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M. Eriksson, G.M. Wahlgren, S. Johansson (Lund Observatory)
When ions in a plasma are subjected to emission lines very close in photon energy to the energy difference between two levels in the ions, the photons can pump the ions through the corresponding channel. This is called photoexcitation by accidental resonance (PAR). The most famous example of PAR is the Bowen mechanism, where the He II Ly\alpha line at 303.783 Åpumps two 2p2 - 2p3d channels in O III, at 303.800 and 303.695 Å, resulting in the so called O III Bowen lines between 2800 and 4000 Å. In IUE spectra of 20 symbiotic stars we observe the O III Bowen lines in 16. The O III Bowen lines are analyzed and the pumping efficiencies are discussed. In spectra of four of the symbiotic stars, AG Peg, RR Tel, HM Sge and V1016 Cyg, we observethe N III 3s-3p lines at 4000 Åand the 3p-3d lines at 4640 Åare observed. It was proposed already by Bowen (1935) that one of the O III lines, \lambda374.432 could photoexcite N III through the channels at 374.434 and 374.442 Å, which would actually lead to the N III lines we observe in the symbiotic stars. We have modeled intensity ratios between the N III lines to test pumping by O III emission as the explanation for the N III 3s-3p and 3p-3d emission. This is valid for AG Peg but can be excluded for HM Sge. For RR Tel and V1016 Cyg the Bowen mechanism seems to be involved in the formation of the N III lines, but additional processes such as radiative recombination or charge-exchange with neutral hydrogen are needed to explain the observed intensity ratios.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.