Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982apj...257l..87d&link_type=abstract
Astrophysical Journal, Part 2 - Letters to the Editor, vol. 257, June 15, 1982, p. L87-L90.
Astronomy and Astrophysics
Astrophysics
27
Astrophysics, Cosmic Plasma, Electron Energy, Nebulae, Plasma Temperature, Atomic Energy Levels, Charge Transfer, Electron Recombination, Forbidden Bands, Oxygen Ions, Oxygen Spectra, Planetary Nebulae, Positive Ions
Scientific paper
Electron temperatures of astrophysical plasmas are usually derived from the relative intensities of lines emitted by metastable levels of positive ions, particularly of O(+2) and N(+), on the assumption that the levels are excited by electron impact. The populations of the metastable levels of O(+2) are augmented by a radiative cascade following charge transfer of O(+3) with hydrogen and are not always a reliable diagnostic of the temperature. An evaluation of the relative contributions of charge transfer to the 1S and 1D level populations shows that the temperatures derived from the O(+2) line intensities tend to be overestimates. A quantitative measure of the effect of charge transfer can be obtained from the intensity of the O(+2) line at 5592 A. In planetary nebulae the effect on the derived electron temperature seems to be small. It will be larger in plasmas produced by nonthermal ionizing sources. Attention is drawn to the possibility that temperatures derived from the O(+2) line at 1663 A may also be too high because of the enhanced population of the 5S level by dielectronic recombination.
Dalgarno Alexander
Sternberg Amiel
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