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 Angstrom)/I(474 0 Angstrom), R-2 = I(7238 Angstrom/I(4711 + 4740 Angstrom), R-3 = I(72 63 Angstrom)/I(4711 + 4740 Angstrom), and R-4 = I(7171 Angstrom)/I(471 1 + 4740) Angstrom), for a range of electron temperatures (T-e = 5000- 20,000 K) and electron densities (N-e = 10-10(6) cm(-3)) appropriate t o gaseous nebulae. These diagrams should, in principle, allow the simu ltaneous determination of T-e and N-e from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planet ary nebulae from (R-1, R-3) and (R-1, R-4), using observational date o btained with the Hamilton echelle spectrograph on the 3 m Shane Telesc ope at the Lick Observatory, are found to show excellent internal cons istency and to be in generally good agreement with the values of T-e a nd N-e estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoreti cal ratios and, hence, the atomic data adopted in their derivation. In addition, they imply that the 7171 Angstrom line is not as seriously affected by telluric absorption as previously thought. However, the ob served values of R-2 are mostly larger than the theoretical high-tempe rature and density limit, which is due to blending of the Ar IV 7237.5 4 Angstrom line with the strong C II transition at 7236 Angstrom.