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Astron. Astrophys. 359, 729-742 (2000)

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7. Solar Fe line asymmetries

Line asymmetries carry additional information to line shifts, since the lines trace the atmospheric structure throughout the line forming region and not only the higher layers where the core is formed and thus the line shift. Fig. 15 shows a few examples of the predicted line asymmetries and the corresponding observed bisectors. It should be noted that both types of bisectors are on an absolute wavelength scale and the synthetic bisectors have therefore not been shifted in velocity to match the observations. In order to achieve such a remarkable agreement it is necessary to have both a very accurate description of the atmospheric structure and the details of line formation as well as very high quality laboratory wavelengths. Clearly the result is very satisfactory. The excellent correspondence in Fig. 15 is not only fortuitous, as is apparent from an inspection of Fig. 16, which shows the differences in observed and predicted line asymmetries for all the 67 computed Fe I lines. Under ideal conditions the differences should all be vertical lines with no velocity offset, which in fact is not far from the truth. In particular the weaker lines show excellent agreement, while the situation becomes progressively worse for the cores of the stronger lines, reflecting the shortcomings already discussed in Sect. 6. This discrepancy also affects the bisectors closer to the continuum for the stronger lines, causing the bisector differences to be predominantly positive. The situation for the Fe II lines are shown in Fig. 17, which again is very satisfactory. Both with an inferior resolution and height extension in the convection simulations the resulting bisectors are of noticably lower quality when comparing with observations (Asplund et al. 2000a). The good overall agreement in terms of line asymmetries therefore lend very strong support to the realism of the convection simulations.

[FIGURE] Fig. 15. The predicted (solid line) and observed (line with error bars) bisectors of the Fe I 680.4, 627.1 and 624.0 nm (in order of increasing line strength) lines. It should be emphasized that the velocity scale is absolute for both the computed and measured line asymmetries, i.e. no arbitrary wavelength shifts have been applied in order to bring the two into agreement. Clearly the predicted bisectors agree rather satisfactory with the observations. In fact, the close resemblance for these lines is partly fortuitous since both the laboratory wavelengths and the wavelength calibration of the solar intensity atlas may only be accurate to about [FORMULA] m s-1. In comparison, classical 1D model atmospheres will of course only produce vertical bisectors at zero absolute velocity

[FIGURE] Fig. 16. Differences between predicted and observed bisectors for the Fe I lines used for the present analysis; a gravitational red-shift of 633 m s-1 has been subtracted for the observed bisectors. Except for a tendency for the predicted strong lines (line depths greater than 0.5) to have slightly too little blueshift or too much redshift, as also obvious in Fig. 11, the agreement is clearly very satisfactory. The line furthest to the left is Fe I 666.8 nm, whose laboratory wavelength therefore can be suspected to be slightly in error

[FIGURE] Fig. 17. Same as Fig. 16 but for the 15 Fe II lines

In fact it is very easy to detect problematic lines due to erroneous laboratory wavelengths (large velocity offset) and blends (discrepant bisector shape) when comparing line asymmetries. It can be mentioned that initially the wavelengths for the Fe I 697.2 and 718.9 nm lines were accidentally taken from the VALD database rather than from Nave et al. (1994), which differed by only 16 and 5 mÅ (corresponding to 0.7 and 0.2 km s-1), respectively, though immediately detected when analysing the line asymmetries. Furthermore, the Fe II wavelengths provided by Johansson (1998, private communication) were found to be of significantly higher quality than the ones given in Hannaford et al. (1992).

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000
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