Planetary Nebulae

The excitation of atoms and ions to metastable levels by electron collision is followed by cascade to lower levels which, in the process, emit the so-called forbidden quanta. The transition probabilities of spectral lines are quite few by comparison to the allowed transition. The allowed transitions are electric dipole radiations, whereas forbidden transitions correspond to magnetic-dipole and/or electric-quadruple radiations. There are three types of transitions which are the result of collisional excitation: nebular, auroral, and transauroral. All the upward transitions are due to collisional excitation only; however, the downward transitions can be one of two types, i.e., superelastic collisions, or radiation of forbidden lines. The level density and atomic constants determine which of the latter transitions is likely to take place in depopulating the level. Also, the forbidden spectra are observed only for ions whose metastable levels lie a few electron volts above the ground state. Collisionally excited lines are observed in low lying levels of the spectra of CIII, CIV, NIII, NIV, NV, SIIII, etc., in the far ultraviolet.

The study of forbidden lines is one of the major areas of investigation in gaseous nebulae since they dominate the spectra of most gaseous nebulae.