Astron. Astrophys. 322, 256-265 (1997)

3. General considerations

Modern analyses utilize line profile fitting for the parameter determination, which is more accurate than the classical approach based on equivalent width measurements. However, the equivalent width is still a useful tool, and used in this article to illustrate the deviations between LTE and NLTE line profiles. For more insight we will also show selected line profiles.

Which deviations of line profiles from LTE are significant and should be considered in model atmosphere analyses? A general answer to this question is difficult, if not impossible. As a guideline we use the results of analyses of hot hydrogen-rich DA white dwarfs by Kidder (1991), Bergeron et al. (1994), and Finley et al. (1996), which were carried out with similar LTE model atmosphere programs. From an intercomparison of their results for the objects in common, we estimate that the parameters of a typical hot DA ( K) with good observational material can be determined to within K and dex. Both error ranges correspond to relative changes in of 5%. To avoid systematic errors of the order of the fitting errors the accuracy of the model calculations should be better than this. We assume that deviations from LTE of the order of % in can easily be neglected for practical purposes.

If one adopts the smaller formal errors given in the quoted analyses articles, one has to consider smaller deviations. On the other hand even larger deviations are unimportant for lower quality spectroscopic material. Therefore we will give overviews of the NLTE effects by plotting the relative differences

(in percent) between calculated from LTE and NLTE for typical parameter sets. This allows a check whether the deviations between both types of models are important for the particular purpose. The comparison is limited to lines with mÅ. Lines of such strength are easily observable for brighter white dwarfs and sdB stars.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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