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Manuscript Title: Computation of line and continuum radiation from thermal radioastronomical sources.
Authors: M. Brocklehurst, M. Salem
Program title: SELECT BN,CN VALUES
Catalogue identifier: AAEH_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 9(1975)258
Programming language: Fortran.
Computer: IBM 360/65.
Operating system: OS.
RAM: 5K words
Word size: 32
Peripherals: magnetic tape, disc.
Keywords: Astrophysics, Radioastronomy, Radio recombination line, HII region, Thermal source, Continuum spectrum.
Classification: 1.4.

Nature of problem:
The departure coefficients from thermodynamic equilibrium, bn, and their derivatives, cn, are required for the computation of intensities and profiles of hydrogen recombination lines from thermal radioastronomical sources. These coefficients have been computed by Brocklehurst(1970) for values of n up to 300, for 6 values of electron temperature, and for 8 values of electron density. As it is not expected that intensity calculations will be required for more than a few lines at a time, and as the information available is very extensive, a short program has been written to extract bn and cn values for up to 15 lines from the complete table, and to write them on a card, disc, or tape dataset in a form which can be read directly by program RCMBLN.

Solution method:
Atomic hydrogen energy level populations have been computed by Brockle- hurst for different values of electron temperature, Te, and electron density, Ne, in the form of coefficients of departure from thermodynamic equilibrium, bn. As the coefficients of absorption and emission depend upon the bn factors, the equation of radiative transfer can be solved numerically for any given distribution of Te and Ne.

The electron temperature is assumed to be constant throughout the source, and clumping is not taken into account. The equation of radiative transfer has been linearized by assuming that, for all lines, the optical depth due to line absorption (positive or negative) is much less, in absolute value, than unity; and that the line emissivity is much less than the continuum emissivity at the same frequency. These assumptions are adequate to within the accuracy of present day observations.

Running time:
The running time depends greatly upon the number of points at which Ne is specified. For 33 points, the running time is of the order of 2 s per line for a spherical model on an IBM 360/65 computer. Compilation time is approximately 20 s.