Litle of program: GENERAL BN PROGRAM
Nature of physical problem Emission lines produced by the recombination ofhydrogen Catalogue number: ACXI and hydrogenic ions are observed from many astronomical sources; maser amplification is frequently present. The re-Program obtainable from: CPC Program Library, Queen's combination line spectrum depends upon the populations University of Belfast, N. Ireland (see application form in this of the energy levels of the emitting species. The present proissue)
gram computes the ratio, b~,of the population of energy leveln to the (known) population in thermodynamic equi-
Computer: IBM 370/165;Installation: University of cam-librium for given values of electron temperature and density bridge Computer Laboratory A background radiation field may be present. The results are accurate for the range of temperatures and densities associated
Operating system: os with cold clouds, H regions, and planetary nebulae (10-20000 K, iO~_iO6 cm 3).
Programming language used: FORTRAN IV
Method of solution High speed storage required: 43000 words
The method is that described by Brocklehurst [1,2], but with the collision cross-sections of Gee et aL [3]. In statistical equi-No. of bits in a word: 32 librium, the rates of population and depopulation of each of the infinitely many energy levels must be equal. The infinite Overlay structure: None system of linear algebraic equations thus defined is truncated, and correction terms are added to compensate for the omitted
No. of magnetic tapes required: None levels. The resulting system is condensed to a smaller size and solved. Other peripherals used: Card reader, line printer, (optional)
The equations of radiative transfer must in principle be card punch solved simultaneously with the population equations. In practice it is usually sufficient to consider the optical depth for No. of cards in combined program and test deck: 1291 each line to be either zero (no absorption) or infinite (on-thespot absorption).