| 1. |
- Brandenburg, A., et al.
(author)
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Homochiral Growth Through Enantiomeric Cross-Inhibition
- 2005
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In: Origins of Life and Evolution of Biospheres. - : Springer Science and Business Media LLC. - 0169-6149 .- 1573-0875. ; 35:3, s. 225-241
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Journal article (peer-reviewed)abstract
- The stability and conservation properties of a recently proposed polymerization model are studied. The achiral (racemic) solution is linearly unstable once the relevant control parameter (here the fidelity of the catalyst) exceeds a critical value. The growth rate is calculated for different fidelity parameters and cross-inhibition rates. A chirality parameter is defined and shown to be conserved by the nonlinear terms of the model. Finally, a truncated version of the model is used to derive a set of two ordinary differential equations and it is argued that these equations are more realistic than those used in earlier models of that form.
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| 2. |
- Lederer, M.T., et al.
(author)
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Towards abundance determination from dynamic atmospheres
- 2006
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In: Memorie della Societa Astronomica Italiana, v.77. ; , s. 1008-1013
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Conference paper (other academic/artistic)abstract
- We summarize our efforts to determine element abundances (e.g. for the elements C and O) and isotopic ratios (e. g. isotope[12]{C}/isotope[13]{C}) along the Asymptotic Giant Branch (AGB). For the modelling of AGB star spectra for stars on the lower part of the AGB, hydrostatic model atmospheres have been shown to reproduce observations quite well. Stars climbing up the AGB get more and more dynamic. For these strongly pulsating stars dynamic model atmospheres have to be used in order to reproduce the observed spectra which are dominated by dynamical effects. Both the developments of hydrostatic and dynamic model atmospheres are considered, referring to the problems encountered in the synthesis of cool star spectra. We emphasize the importance of AGB stars in globular clusters, which provide a method to get a homogenous sample of stars well-defined in mass, metallicity, and luminosity. Finally, we point out the relevance of our investigations for stellar evolutionary models.
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| 3. |
- Nilsson, M., et al.
(author)
-
Uni-directional polymerization leading to homochirality in the RNA world
- 2005
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In: International Journal of Astrobiology. ; 4, s. 233-239
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Journal article (peer-reviewed)abstract
- The differences between uni-directional and bi-directional polymerization are considered. The uni-directional case is discussed in the framework of the RNA world. Similar to earlier models of this type, where polymerization was assumed to proceed in a bi-directional fashion (presumed to be relevant to peptide nucleic acids), left-handed and right-handed monomers are produced via an autocatalysis from an achiral substrate. The details of the bifurcation from a racemic solution to a homochiral state of either handedness is shown to be remarkably independent of whether the polymerization in uni-directional or bi-directional. Slightly larger differences are seen when dissociation is allowed and the dissociation fragments are being recycled into the achiral substrate.
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| 4. |
- Nowotny, W., et al.
(author)
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Atmospheric dynamics in carbon-rich Miras : II. Models meet observations
- 2005
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 437:1, s. 285-296
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Journal article (peer-reviewed)abstract
- Originating in different depths of the very extended atmospheres of AGB stars, various molecular spectral lines observable in the near-infrared show diverse behaviours and can be used to probe atmospheric dynamics throughout the outer layers of these pulsating red giants. In Nowotny et al. (2005b, A&A, 437, 273, Paper I) time series of synthetic high-resolution spectra were presented, computed from a dynamic model atmosphere for a typical carbon-rich Mira. In this work, line profile shapes, their variations during the lightcycle and radial velocities derived from wavelength shifts are analyzed and compared with results from observed FTS spectra of the C-rich Mira S Cep and other Miras. It is found that the global velocity structure of the model is in qualitative agreement with observations. Radial velocities of molecular lines sampling different layers behave comparably, although some differences are apparant concerning absolute values. A correction factor of p ≈ 1.36 between measured RVs and actual gas velocities is derived for CO Δ v=3 lines. It is shown that dynamic model atmospheres are capable of reproducing Mira spectra without introducing an additional "static layer" proposed by several authors.
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| 5. |
- Nowotny, W., et al.
(author)
-
Atmospheric dynamics in carbon-rich Miras : I. Model atmospheres and synthetic line profiles
- 2005
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 437:1, s. 273-284
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Journal article (peer-reviewed)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.
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| 6. |
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