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Published online by Cambridge University Press: 14 August 2015
The following processes have to be considered.
(a) Radiative capture,
The number of captures on the level i of the atom X, per unit volume per unit time, is NeN(X+)αi(Te), where the recombination coefficient αi (cm3 sec−1) may be expressed in terms of the cross-section for photo-ionization from level i.
(b) Cascade,
The coefficients α(c)i and αi′i are defined to be such that NeN(X+) α(c)i is the number of atoms entering i due to capture and cascade, and that NeN(X+) αi′i is the number of photons emitted in the i′→ i line.
(c) Collisional transitions between excited states, including: collisional redistribution of angular momentum; collisional redistribution of energy between highly excited states; and collision processes which depopulate metastable states.
(d) Effects of radiative transfer. The following limiting cases have been considered:
Case A: Nebulae optically thin in all spectrum lines.
Case B: Nebulae optically thick for transitions to the ground state.
Case C: As Case B, but with absorption of stellar quanta in transitions from the ground state, assuming the central star to radiate as a black body.