A New Mechanism for Excitation of the [N II] Temperature Diagnostic Emission Lines at 6548 \AA\ and 6584 \AA
Session 29 -- General Interstellar Medium
Display presentation, Tuesday, 31, 1994, 9:20-6:30

## [29.07] A New Mechanism for Excitation of the [N II] Temperature Diagnostic Emission Lines at 6548 \AA\ and 6584 \AA

Todd M. Tripp, John S. Gallagher, and Alan Watson (Washburn Observatory, U. Wisconsin)

This paper discusses the possibility that the intensities of the [N II] emission lines at 6548 and 6584 \AA\ are boosted, in a variety of astrophysical nebulae, by the $(2s^{2}2p^{2}) \ ^{1}D_{2}$ --- $(2s^{2}2p3s) \ ^{3}P^{o}_{1}$ transition at 748 \AA. According to theoretical calculations by Fawcett and experimental measurement of the branching ratio, this intersystem ($\Delta S \neq$ 0) transition is as fast as a typical electric dipole allowed transition. The $(2s^{2}2p3s) \ ^{3}P^{o}_{1}$ level is directly populated by a resonance transition from the $(2s^{2}2p^{2}) \ ^{3}P$ ground state, and one out of three electrons that enter the $(2s^{2}2p3s) \ ^{3}P^{o}_{1}$ level will spontaneously decay through the 748 \AA \ transition. Since the 748 \AA\ transition deposits electrons in the $(2s^{2}2p^{2}) \ ^{1}D_{2}$ level from which the [N II] $\lambda \lambda$6548,6584 emissions originate, this remarkably strong intersystem transition will increase the intensities of the famous [N II] temperature diagnostic emission lines if significant excitation of the $(2s^{2}2p3s) \ ^{3}P^{o}_{1}$ level occurs. We discuss the circumstances in which the $(3s) \ ^{3}P^{o}_{1}$ upper level could be significantly populated by photoexcitation, collisional excitation, or recombination. We point out that detection of recombination lines that feed the $(3s) \ ^{3}P^{o}_{1}$ level provides direct evidence that this process is important in nova shells. We also discuss the possible importance of this process in the production of anomalously large [N II]/H$\alpha$ ratios in cooling flow emission line filaments and starburst galaxy superwinds.