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Manuscript Title: Radio recombination lines from H+ regions and cold interstellar clouds: computation of the bn factors.
Authors: M. Brocklehurst, M. Salem
Program title: GENERAL BN PROGRAM
Catalogue identifier: ACXI_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 13(1977)39
Programming language: Fortran.
Computer: IBM 370/165.
Operating system: OS.
RAM: 43K words
Word size: 32
Keywords: Astrophysics, Radioastronomy, Hii region, Cold cloud, Thermal source, Recombination line, Interstellar medium.
Classification: 1.4, 1.8.

Nature of problem:
Emission lines produced by the recombination of hydrogen and hydrogenic ions are observed from many astronomical sources; maser amplification is frequently present. The recombination line spectrum depends upon the populations of the energy levels of the emitting species. The present program computes the ratio, bn, of the population of energy level n to the (known) population in thermodynamic equilibrium for given values of electron temperature and density. A background radiation field may be present. The results are accurate for the range of temperatures and densities associated with cold clouds, H+ regions, and planetary nebulae (10 - 20,000K, 10**-4 - 10**6 cm-3).

Solution method:
The method is that described by Brocklehurst, but with the collision cross-sections of Gee et al. In statistical equilibrium, the rates of population and depopulation of each of the infinitely many energy levels must be equal. The infinite system of linear algebraic equations thus defined is truncated, and correction terms are added to compensate for the omitted levels. The resulting system is condensed to a smaller size and solved. The equations of radiative transfer must in principle be solved simultaneously with the population equations. In practice it is usually sufficient to consider the optical depth for each line to be either zero (no absorption) or infinite (on-the-spot absorption).