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Manuscript Title: A computer program for calculating non-LTE model stellar atmospheres.
Authors: I. Hubeny
Program title: TLUSTY
Catalogue identifier: ABFK_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 52(1988)103
Programming language: Fortran.
Computer: VAX 8600.
Operating system: VMS.
RAM: 960K words
Word size: 32
Peripherals: disc.
Keywords: Stellar atmospheres, Transfer radiative, Spectral line formation, Astrophysics, Stars.
Classification: 1.7.

Nature of problem:
The program computes model stellar atmospheres, assuming plane-parallel, horizontally homogeneous atmosphere in radiative and hydrostatic equilibrium, and allowing for departures from local thermodynamic equilibrium (LTE) for a set of occupation numbers of selected atomic and ionic energy levels. The present program is similar in approach to the computer code by Mihalas, Heasley and Auer, but is completely independent. It differs from this code in many ways, such as the inclusion of an optional, and generally much larger, number of chemical species, ionization stages, energy levels, and transitions; allowing for the wind blanketing, etc. Also, the program is able to compute semi- empirical models (models with a fixed temperature distribution).

Solution method:
A highly coupled, non-linear set of the radiative transfer, hydrostatic equilibrium, radiative equilibrium, and statistical equilibrium equations is solved by the complete-linearization technique of Auer and Mihalas. Substantial computer time is saved by introducing so-called fixed-option atomic transitions, which are treated essentially exactly, but are not linearized. There are several difference-equation representations of the transfer equation available in the program, namely the standard Feautrier scheme, the cubic spline scheme, and the fourth-order Hermitian scheme. The program offers several options to avoid possible divergences or to accelerate convergence of the complete linearization iterations, based on a set of the equivalent-two-level- atom procedures for selected atomic transitions performed between two consecutive iterations of complete linearization. A brief description of a previous version of the program is presented in Astron. Astrophys. 98 (1981) 96.

Restrictions:
The basic restriction is the assumption of plane-parallel, horizontally homogeneous atomsphere, with no macroscopic velocity fields. Although the present version assumes radiative equilibrium, it is not difficult to modify the code to consider empirical additional heating or cooling rates in the atomsphere, or to consider the more general radiative + convective equilibrium. The number of energy levels and atomic transitions in the statistical equilibrium equations is not limited; the only restriction is posed by the computer time available. The spectral lines are assumed to be formed with complete frequency redistribution.

Running time:
For a case with 70 depth points and 43 explicit frequencies, the execution time (on VAX 8600) is about 160 s per iteration; for a case with 90 explicit frequencies it is about 11 minutes per iteration. Roughly, the running time scales linearly with the number of depth points, and as the cube of the number of explicit frequencies.