NLTE Model Stellar Atmospheres: \\ The Hybrid CL/ALI Method

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Session 40 -- Computational Astrophysics I
Display presentation, Wednesday, 1, 1994, 9:20-6:30

[40.11] NLTE Model Stellar Atmospheres: \\ The Hybrid CL/ALI Method

T. Lanz (NASA/GSFC), I. Hubeny (NASA/GSFC, USRA)

A new numerical method for computing sophisticated NLTE model stellar atmospheres is presented. The method, called the hybrid Complete Linearization/Accelerated Lambda Iteration (CL/ALI) method, combines advantages of both its constituents. Its rate of convergence is virtually as high as for the standard CL method, while the computer time per iteration is almost as low as for the standard ALI method. The method is formulated as the standard complete linearization; the only difference being that the radiation intensity at selected frequency points is not explicitly linearized; instead, it is treated by means of the ALI approach. The scheme offers a wide spectrum of options, ranging from the full CL to the full ALI method. We demonstrate that the method works optimally if the majority of frequency points are treated in the ALI mode, while the radiation intensity at a few (typically 2 - 30) frequency points is explicitly linearized.

We show how this method can be applied to calculate metal line blanketed NLTE model atmospheres of hot stars. We use the idea of ``superlevels'' and ``superlines'' introduced originally by Anderson (1989). We calculate several illustrative models taking into account several tens of thousand of lines of Fe~{\sc iii} to Fe~{\sc vi}, and show that the hybrid CL/ALI method provides a robust method for calculating NLTE line blanketed model atmospheres for a wide range of stellar parameters.

This work was supported in part by NASA grants NAGW-3025 and NAGW-3834.

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