1995A&A...294..555L

Photon pumping by UV continua is investigated as the cause of anomalously strong emission by many gaseous nebulae in the lines λλ6667, 7378 and 7412A belonging to the 2F multiplet of Ni II. For the circumstellar ejecta of P Cygni, the enhancement of these lines by factors ∼10³ is explained by this mechanism as also are the relative intensities within the multiplet. For the Orion nebula, the strongest line at 7378A has its emissivity increased by a factor ∼5 and this correction, together with a crude allowance for the corresponding effect for [Fe II] lines, removes a significant disparity between the empirically-determined ionization ratios Ni⁺⁺/Ni⁺ and Fe⁺⁺/Fe⁺. In addition, the previously anomalous intensity ratio 7378:7412 is accurately predicted. However, for filaments in the Crab nebula, pumping by the non-thermal synchrotron spectrum or by the H and He recombination continua fall far short of explaining the observed strong 7378A emission line. This failure and the successful explanation of the Orion anomalies together greatly strengthen the straightforward abundance interpretation of the Crab's Ni/Fe problem. Observational tests of this interpretation as well as of the remaining possible excitation mechanisms are proposed.