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 A)/I(4740 A), R_2 = I(7238 A)/I(4711 + 4740 A), R_3 = 7(7263 A)/I(4711 + 4740 A), and R_4 = I(7171 A)/I(4711 + 4740 A), 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 A 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 A line with the strong C Ⅱ transition at 7236 A
展开▼
