F. C. McKenna

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 (R1, R2), (R1, R3), and (R1, R4), where R1 = I(4711 ?)/I(4740 ?), R2 = I(7238 ?)/I(4711 + 4740 ?), R3 = I(7263 ?)/I(4711 + 4740 ?), and R4 = I(7171 ?)/I(4711 + 4740 ?), for a range of electron temperatures (Te = 5000-20,000 K) and electron densities (Ne = 10-106 cm-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae from (R1, R3) and (R1, R4), 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 Te and Ne 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 ? line is not as seriously affected by telluric absorption as previously thought. However, the observed values of R2 are mostly larger than the theoretical high-temperature and density limit, which is due to blending of the Ar IV 7237.54 ? line with the strong C II transition at 7236 ?.

[N II] AND [O III] MEAN ELECTRON TEMPERATURES IN PLANETARY NEBULAE

Mean electron tempertures for 106 planetary nebulae are presented, which have been derived using calculations of the values of electron temperature-sensitive line ratios involving forbidden transitions among the 2s2 2p2 3P, 1D, and 1S levels of N+ and O++, based on new electron impact rates and transition probabilities. Comparison of these results with values of Te[N II] and Te[O III] determined previously by Kaler reveal that the present electron temperatures are systematically lower for both ions, and that this discrepancy is correlated with the electron density in the nebula. It is also shown that the average difference tween Te[N II] and Te[O III] in a planetary nebula is somewhat smaller than that derived by Kaler, with the present results implying that the N II and O III temperatures disagree on average by 2070 K as opposed to the 2210 K average found by Kaler.

[O V] in the Ultraviolet Spectra of Gaseous Nebulae

Theoretical O V electron-density-sensitive emission line ratios for R = I(2s21S0 - 2s2p3P2)/I(2s21S0 - 2s2p3P1) are presented. Inspection of Goddard High Resolution Spectrograph Hubble Space Telescope spectra of RR Tel reveals the presence of the [O V] 2s21S - 2s2p3P2 line at 1213.80 A, which is 4.62 ± 0.12 A away from the 2s21S - 2s2p3P2 intercombination transition at 1218.42 A, in good agreement with the theoretical prediction of Δλ = 4.54 A. The resultant value of R = 0.82 ± 0.11 implies a logarithmic electron density, log Ne, of 5.2 ± 0.2 cm-3, in good agreement with that found from other ions with high electron temperature, such as Ne VI, which also provides support for the identification.

The Identification of the O V Forbidden Line in the Ultraviolet Spectrum of Gaseous Nebulae

The 2s2 1S0 – 2s2p 3P1 intercombination line at 1218.34 A of Be-like O V has been observed in IUE spectra of gaseous nebulae such as RR Tel (Doschek & Feibelman 1993). However, the forbidden line at 1213 A has not been detected to date in any astrophysical object, with the possible exception of the Sun, where Sandlin, Brueckner & Tousey (1977) very tentatively identify the line at 1213.90 A in an off-limb spectrum.

S II emission lines in planetary nebulae

Emission lines arising from transitions in S II have been detected in a wide variety of astronomical sources, including planetary nebulae (Hyung, Keyes & Aller 1995). These transitions are used to derive information on emitting plasmas parameters (Te, Ne) through diagnostic line ratios, although to calculate these quantities reliably, accurate atomic data must be employed, especially for electron impact excitation rates.