7. NLTE line formation calculations on LTE atmospheres
Since NLTE calculations can be very (computer) time consuming, one is interested in performing them as cheaply as possible. One way which is often applied for e.g. main sequence A and B stars is a NLTE line formation of a certain element on a LTE atmosphere with the temperature structure kept fixed. For this purpose one has to assume that the influence on the atmospheric structure of modified occupation numbers of this element can be neglected. It is obvious that this is not always guaranteed for hydrogen and helium. To check the validity of this assumption we performed a few NLTE line formation calculations on our LTE atmospheres.
We selected models representative for the stellar classes discussed in the previous sections. We derive different results for the different classes and parameter ranges. For the sdB stars NLTE line formation on LTE atmospheres seems to be a very successful approach. The remaining deviations of the sdB model ( K, , ) from the self-consistent NLTE atmosphere amount to only 2%, except for the very temperature sensitive He I 4471 Å line with 4%. The hot DAO LTE models ( K, and 7.5, ) are considerably improved by NLTE line formation. The original deviations of 50% and 30% of the and 7.5 models, respectively, are reduced to % (H not considered). However, this is still uncomfortably large. The same amount of deviation remained for the DA model with trace helium ( K, , ) after NLTE line formation, so there was no substantial improvement in this case. Virtually no improvement could be derived for the DO model ( K, , ). The reason for the failure in the latter two cases is that the atmospheric structure is substantially modified due to NLTE effects. These few examples demonstrate that NLTE line formation calculations on LTE atmospheres can be a useful and time saving tool. However, it has to be verified whether the atmospheric structure is influenced by NLTE deviations. One indicator for a violation of this basic assumption of line formation calculations is flux conservation. If the emergent flux of the atmosphere is considerably changed after line formation has been performed, some feedback on the structure is to be expected.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998