title: Nebular and auroral emission lines of [Cl III] in the optical spectra of planetary nebulae.
abstract: Electron impact excitation rates in Cl III, recently determined with the R-matrix code, are used to calculate electron temperature (T(e)) and density (N(e)) emission line ratios involving both the nebular (5517.7, 5537.9 A) and auroral (8433.9, 8480.9, 8500.0 A) transitions. A comparison of these results with observational data for a sample of planetary nebulae, obtained with the Hamilton Echelle Spectrograph on the 3-m Shane Telescope, reveals that the R(1) = I(5518 A)/I(5538 A) intensity ratio provides estimates of N(e) in excellent agreement with the values derived from other line ratios in the echelle spectra. This agreement indicates that R(1) is a reliable density diagnostic for planetary nebulae, and it also provides observational support for the accuracy of the atomic data adopted in the line ratio calculations. However the [Cl iii] 8433.9 A line is found to be frequently blended with a weak telluric emission feature, although in those instances when the [Cl iii] intensity may be reliably measured, it provides accurate determinations of T(e) when ratioed against the sum of the 5518 and 5538 A line fluxes. Similarly, the 8500.0 A line, previously believed to be free of contamination by the Earth's atmosphere, is also shown to be generally blended with a weak telluric emission feature. The [Cl iii] transition at 8480.9 A is found to be blended with the He i 8480.7 A line, except in planetary nebulae that show a relatively weak He i spectrum, where it also provides reliable estimates of T(e) when ratioed against the nebular lines. Finally, the diagnostic potential of the near-UV [Cl iii] lines at 3344 and 3354 A is briefly discussed.
authors.affiliation: Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland.
authors.forename: F P
authors.initials: FP
authors.lastname: Keenan
authors.forename: L H
authors.initials: LH
authors.lastname: Aller
authors.forename: C A
authors.initials: CA
authors.lastname: Ramsbottom
authors.forename: K L
authors.initials: KL
authors.lastname: Bell
authors.forename: F L
authors.initials: FL
authors.lastname: Crawford
authors.forename: S
authors.initials: S
authors.lastname: Hyung
mesh_terms: D001256:Astronomy / Q000379:methods; D004583:Electrons; D008962:Models, Theoretical; D016083:Planets