W. Windsteig

University of Vienna, Wien, Vienna, Austria

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Publications (17)10.17 Total impact

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    ABSTRACT: Pulsation influences atmospheric structures of variable AGB stars (Miras) considerably. Spectral lines of the CO \varDelta v\varDelta v =3 vibration–rotation bands (at ≈ 1.6 μm) therefore have a very characteristic appearance in time series of high-resolution spectra. Coupled to the light cycle they can be observed blue- or red-shifted, for some phases even line doubling is found. This is being explained by radial pulsations and shock fronts emerging in the atmospheres. Based on dynamic model atmospheres synthetic CO line profiles were calculated consistently, reproducing this scenario qualitatively.
    09/2006: pages 281-284;
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    ABSTRACT: Atmospheres of evolved AGB stars are heavily affected by pulsation, dust formation and mass loss, and they can become very extended. Time series of observed high-resolution spectra proved to be a useful tool to study atmospheric dynamics throughout the outer layers of these pulsating red giants. Originating at various depths, different molecular spectral lines observed in the near-infrared can be used to probe gas velocities there for different phases during the lightcycle. Dynamic model atmospheres are needed to represent the complicated structures of Mira variables properly. An important aspect which should be reproduced by the models is the variation of line profiles due to the influence of gas velocities. Based on a dynamic model, synthetic spectra (containing CO and CN lines) were calculated, using an LTE radiative transfer code that includes velocity effects. It is shown that profiles of lines that sample different depths qualitatively reproduce the behaviour expected from observations. Comment: accepted by A&A, 12 pages, 9 figures
    Astronomy and Astrophysics 03/2005; · 5.08 Impact Factor
  • Proceedings of the International Astronomical Union 01/2003; 210.
  • Proceedings of the International Astronomical Union 01/2000; 177.
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    ABSTRACT: The ISO-SWS spectra of several representative carbon- and oxygen-rich AGB variables (R Aql, R Cas, T Dra, V Cyg, V CrB and R Scl) were obtained at several phases of their pulsational cycle. This allows us to study the molecular features and the dust emission of these objects as a function of stellar parameters as well as pulsational phase. The observational data are compared with synthetic molecular spectra based on hydrostatic models (MARCS) and on dynamical atmospheres, which include such phenomena as pulsation, mass loss and dust formation. Some aspects of the SWS spectra like their variations can only be explained by dynamical atmospheres. To be able to analyse also the targets with significant mass loss, we also included a dust component in the synthetic spectra. On the basis of these synthetic spectra we also investigate the influence of molecular features on the interpretation of the dust emission and vice versa. This is of special importance in the 8-16 μm region.
    02/1999; 427:365.
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    ABSTRACT: We present ISO-SWS observations of cool oxygen-rich AGB variables and compare them to synthetic molecular and dust spectra based on hydrostatic and dynamic model atmospheres.
    Proceedings of the International Astronomical Union 01/1999; 191:169.
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    ABSTRACT: The first overtone rotation-vibration transitions of SiO give rise to prominent bandheads in the wavelength range between 4.0 and 4.5 mu m. In order to study the behaviour of these features in AGB stars we observed the 3.94 to 4.12 mu m spectra for a sample of 23 oxygen-rich late-type variables. In contrast to the SRb objects, the Miras show a very large scatter of the equivalent widths of the SiO bands. Despite their cool temperatures some of them have only weak or no SiO absorption, which seems to be related to their strong pulsations producing a large variability of the features. When comparing the band intensities with photometric data, we found a general decrease with bluer IRAS (12-25) colors. However, this trend may only reflect the different behaviour of the Miras and SRb stars in our sample. We did not discover any correlation of the equivalent widths with the effective temperatures derived from (J-K), or with the (K-12) color and the IRAS-LRS class, both of which can be regarded as a rough measure for the thickness of the circumstellar shell. In Paper I of this series (Aringer et al. \cite{siop}) we have shown that synthetic spectra calculated from hydrostatic MARCS atmospheres are successful in reproducing the observed band intensities of giants with spectral types earlier than about M5 III and M2 II\@. However, they generally predict too strong features for very cool and extended objects, as they are discussed in this work. And they fail completely when it comes to Miras with weak or no SiO absorption. These stars are dominated by dynamical phenomena and, not surprisingly, they can therefore not be described by hydrostatic structures. Thus, we have also computed synthetic spectra based on experimental dynamical models. Although they still have some shortcomings, we demonstrate that, in principle, they are able to explain the whole range of equivalent widths of the observed SiO bandheads and their variations. Based on observations made at the European Southern Observatory (ESO), La Silla (Chile)}
    Astronomy and Astrophysics 01/1999; 342:799-808. · 5.08 Impact Factor
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    ABSTRACT: . The first overtone rotation-vibration transitions of SiO give rise to prominent bandheads in the wavelength range between 4.0 and 4:5 m. In order to study the behaviour of these features in AGB stars we observed the 3.94 to 4:12 m spectra for a sample of 23 oxygen-rich latetype variables. In contrast to the SRb objects, the Miras show a very large scatter of the equivalent widths of the SiO bands. Despite their cool temperatures some of them have only weak or no SiO absorption, which seems to be related to their strong pulsations producing a large variability of the features. When comparing the band intensities with photometric data, we found a general decrease with bluer IRAS (12Gamma25) colors. However, this trend may only reflect the different behaviour of the Miras and SRb stars in our sample. We did not discover any correlation of the equivalent widths with the effective temperatures derived from (JGammaK), or with the (KGamma12) color and the IRAS-LRS class, both of which ca...
    12/1998;
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    ABSTRACT: . We have calculated spectral energy distributions and synthetic IRAS colours of carbon-rich long-period variables. On top of dynamical models for the stellar atmosphere and circumstellar envelope which consistently treat the timedependent hydrodynamics, the formation of amorphous carbon grains and grey radiative transfer, frequency-dependent radiative transfer calculations have been carried out in the range between 1 ¯m and 290 ¯m. Calculating the IRAS colours we find that the models lie in regions I (stars without circumstellar shells) and VII (variable stars with carbon-rich circumstellar shells) of the IRAS two colour diagram in accordance with observational results for carbon stars. They form an almost linear sequence (near the black body line) reflecting the different mass loss rates. We compare our results to empirical formulae which link the mass loss rate to the observed flux at 60 ¯m. Furthermore, we have also investigated qualitatively the effects of dust with ff-SiC grains ...
    12/1998;
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    ABSTRACT: The ISO-SWS spectra of several representative carbon- and oxygen-rich AGB variables (R Aql, R Cas, T Dra, V Cyg, V CrB and R Scl) were obtained at several phases of their pulsational cycle. This allows us to study the molecular features and the dust emission of these objects as a function of stellar parameters as well as pulsational phase. The observational data are compared with synthetic molecular spectra based on hydrostatic models (MARCS) and on dynamical atmospheres, which include such phenomena as pulsation, mass loss and dust formation. Opacities of CO, CN, C_2, C_3, C_2H_2, HCN, SiO, H_2O, TiO, OH and CO_2 have been taken into account. As it turns out, a complete treatment of molecular opacities including a large number of lines of species like C_3, C_2H_2, HCN, H_2O and TiO is crucial for the construction of model atmospheres, the interpretation of dust features and the appearance of the pseudo-continuum. Some aspects of the SWS spectra like their variations can only be explained by dynamical atmospheres. To be able to analyse also the targets with significant mass loss, we included a dust component in the synthetic spectra. The dust component either comes directly from the dynamical models or is modelled with the DUSTY code. On the basis of these synthetic spectra we also investigate the influence of molecular features on the interpretation of the dust emission and vice versa. This is of special importance in the 8 to 16 micron region. This work was supported by the Austrian Science Fund project S7308-AST. FK is supported by APART (Austrian Academy of Sciences).
    01/1998;
  • 01/1998;
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    ABSTRACT: We have calculated synthetic high resolution spectra in the wavelength range around 4 microns, which are based on dynamical models for AGB stars (see review R2-03). We investigate the effects of sphericity and velocity fields on the molecular spectra of these objects. Since the atmospheres are strongly affected by dynamical effects (pulsation, shock waves, stellar winds) the resulting line profiles, especially of CO and SiO in O-rich atmospheres, are both complex and time-dependent. The comparison of our theoretical spectra with high resolution FTS observations demonstrates the crucial role of shock waves, realistic opacities, and sphericity effects for the modelling of AGB star atmospheres. In addition, we want to emphasize the importance of including velocity fields into the spectral synthesis in order to be able to reproduce line profiles and intensities.
    Proceedings of the International Astronomical Union 01/1998; 191.
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    ABSTRACT: We present synthetic spectra for the molecular and dust components of AGB star atmospheres. The spectra are based on dynamical model atmospheres, the dust component is either modeled with the DUSTY code or taken directly from the dynamical models. We compare these spectra with ISO-SWS observations of C-rich and O-rich AGB variables.
    01/1997; 419:213.
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    W. Windsteig, R. Loidl, S. Hoefner, E. A. Dorfi
    Proceedings of the International Astronomical Union 01/1997; 189:172P.
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    ABSTRACT: We have calculated spectral energy distributions and synthetic IRAS colours of carbon-rich long-period vari-ables. On top of dynamical models for the stellar atmosphere and circumstellar envelope which consistently treat the time-dependent hydrodynamics, the formation of amorphous carbon grains and grey radiative transfer, frequency-dependent radia-tive transfer calculations have been carried out in the range between 1 m and 290 m. Calculating the IRAS colours we find that the models lie in regions I (stars without circumstellar shells) and VII (variable stars with carbon-rich circumstellar shells) of the IRAS two colour diagram in accordance with ob-servational results for carbon stars. They form an almost linear sequence (near the black body line) reflecting the different mass loss rates. We compare our results to empirical formulae which link the mass loss rate to the observed flux at 60 m. Further-more, we have also investigated qualitatively the effects of dust with -SiC grains and of detached shells. The SiC-contribution causes mainly a blue-shift of about 0:2 mag in the [12] ?[25] colour. Adding a detached shell results in higher [25] ?[60] values partly shifting the models into region VIa (non-variable stars with relatively cold dust at large distances) of the IRAS two colour diagram as expected from observations.
    Astronomy & Astrophysics - ASTRON ASTROPHYS. 01/1997; 324.
  • 01/1997;
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    ABSTRACT: We present ISO-SWS spectra for several representative AGB carbon stars obtained at several phases of the pulsational cycle. For stars with significant mass loss we correct the spectra for the dust emission by using DUSTY models. We then compare these corrected data to synthetic spectra based on hydrostatic and dynamical model atmospheres. We also discuss the influence of the fundamental model parameters on the resulting synthetic spectra.
    Proceedings of the International Astronomical Union 191:181.