Dynamic model atmospheres of AGB stars. III. Effects of frequency-dependent radiative transfer (original) (raw)
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Dynamic model atmospheres of AGB stars I. Atmospheric structure and dynamics
The strong interactions of shock waves caused by stellar pulsation, the formation of molecules and dust grains and a variable radiation field present a considerable challenge when modelling the atmospheres and circumstellar envelopes of pulsating asymptotic giant branch stars. In this paper we present dynamic model atmospheres of long-period variables which allow a consistent computation of near-infrared molec- ular features and their variability with phase. We discuss the effects of grey radiative transfer, of molecular opacities and of shock waves on the atmospheric structures and on the resulting wind properties. We find that the gas absorption coefficient used in the dynamical calculation has a considerable influence on the structure of the atmosphere, the mass loss and the observable spectral features. Therefore, we stress the importance of using reasonable mean gas opacities in grey dynamic models. Most topics discussed in this paper concern both C- and O-rich atmo- spheres but the quantitative results are mainly based on C-rich models. Synthetic spectra resulting from selected C- and O-rich models have been presented and compared to observations in several recent papers. A systematic investigation of observable properties of our C-rich models will be the subject of a second paper in this series.
Dynamical Atmospheres and Winds of AGB Stars
Symposium - International Astronomical Union
We summarize the current status of our latest generation of model atmospheres for pulsating asymptotic giant branch stars, discussing effects of non-grey radiative transfer, dust grain properties and drift between gas and dust on the atmospheric structures and wind characteristics. In addition, we give an overview of the resulting synthetic spectra and how they compare with observations.
Astronomy & Astrophysics, 2004
We have calculated synthetic opacity sampling spectra for carbon-rich Asymptotic Giant Branch (AGB) stars based on dynamic model atmospheres which couple time-dependent dynamics and frequency-dependent radiative transfer, as presented in the third paper of this series. We include the molecules CO, CH, CN, C2, CS, HCN, C2H2 and C3 in our calculations, both when computing the atmospheric structures, and the synthetic spectra. A comparison of the synthetic spectra with various observed colours and spectra in the wavelength range between 0.5 and 25 μm,of TX Psc, WZ Cas, V460 Cyg, T Lyr and S Cep is presented. We obtain good agreement between observations gathered at different phases and synthetic spectra of one single hydrodynamical model for each star in the wavelength region between 0.5 and 5 μm. At longer wavelengths our models showing mass loss offer a first self-consistent qualitative explanation of why a strong feature around 14 μm, which is predicted by all hydrostatic models as well as dynamical models showing no mass loss, is missing in observed AGB carbon star spectra.
2D dynamical models for dust-driven winds of AGB stars
New multi-dimensional models for C-star winds, comprising hydrodynamics with radiation pressure on dust grains, chemistry, time-dependent dust formation and simplified grey radiative transfer are presented. These models reveal an even more complex picture of the dust formation and wind acceleration compared to existing 1D models. Excited by hydrodynamical, radiative and thermal instabilities, dust forms from time to time in limited areas close to the star. These clouds are then accelerated outward by radiation pressure which creates gaps in the dust formation zone. These gaps are not only refilled by matter sliding in from the inside (the very reason for the dust-driven mass loss), but also by matter which is falling back from the outside at different places. A highly dynamical and turbulent dust formation zone is created in this way, whichin return -again creates a strongly inhomogeneous dust distribution. Further away from the star, flow instabilities (e.g. Rayleigh-Taylor) have time to modify and further shape the dust clouds produced in the dust formation zone.
Dynamic model atmospheres of AGB stars II. Synthetic near infrared spectra of carbon stars
We have calculated synthetic opacity sampling spec- tra for carbon-rich Asymptotic Giant Branch (AGB) stars based on dynamic model atmospheres presented in the first paper of this series. We discuss how different model parameters influence the resulting synthetic spectra and how the spectra vary with phase. The molecules included are: CO, CH, CN, C 2 , HCN, C 2 H 2 and C 3 . We show in which atmospheric layers the dif- ferent molecules form, in an attempt to understand the qual- itatively different variation with pulsation phase exhibited by various spectral features. Almost all features are blends of tran- sitions from more than one molecule, and we therefore identify the most important transitions and molecules that contribute to the main spectral features from 0.5 to 12 μm. Furthermore, we demonstrate the effect on the individual spectral features due to the carbon depletion when dust is formed in the atmosphere.
Dust Grain Properties in Atmospheres of AGB Stars
Proceedings of The International Astronomical Union, 2003
We present self-consistent dynamical models for dust driven winds of carbon-rich AGB stars. The models are based on the coupled system of frequency-dependent radiation hydrodynamics and time-dependent dust formation. We investigate in detail how the wind properties of the models are influenced by the micro-physical properties of the dust grains that enter as parameters. The models are now at a level where it is necessary to be quantitatively consistent when choosing the dust properties that enters as input into the models. At our current level of sophistication the choice of dust parameters is significant for the derived outflow velocity, the degree of condensation and the estimated mass loss rates of the models. In the transition between models with and without mass-loss the choice ofmicro-physical parameters turns out to be very significant for whether a particular set of stellar parameters will give rise to a dust-driven mass loss or not.
Spectral energy distributions in presence of shells of dust around AGB stars
We present theoretical Spectral Energy Distributions (SEDs) of Single Stellar Populations (SSPs) of intermediate and old ages, in which the effect of the dust shells en-shrouding Asymptotic Giant Branch (AGB) stars is taken into account. These dust-rich shells are produced by the stellar wind from AGB stars. They absorb the radiation coming from the star underneath and re-emit it in the mid and far infrared (MIR/FIR). To this aim, we follow in detail the evolution of the AGB stars and solve the radiative transfer equa-tion for a realistic model of the dust shell. We show how important features of the SEDs, such as the 9.7µm S i − O and the 11.3µm S iC, evolve with time. The theoretical results are compared to observational data for stars and clusters of the Magellanic Clouds.
Winds of M- and S-type AGB stars: an unorthodox suggestion for the driving mechanism
Astronomy & Astrophysics, 2007
Context. Current knowledge suggests that the dust-driven wind scenario provides a realistic framework for understanding mass loss from C-rich AGB stars. For M-type objects, however, recent detailed models demonstrate that radiation pressure on silicate grains is not sufficient to drive the observed winds, contrary to previous expectations. Aims. In this paper, we suggest an alternative mechanism for the mass loss of M-type AGB stars, involving the formation of both carbon and silicate grains due to non-equilibrium effects, and we study the viability of this scenario. Methods. We model the dynamical atmospheres and winds of AGB stars by solving the coupled system of frequency-dependent radiation hydrodynamics and time-dependent dust formation, using a parameterized description of non-equilibrium effects in the gas phase. This approach allows us to assess under which circumstances it is possible to drive winds with small amounts of carbon dust and to get silicate grains forming in these outflows at the same time. Results. The properties of the resulting wind models, such as mass-loss rates and outflow velocities, are well within the observed limits for M-type AGB stars. Furthermore, according to our results, it is quite unlikely that significant amounts of silicate grains will condense in a wind driven by a force totally unrelated to dust formation, as the conditions in the upper atmosphere and wind acceleration region put strong constraints on grain growth. Conclusions. The proposed scenario provides a natural explanation for the observed similarities in wind properties of M-type and C-type AGB stars and implies a smooth transition for stars with increasing carbon abundance, from solar-composition to C-rich AGB stars, possibly solving the longstanding problem of the driving mechanism for stars with a C/O close to one.
Commission 36: Theory of Stellar Atmospheres
Proceedings of the International Astronomical Union, 2010
Pulsating stars and asteroseismology Classically variable stars, and 'ordinary' solar-type ones. Inverting observed pressure-mode frequencies into atmospheric structure. Mass-loss mechanisms in pulsating stars. Effects of rapid rotation on pulsation.
Shells of dust around AGB stars: Effects on the integrated spectrum of Single Stellar Populations
Astronomy & Astrophysics, 2003
In this paper we present models for Single Stellar Populations (SSPs) of intermediate and old ages where dust enshrouded Asymptotic Giant Branch (AGB) stars are introduced. As long known AGB stars are surrounded by dust-rich shells of matter caused by their own stellar wind, which absorb the radiation coming from the central object and re-emit it in the far infrared (IR). To this aim, particular care is devoted to follow the evolution of the AGB stars throughout the quiet and thermally pulsing regimes, to evaluate the effect of self contamination in the outermost layers by the third dredge-up mechanism, to follow the transition from oxygen-rich to carbon-rich objects (as appropriate to their initial mass and chemical composition), and finally to estimate the efficiency of mass-loss by stellar winds, all aspects that concur to the formation and properties of the dusty shells around. In addition to this, accurate physical models of the dusty shells are presented in which the reprocessing of radiation from the central stars is calculated by solving the radiative transfer equations in presence of dust particles of different chemical composition. The resulting spectral energy distribution (SED) is examined to show how important features, like the 10 µm Si−O stretching mode feature and the 11 µm SiC feature, evolve with time. The SEDs are then convolved with the IRAS filters to obtain the flux in various pass-bands, i.e. 12, 25 and 60 µm, for individual AGB stars of different mass, chemical composition, and age. The comparison is made by means of SSPs along which AGB stars of the same age but different initial masses are located. This allows us to explore the whole range of masses and ages spanned by AGB stars. The theoretical results are compared to the observational data for selected groups of stars. The same is made for the J, H, K, L pass-bands of the Johnson system. Finally, from the integrated SEDs of the SSPs, we derive the integrated Johnson J, H, K, L magnitudes and colors to be compared to infrared data for star clusters of the Magellanic Clouds. In general good agreement with the data is possible if the effects of the circumstellar shells of dust are taken into account.