Nebular and Auroral Emission Lines of [Ar iv ] in the Optical Spectra of Planetary Nebulae

Recent R -matrix calculations of electron impact excitation rates in Ar IV are used to calculate the emission-line ratio: ratio diagrams ( R 1 , R 2 ), ( R 1 , R 3 ), and ( R 1 , R 4 ), where R 1 = I (4711 Å)/ I (4740 Å), R 2 = I (7238 Å)/ I (4711 + 4740 Å), R 3 = I (7263 Å)/ I (4711 + 4740 Å), and R 4 = I (7171 Å)/ I (4711 + 4740 Å), for a range of electron temperatures ( T e = 5000-20,000 K) and electron densities ( N e = 10-10 6 cm -3 ) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of T e and N e from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae from ( R 1 , R 3 ) and ( R 1 , R 4 ), using observational date obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope at the Lick Observatory, are found to show excellent internal consistency and to be in generally good agreement with the values of T e and N e estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoretical ratios and, hence, the atomic data adopted in their derivation. In addition, they imply that the 7171 Å line is not as seriously affected by telluric absorption as previously thought. However, the observed values of R 2 are mostly larger than the theoretical high-temperature and density limit, which is due to blending of the Ar IV 7237.54 Å line with the strong C II transition at 7236 Å.