THE OPAL OPACITY CODE

The OPAL code was developed at the Lawrence Livermore National Laboratory to compute opacities of low- to mid-Z elements. Briefly, the calculations are based on a physical picture approach that carries out a many-body expansion of the grand canonical partition function. The method includes electron degeneracy and the leading quantum diffraction term as well as systematic corrections necessary for strongly-coupled plasma regimes. The atomic data are obtained from a parametric potential method that is fast enough for in-line calculations while achieving an accuracy comparable to single configuration Dirac-Fock results. The calculations use detailed term accounting; for example, the bound-bound transitions are treated in full intermediate or pure LS coupling depending on the element. Degeneracy and plasma collective effects are included in inverse bremsstrahlung and Thomson scattering. Most line broadening is treated with a Voigt profile that accounts for Doppler, natural width, electron impacts, and for neutral and singly ionized metals broadening by H and He atoms. The exceptions are one-, two-, and three-electron systems where linear Stark broadening by the ions is included.

Rosseland Mean Opacity Tables

The monochromatic opacities for a mixture of 21 elements have been computed assuming local thermodynamic equilibrium and archived as a function of photon frequency on a R-T6 mesh where T6 is the temperature in millions of degrees, and R=r/T63 with r the density in g/cm3. The elements included are: H, He, C, N, O, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Ti, Cr, Mn, Fe, and Ni. Although the calculations were done for a specific composition, opacities for different element abundances can be readily generated from the same monochromatic data by the principle of corresponding states.

It is anticipated that most needs for astrophysical opacities can be met with a standard set of tables. Those currently available can be obtained by following this link to existing OPAL Opacity Tables. Requests for new tables can be made by following this link to Generate New OPAL Opacity Tables.

Equation of State Tables

Tabulation of thermodynamic variables poses different problems compared to opacity. Whereas heavier elements are crucial for the latter, elements heavier than O generally play a minor role in astrophysical equations of state. Nevertheless, analysis of helioseismic data requires highly accurate derivatives of the equation of state. To minimize the computer requirements, the astrophysical mixture was reduced to H, He, C, O and Ne where the abundance of Ne accounts for all heavier elements. More complete calculations were performed to verify that only small errors result from the reduced composition.

The equation of state data is tabulated on a temperature-density grid. Three sets of data, along with interpolation codes, are available. The EOS and EOSPLUS sets where calculated with an earlier version of the OPAL equation of state code. These two sets differ only in the grid density; EOSplus has the denser grid. The EOS_2001 set was calculated with an improved version of the OPAL code. The main physics differences are that the electrons are now treated relativistically and the activity expansion method has been improved for repulsive inter-particle interactions. The spacing of the EOS_2001 temperature-density grid is similar to EOSplus, but covers a significantly larger range in temperature and density.


For more information about OPAL, contact:

Carlos Iglesias -- iglesias1@llnl.gov

Information date 2001 December 14.

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