Abundance analysis model atmosphere

Astronomical Observatory, were used to carry out the calculations of theoretical equivalent widths of lines, synthetic spectra and a set of plane parallel, line-blanketed, flux constant LTE model atmospheres. The effective temperatures of the stars were determined from photometry, the infrared flux method and corrected, if needed, in order to achieve the LTE excitation balance in the iron abundance results. The gravities were found by forcing Fe I and Fe II to yield the same iron abundances. The microturbulent velocities were determined by forcing Fe I line abundances to be independent of the equivalent width. For more details on the method of analysis and atomic data see Tautvaisiene et al. (2001). 

A weak but useful carbon line [Cl] 8727.13 A disappears in halo dwarfs with metallicities below —1. To measure carbon abundance in halo stars one can use four Cl high excitation lines near 9100 A and the CH band at 4300 A. The Cl lines at 9100 A together with the OI triplet at 7771 A have been used by Tomkin et al. (1992) and Akerman et al. (2004) to study the behaviour of C/O versus metallicity. However, Cl and OI lines employed in these papers are sensitive to a non-LTE effects and one has to bare in mind that this sensitivity is different for C and O. The CH band at 3145 A used by Israelian et al. (1999) is almost saturated in disk stars and several blends makes the abundance analysis less accurate. To ensure a homogeneous analysis of the C/O and N/O ratio from NH,CH and OH lines in the near-UV, we used the same model atmospheres and tools as in our previous studies. The oxygen abundances were compiled from Israelian et al. (1998, 2001) and Boesgaard et al. (1999). 

Abstract. The chemical composition of B 12, a Be star in the SMC cluster NGC 330, is analysed using high-resolution UVES/VLT spectra and the non-LTE model atmosphere code TLUSTY. A differential analysis relative to a SMC standard star AV 304 revealed (1) a general under-abundance of metals compared with that expected for the SMC, and (2) the lack of nitrogen enhancement. The former is attributed to the presence of a disk, and its contribution to the overall emission is estimated. Possible explanations for the lack of rotational mixing in the apparently rapidly rotating star are discussed. 

Fig. 3.3. Cartoon indicating steps in abundance analysis using model atmospheres. After Gustafsson (1980).

Bengt Stromgren and A. Unsold make quantitative analysis of solar abundances with fairly realistic model atmosphere, including H opacity source just discovered by R. Wildt. 

Modern quantitative spectroscopy of hot stars has two aspects the analysis of photospheric lines and stellar wind lines The first one is meanwhile established as an almost classical tool to determine stellar parameters. NLTE model atmosphere and line formation calculations yield Te , log g and abundances with high precision (see recent reviews by Husfeld (this meeting), Kudritzki (1987), Kudritzki and Hummer (1986), Kudritzki, (1985).) The second aspect, however, the quantitative analysis of stellar wind lines is still at its very beginning. For long time the stellar wind lines have been used to determine mass-loss rates M and terminal velocities v only. While these studies were pioneering and of enormous importance, it was also clear that very approximate calculations were done with respect to NLTE ionization and excitation and the radiative transfer in stellar winds. Thus, stellar wind lines could be used only in a more qualitative comparative sense, with no theory behind, which allowed the determination of precise and reliable numbers. 

Casual reading of the literature may give the impression that secrets of stellar nucleosynthesis are revealed only by detailed quantitative analysis of a stellar spectrum involving model atmospheres and spectrum synthesis with perhaps attention to non-LTE and other effects. Exacting but not exciting work may be the verdict. Although there is important information in abundances that can come only from a detailed and precise analysis, there have been and surely remain startling results which can be unearthed from quite simple analyses and even from mere inspection of a spectrum. I discuss two such examples. 

Lack of an abundance estimate for a trace element like boron has no effect on the accuracy of the abundance analysis for other elements but merely restricts astrophysical interpretations involving B. On the other hand, helium is an abundant elements with effects on the atmospheric structure and through this on the derived abundances of other elements. Although rarely stated explicitly, abundance analyses of cool stars are dependent on an assumption about the He/H ratio the assumption enters both into the model atmosphere and synthetic spectrum calculations. For normal stars, ignorance about the He/H ratio is mitigated by the fact that the He/H ratio is surely constrained within tight limits (Y = 0.24 to 0.26, see above). 

In the case of climate modeling such series are abundant, and since atmospheric phenomena present very complex and sophisticated structure, it is not possible easily to model them through analytical and physical approaches. The remaining open door for their behavior assessment is the use of time series analysis. The classical time series analysis does not provide any insight into the dynamism of the phenomenon but only about its mechanical decomposition into various trends. 

For the Sgr dSph we present the UVES DIC1 spectra for 12 giants. Complete analysis of two of them has already been published [2], while for the other ten only iron and a-elements abundances have been published so far (see [3]). Details on the reduction and analysis procedures, and physical parameters for the stars are provided in [3], but they can be briefly resumed here the spectra have been analyzed by means of LTE, one dimensional atmosphere models, using ATLAS, WIDTH and SYNTHE codes (see [7] and [10]). Te// for the stars are in the range 4800 - 5050 K, log g between 2.3 and 2.7. We analyzed abundances of proton capture (Na, Al, Sc, V), a (Mg, Si, Ca, Ti), Iron-peak (Cr, Fe, Co, Ni, Zn) and heavy neutron-capture (Y, La, Ce, Nd) elements. 

Preliminary simulations of convection in red giants stellar atmospheres indicate that cooler surface layers are expected in 3D models than in ID. Therefore, in the LTE case, for lines forming in those layers corrections to abundances derived with ID analysis have to be applied in the same direction as for dwarfs and subgiants. The magnitude of the corrections though appears to be lower for red giants. 

The above inference concerns all chemical pollutants except Fe, Cu, Si, and Ni. The abundance of these elements in melted-snow samples is beyond the limits predicted by the above model of vaporization and consequently can be attributed to non-molecular forms of mass-transfer of these elements. This discrepancy can be explained by some additional sources of contaminants within the considered technology. A comprehensive comparative analysis shows that the most probable form of transferring such elements as Fe, Cu, Si, Ni into the atmosphere and snow are the matte and dust, where they are major chemical elements. A rather strong correlation... 

While significant uncertainty exists in determining the absolute stellar abundances of the heavy elements, the relative isotopic abundance of an atomic species can be determined somewhat independently of the atmospheric modelling uncertainties [4, 29]. Observations of different ZrO bands in a star do produce slightly different surface abundance values, but the uncertainty this introduces into our analysis of stellar nucleosynthesis is small compared to other uncertainties we have already discussed. In addition, the heavy element... 

Another important source for potential systematic uncertainty stems from the fact that the Li abundance is not directly observed but rather, inferred from an absorption line strength and a model stellar atmosphere. Its determination depends on a set of physical parameters and a model-dependent analysis of a stellar spectrum. Among these parameters, are the metallicity characterized by the iron abundance (though this is a small effect), the surface gravity which for hot stars can lead to an underestimate of up to 0.09 dex if log g is overestimated by 0.5, though this effect is negligible in cooler stars. Typical uncertainties in log g are 0.1 0.3. The most important source for error is the surface... 

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