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Modern Solar Facilities – Advanced Solar Science, 307–310 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 The Ba ii λ4554 resonance line and solar granulation V. L. Olshevsky* and N. G. Shchukina Main Astronomical Observatory, National Academy of Sciences, Kyiv, Ukraine * Email: sya@mao.kiev.ua Abstract. We present the results of an investigation of the impact of NLTE effects and of granulation inhomogeneities on the solar Ba ii λ4554 Å line. Our analysis is based on both the classical one-dimensional (1D) solar atmosphere models and on the new generation of three-dimensional (3D) hydrodynamical models. We show that NLTE and 3D effects have to be taken into account for reliable diagnostics of the solar atmosphere using this line. We analyse the influence of different parameters on the line shape. It turns out to be most sensitive to collisional broadening and barium abundance. Uncertainties in the oscillator strength, micro- and macroturbulence (in 1D-case) have a secondary importance. We have derived the barium abundance assuming NLTE. We find ABa = 2.16 in good agreement with the recent result of Asplund et al. (2005). 1 Introduction There are at least two attractive features of the Ba ii λ4554 Å resonance line as a diagnostic tool. Firstly, the line shows conspicuous linear polarization signal around 0.6% in the atlas of Gandorfer (2002). Apparently, this line seems to be suitable for investigating the Sun’s hidden magnetism via the Hanle effect. Secondly, the Ba ii λ4554 Å line is sensitive to the non-thermal motions because its thermal broadening is small. The line formation heights “trace” the photosphere and the lower chromosphere making this line a good indicator of the evolution of granulation structure with height. An example of such a diagnostic is the analysis by Sütterlin et al. (2001) of observations obtained with a barium filter at DOT (Skomorovsky et al. 1976). Diagnostic usage of the Ba ii resonance line requires understanding of its NLTE formation in inhomogeneous models of the solar atmosphere. We stress that after the NLTE analyses made by Rutten (1977, 1978) and Rutten & Milkey (1979), interest on the solar barium spectra has in fact faded. Note that more numerous stellar studies (see, e.g., Mashonkina & Gehren 2000; Mashonkina et al. 2003; Mashonkina & Zhao 2006) are restricted by the narrow constraints of 1D classical modeling and by using NLTE abundance corrections without careful analysis of the physics of formation of the barium spectrum. This is precisely the aim of our contribution where we show results of our multilevel radiative transfer simulations of the Ba ii λ4554 Å line both in 1D and 3D models of the solar atmosphere. 308 V. L. Olshevsky and N. G. Shchukina: Ba ii λ4554 resonance line and solar granulation 2 The method and atomic model We carry out our investigation using the NLTE numerical code ”NATAJA”. The code was developed by Shchukina & Trujillo Bueno (2001) to facilitate radiative transfer simulations with complex atoms in realistic solar and stellar atmospheric models. Our atomic model includes 40 energy levels of Ba i and Ba ii. The levels are interconnected by 99 bound-bound and 39 bound-free radiative transitions. We have also taken into account the hyperfine structure and isotopic shift for the Ba ii λ4554 Å line. 3 One-dimensional case We performed our calculations using different ”standard” 1D atmospheric models: MACKKL (Maltby et al. 1986), VALC (Vernazza et al. 1981), HOLMUL (Holweger & Müller 1974). We found that the line shape depends insignificantly on the choice of the model. Thus we present our results only for the MACKKL model. Figure 1 (right panel) shows that the departure coefficiens β of the upper levels of the Ba ii resonance doublet (λ4554 Å and λ4934 Å) and the upper level of the Ba i λ5535 resonance line drop rapidly below unity as the height increases. This level underpopulation is mainly due to resonance line scattering and photon losses. Concerning the lower (ground) level of the Ba i λ5535 line its underpopulation is caused by another NLTE mechanism known as ultraviolet overionization (Shchukina & Trujillo Bueno 2001). At the same time the lower level of the Ba ii resonance doublet is insensitive to this mechanism. So its population is close to the LTE value. The levels with excitation potential of more than 2 eV are overpopulated because of ultraviolet line pumping. The main consequence of such a behaviour of the departure coefficients is significant deviation of the line source function of...

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