Nebular and auroral emission lines of [Cl III] in the optical spectra of planetary nebulae

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 Angstr om) and auroral (8433.9, 8480.9, 8500.0 Angstrom) 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 Telescop e, reveals that the R-1 = /(5518 Angstrom)/I(5538 Angstrom) intensity ratio provides estimates of N-e in excellent agreement with the values derived f rom other line ratios in the echelle spectra. This agreement indicates that R-1 is a reliable density diagnostic for planetary nebulae, and it also pr ovides observational support for the accuracy of the atomic data adopted in the line ratio calculations. However the [Cl III] 8433.9 Angstrom line is found to be frequently blended with a weak telluric emission feature, altho ugh in those instances when the [Cl III] intensity may be reliably measured , it provides accurate determinations of T-e when ratioed against the sum o f the 5518 and 5538 Angstrom line fluxes. Similarly, the 8500.0 Angstrom li ne, previously believed to be free of contamination by the Earth's atmosphe re, is also shown to be generally blended with a weak telluric emission fea ture. The [CI III] transition at 8480.9 Angstrom is found to be blended wit h the He I 8480.7 Angstrom line, except in planetary nebulae that show a re latively weak He I spectrum, where it also provides reliable estimates of T -e when ratioed against the nebular lines. Finally, the diagnostic potentia l of the near-UV [Cl III] lines at 3344 and 3354 Angstrom is briefly discus sed.