Plane-Parallel Codes

I compute the structure of stellar atmospheres using computer programs based on the Atlas codes developed by Robert Kurucz. Kurucz has two versions of his program. The older version is named Atlas9, and it assumes traditional plane-parallel geometry, hydrostatic pressure equilibrium and LTE populations. Its important contribution is the inclusion of the opacity from millions of atomic, ionic and molecular spectral lines using opacity distribution functions (ODFs) that have been pre-computed for a wide range of temperatures, pressures, compositions and microscopic velocity fields. The newer Atlas12 version differs from the Atlas9 code by employing an opacity sampling (OS) of the spectral lines, which eliminates the discrete values of temperature and pressure imposed by the ODFs, as well as providing complete flexibility in choosing the composition and the microscopic velocity fields, both of which might vary with depth.

My versions of the Atlas codes try to remain true to the intent of the originals available from Kurucz, but they have been completely rewritten to modify the programs in a number of ways, including:

• Where Atlas9 differs from Atlas12, except if this is required by the different treatment of line opacity, the coding from the newer Atlas12 has been adopted.
• The code is rewritten in modern Fortran, currently version 2008, although there are features of modern Fortran that have not yet been employed.
• All "go to" statements have been removed because this structure is deprecated in modern Fortran. This necessitated rewriting many routines to eliminate discontinuous jumps. The goal is to have every routine flow continuously from top-to-bottom.
• All "equivalence" statements have been removed because this structure is deprecated in modern Fortran. This avoids any confusion about what a particular variable represents.
• All "common" statements have been replaced by modules. In particular, program dimensions and physical constants are defined in modules that can be edited once and these changes then propagate to the whole program.
• Alternative utility routines, often taken from Numerical Recipes, have been added. These have been tested against the original routines to determine if they are faster or more accurate.
• Kurucz's original integral equation method for radiative transfer has been supplemented with two additional routines. One uses the second-order differential equation method of Feautrier, and the other uses Rybicki's restructuring of the Feautrier method.
• Routines that are used in only one place have been made internal to the routine that calls them.
• The codes have been broken into a series of units, largely following the approach suggested by Kurucz in his original publication (SAO Special Report 309). These units can be edited individually, then compiled and relinked into the executable by a Makefile.
  • main - this unit contains the top level of the program, as well as the output routine, among others.
  • read - this unit reads in the input instructions and data, and also contains routines needed to compute starting data from the input.
  • conv - this unit contains a single routine to compute the convective energy flow.
  • kapp - this unit contains the routines that determine the radiative opacity from all sources at all temperatures.
  • pops - this unit calculates the populations of all the atoms, ions and molecules at every temperature and pressure of the atmosphere.
  • josh - this unit has the three routines mentioned above that calculate the transfer of radiation.
  • tcorr - this unit has a single routine that determines the temperature correction needed at the end of an iteration to improve the conservation of energy from level-to-level in the atmosphere, as well as improve the gradient of the energy conservation.
  • utils - this unit holds various utility routines that are used by other routines throughout the code.

To avoid confusion, the modified version of Atlas9 is called Atlas_ODF and the modified version of Atlas12 is called Atlas_OS. Compressed tar files of the source files can be accessed at Atlas_ODF and Atlas_OS.

ODFs are available from either Kurucz or from Fiorella Castelli, which are recommended by Kurucz. The Castelli and Kurucz ODF files have different dimensions for the pressures and temperatures. Instructions have been added to Atlas_ODF to identify which dimensions to use, but the defaults are the Castelli dimensions.

The binary ODFs files provided by either Castelli or Kurucz are read and rewritten as binary files using the program bdf2bin. The purpose of this is to:

• use array element order (as Kurucz does) but with 2-byte integers
• use a record length specified in bytes, not words

Atlas_OS uses the line lists given on the Kurucz web site.

In addition to the programs to compute model atmospheres, the Kurucz programs to compute synthetic spectra have also been modified in the same way as the Atlas_ODF and Atlas_OS programs. These are included in the Atlas_ODF distribution.

Spherical Codes

Spherical versions of both Atlas_ODF and Atlas_OS have been developed, called SAtlas_ODF and SAtlas_OS and described in Lester & Neilson, 2008, Astronomy & Astrophysics, vol. 491, p. 633. Because these programs use spherical geometry for the atmospheres, only the Rybicki radiative transfer routine is appropriate. The source code files are available at SAtlas_ODF and SAtlas_OS, and the SAtlas_ODF distribution has spherical versions of the spectrum synthesis programs.