Fluorescent excitation of (Ni II) lines in the spectra of gaseous nebulae

Details Fluorescent excitation of (Ni II) lines in the spectra of gaseous nebulae Fluorescent excitation of (Ni II) lines in the spectra of gaseous nebulae

Feb 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...294..555l&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 294, no. 2, p. 555-567

Astronomy and Astrophysics

Astrophysics

62

Crab Nebula, Emission Spectra, Fluorescence, Iron, Line Spectra, Nickel, Orion Nebula, Photons, Ultraviolet Radiation, H Ii Regions, Interstellar Gas, Stellar Envelopes, Supernova Remnants

Scientific paper

Photon pumping by UV continua is investigated as the cause of anomalously strong emission by many gaseous nebulae in the lines lambda lambda 6667, 7378 and 7412 A belonging to the 2F multiplet of Ni II. For the circumstellar ejecta of P Cygni, the enhancement of these lines by factors approximately 103 is explained by this mechanism as also are the relative intensities within the multiplet. For the Orion nebula, the strongest line at 7378 A has its emissivity increased by a factor approximately 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 7378 A 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.

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