| 1. |
- Lebzelter, T., et al.
(författare)
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Comparative modelling of the spectra of cool giants
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 547, s. A108-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. Aims. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Methods. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. Results. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Conclusions. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.
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| 2. |
- Lebzelter, T., et al.
(författare)
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Abundance analysis for long period variables Velocity effects studied with O-rich dynamic model atmospheres
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 517, s. A6-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Measuring the surface abundances of AGB stars is an important tool for studying the effects of nucleosynthesis and mixing in the interior of low-to intermediate mass stars during their final evolutionary phases. The atmospheres of AGB stars can be strongly affected by stellar pulsation and the development of a stellar wind, though, and the abundance determination of these objects should therefore be based on dynamic model atmospheres. Aims. We investigate the effects of stellar pulsation and mass loss on the appearance of selected spectral features (line profiles, line intensities) and on the derived elemental abundances by performing a systematic comparison of hydrostatic and dynamic model atmospheres. Methods. High-resolution synthetic spectra in the near infrared range were calculated based on two dynamic model atmospheres (at various phases during the pulsation cycle) as well as a grid of hydrostatic COMARCS models with effective temperatures T-eff and surface gravities log g over an adequate range. Equivalent widths of a selection of atomic and molecular lines (Fe, OH, CO) were derived in both cases and compared with each other. Results. In the case of the dynamic models, the equivalent widths of all investigated features vary over the pulsation cycle. A consistent reproduction of the derived variations with a set of hydrostatic models is not possible, but several individual phases and spectral features can be reproduced well with the help of specific hydrostatic atmospheric models. In addition, we show that the variations in equivalent width that we found on the basis of the adopted state-of-the-art dynamic model atmospheres agree qualitatively with observational results for the Mira R Cas over its light cycle. Conclusions. The findings of our modelling form a starting point to deal with the problem of abundance determination in strongly dynamic AGB stars (i.e., long-period variables). Our results illustrate that some quantities such as the C/O ratio can probably still be determined to a reasonable accuracy, but the measurement of other quantities will be hampered by the dynamics. The qualitative agreement with observations of R Cas opens promising possibilities for a forthcoming quantitative comparison of our synthetic spectra with observed ones of AGB variables in the globular cluster 47 Tuc.
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| 3. |
- Heiter, Ulrike, et al.
(författare)
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Gaia spectroscopy overview and comparative spectrum modeling for cool giants
- 2011
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 328:Conference 1, s. 012001-
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Tidskriftsartikel (refereegranskat)abstract
- The Gaia mission will provide spectroscopic and spectrophotometric observations for a vast number of stars, allowing a characterization in terms of stellar parameters along with the astrometric measurements. The pipeline for astrophysical parameter determination relies on libraries of model spectra for mapping stellar parameters to flux distributions. To maximize the reliability of the parametrization, adequate efforts are required to assess the realism of the model spectra and to calibrate the analysis methods. We describe a spectrum modelling experiment, in which high-resolution optical and infrared spectra of four cool giant stars were analysed by 14 different groups. The resulting variance in derived stellar parameters illustrates the need to be cautious when comparing or combining stellar parameters from different model atmospheres and analysis strategies. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method.
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| 4. |
- Lebzelter, T., et al.
(författare)
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Abundance analysis for long-period variables II. RGB and AGB stars in the globular duster 47 Tucanae
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 567, s. A143-
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Tidskriftsartikel (refereegranskat)abstract
- Context Asymptotic giant branch (AGB) stars play a key role in the enrichment of galaxies with heavy elements. Due to their large amplitude variability, the measurement of elemental abundances is a highly challenging task that has not been solved in a satisfactory V ay yet. Aims. Following our previous work we use hydrostatic and dynamical model atmospheres to simulate observed high-resolution near-infrared spectra of 12 variable and non variable red giants in the globular cluster 47 Tuc. The 47 Tuc red giants are independently well characterized in important parameters (mass, metallicity, luminosity). The principal aim was to compare synthetic spectra based on the dynamical models with observational spectra of 47 Ric variables. Assuming that the abundances are unchanged on the upper giant branch in these low-mass stars, our goal is to estimate the impact of atmospheric dynamics on the abundance determination. Methods. To estimate abundances we measured the equivalent widths of selected features in observed spectra and compared the results with predictions from a set of hydrostatic and dynamical model atmospheres resembling 47 Tun AGB stars in their fundamental parameters. Our study includes lines of (CO)-C-12, (CO)-C-13. OH, and Na. Furthermore, we investigated the variations in line intensities over a pulsation cycle. Results. We present new measurements of the C/O and C-12/C-13 ratio for 5 non variable red giants in 47 Tuc. The equivalent widths measured for our 7 variable star, strongly differ from the non variable stars arid cannot be reproduced by either hydrostatic or dynainical model atmospheres. Nevertheless, the dynamical models fit the observed spectra of long-period variables much better than any hydrostatic model. For some spectral features. the variations in the line intensities predicted by dynamical models over a pulsation cycle give similar values as a sequence of hydrostatic models with varying temperature and constant surface gravity. Conclusions. Our study of the dynamical effects on abundance determination visible in these well characterized cluster stars prepares the ground for the long-term goal of deriving abundarices for variable AGB stars in general.
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| 5. |
- Lebzelter, T., et al.
(författare)
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CRIRES-POP A library of high resolution spectra in the near-infrared
- 2012
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 539
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Tidskriftsartikel (refereegranskat)abstract
- Context. New instrumental capabilities and the wealth of astrophysical information extractable from the near-infrared wavelength region have led to a growing interest in the field of high resolution spectroscopy at 1-5 mu m. Aims. We aim to provide a library of observed high-resolution and high signal-to-noise-ratio near-infrared spectra of stars of various types throughout the Hertzsprung-Russell diagram. This is needed for the exploration of spectral features in this wavelength range and for comparison of reference targets with observations and models. Methods. High quality spectra were obtained using the CRIRES near-infrared spectrograph at ESO's VLT covering the range from 0.97 mu m to 5.3 mu m at high spectral resolution. Accurate wavelength calibration and correction for telluric lines were performed by fitting synthetic transmission spectra for the Earth's atmosphere to each spectrum individually. Results. We describe the observational strategy and the current status and content of the library which includes 13 objects. The first examples of finally reduced spectra are presented. This publication will serve as a reference paper to introduce the library to the community and explore the extensive amount of material.
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| 6. |
- Lorenz, D., et al.
(författare)
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Long-period variables in NGC 147 and NGC 185
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 532
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Tidskriftsartikel (refereegranskat)abstract
- Context. Previous studies on the stellar content of the two nearby dwarf galaxies NGC 147 and NGC 185 reveal a rich population of late-type giants in both systems, including a large number of carbon-rich objects. These stars are known to show pronounced photometric variability, which can be used for a more detailed characterisation of these highly evolved stars. Owing to their well-studied parameters, these Local Group members are ideal candidates for comparative studies. Aims. Through photometric monitoring, we attempt to provide a catalogue of long-period variables (LPVs), including Mira variables, semi-regular variables, and even irregular variables in NGC 147 and NGC 185. We investigate the light variations and compare the characteristics of these two LPV populations with the results found for other galaxies, such as the LMC. Methods. We carried out time-series photometry in the i-band of the two target galaxies with the Nordic Optical Telescope (NOT), covering a time span of approximate to 2.5 years. More than 30 epochs were available for a period search. These data were then combined with single-epoch K-band photometry, also obtained with the NOT. Narrow-band photometry data from the literature was used to distinguish between O-rich and C-rich stars. Results. We report the detection of 513 LPVs in NGC 185 and 213 LPVs in NGC 147, showing amplitudes Delta i of up to approximate to 2(mag) and periods ranging between 90 and 800 days. The period-luminosity diagram for each of our target galaxies exhibits a well populated sequence of fundamental mode pulsators. The resulting period-luminosity relations we obtained are compared to relations from the literature. We discuss the universality of those relations because of which, as a side result, a correction of the distance modulus of NGC 185 may be necessary. A value of (m - M) = 24.(m)30 seems to be more appropriate to match the observed data. Only one of our two galaxies, namely NGC 185, has a significant fraction of possibly first overtone pulsators. An interpretation of this finding in terms of differences in the star-formation histories is suggested.
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| 7. |
- Lorenz, D., et al.
(författare)
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LPVs as possible distance indicators in NGC147 and NGC185
- 2012
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Ingår i: Astrophysics and Space Science Proceedings. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1570-6591 .- 1570-6605. - 9783642220173 ; , s. 169-172
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Konferensbidrag (refereegranskat)abstract
- NGC147 and NGC185 are both dwarf galaxies and companions of the M31 Galaxy. These satellites of Andromeda show similar properties in their color magnitude diagram but also striking differences concerning their stellar content of intermediate age stars as well as their amount of gas and dust. This and the fact that their small apparent separation on the sky of approximately one degree make them very interesting for comparative studies. Photometric monitoring of stars in the i-band of the two dwarf galaxies NGC147 and NGC185 covering ≈ 2.5 years resulted in the discovery of a large number of Long Period Variables (LPVs). A total of 323 LPVs in NGC 185 and 147 LPVs in NGC147 were available for further analysis. Additional single-epoch Ks-band photometry was obtained in order to compare the resulting period luminosity diagrams with those of other systems like, e.g., the Large Magellanic Cloud (LMC). Making use of narrow-band photometry a significant fraction of the detected LPVs could be identified as carbonrich. To establish a universal relation between period and luminosity for LPVs, such variables need to be studied in different galaxies. Therefore, the resulting Ks - logP relations (PLRs) were compared with those from the LMC. The PLRs of NGC147 are in good agreement with the LMC relations. In contrast, a systematic shift of the sequence of fundamental mode pulsators (labeled as C) and the sequence for first overtone pulsators (C0) suggests a revised distance estimate for NGC185.
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