations in model phase and dust shell parameters to investigate the expected variability of our mid-infrared photometric and interferometric data. Results. The observed visibility spectra do not show any indication of variations as a function of pulsation phase and cycle. The observed photometry spectra may indicate intracycle and cycle-to-cycle variations at the level of 1-2 standard deviations. The photometric and visibility spectra of RR Aql can be described well by the radiative transfer model of the dust shell that uses a dynamic model atmosphere describing the central source. The best-fitting model for our average pulsation phase of Φ V = 0.64 ± 0.15 includes the dynamic model atmosphere M21n (T model = 2550 K) with a photospheric angular diameter of θ Phot = 7.6 ± 0.6 mas, and a silicate dust shell with an optical depth of τ V = 2.8 ± 0.8, an inner radius of R in = 4.1 ± 0.7 R Phot , and a power-law index of the density distribution of p = 2.6 ± 0.3. The addition of an Al 2 O 3 dust shell did not improve the model fit. However, our model simulations indicate that the presence of an inner Al 2 O 3 dust shell with lower optical depth than for the silicate dust shell can not be excluded. The photospheric angular diameter corresponds to a radius of R phot = 520 +230 −140 R and an effective temperature of T eff ∼ 2420 ± 200 K. Our modeling simulations confirm that significant intracycle and cycle-to-cycle visibility variations are not expected for RR Aql at mid-infrared wavelengths within our uncertainties. Conclusions. We conclude that our RR Aql data can be described by a pulsating atmosphere surrounded by a silicate dust shell. The effects of the pulsation on the mid-infrared flux and visibility values are expected to be less than about 25% and 20%, respectively, and are too low to be detected within our measurement uncertainties.

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Dust formation in winds of long-period variables

Cited by 38 publications

References 28 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

Mid-infrared interferometric monitoring of evolved stars

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