Nebular and auroral emission lines of [Cl III]inthe optical spectra of planetary nebulae Francis P. Keenan*, Lawrence H. Aller†‡, Catherine A. Ramsbottom§, Kenneth L. Bell§, Fergal L. Crawford*, and Siek Hyung¶ *Department of Pure and Applied Physics, and §Department of Applied Mathematics and Theoretical Physics, The Queen’s University of Belfast, Belfast BT7 1NN, Northern Ireland; †Astronomy Department, University of California, Los Angeles, CA 90095-1562; and ¶BohyunSan Observatory, P.O. Box 1, Jacheon Post Office, Young-Cheon, KyungPook, 770-820, South Korea Contributed by Lawrence H. Aller, December 31, 1999 Electron impact excitation rates in Cl III, recently determined with vations. Specifically, we assess the usefulness of [Cl III] line ratios the R-matrix code, are used to calculate electron temperature (Te) as temperature and density diagnostics for planetary nebulae. and density (Ne) emission line ratios involving both the nebular (5517.7, 5537.9 Å) and auroral (8433.9, 8480.9, 8500.0 Å) transi- Atomic Data and Theoretical Line Ratios tions. A comparison of these results with observational data for a The model ion for Cl III consisted of the three LS states within sample of planetary nebulae, obtained with the Hamilton Echelle the 3s23p3 ground configuration, namely 4S, 2D, and 2P, making ؍ Spectrograph on the 3-m Shane Telescope, reveals that the R1 a total of five levels when the fine-structure splitting is included. ͞ I(5518 Å) I(5538 Å) intensity ratio provides estimates of Ne in Energies of all these levels were taken from Kelly (8). Test excellent agreement with the values derived from other line ratios calculations including the higher-lying 3s3p4 terms were found to in the echelle spectra. This agreement indicates that R1 is a reliable have a negligible effect on the 3s23p3 level populations at the density diagnostic for planetary nebulae, and it also provides electron temperatures and densities typical of planetary nebulae, observational support for the accuracy of the atomic data adopted and hence these states were not included in the analysis. in the line ratio calculations. However the [Cl III] 8433.9 Å line is Electron impact excitation rates for transitions in Cl III were found to be frequently blended with a weak telluric emission obtained from Ramsbottom et al. (5), whereas for Einstein feature, although in those instances when the [Cl III] intensity may A-coefficients the calculations of Mendoza and Zeippen (9) be reliably measured, it provides accurate determinations of T e were adopted. Our previous work on the isoelectronic ions S II when ratioed against the sum of the 5518 and 5538 Å line fluxes. (3) and Ar IV (4) has shown that excitation by protons is Similarly, the 8500.0 Å line, previously believed to be free of unimportant under nebular conditions, and hence this process contamination by the Earth’s atmosphere, is also shown to be has not been included in the present analysis. generally blended with a weak telluric emission feature. The [Cl III] Using the atomic data discussed above in conjunction with the transition at 8480.9 Å is found to be blended with the He I 8480.7 statistical equilibrium code of Dufton (10), we derived relative Å line, except in planetary nebulae that show a relatively weak Cl III level populations and hence emission line strengths for a He I spectrum, where it also provides reliable estimates of Te when ϭ ratioed against the nebular lines. Finally, the diagnostic potential range of electron temperatures (Te 5,000–20,000 K) and densities (N ϭ 101.5 to 105.5 cmϪ3). Details of the procedures of the near-UV [Cl III] lines at 3344 and 3354 Å is briefly discussed. e involved and approximations made may be found in Dufton (10) and Dufton et al. (11). Given errors of typically Ϯ10% in both t was recognized in the 1940s that forbidden emission line the adopted electron excitation rates and A-values, we estimate Iratios from nebular plasmas would provide a means for that our derived theoretical line ratios should be accurate to estimating electron densities (Ne) and temperatures (Te)inthe Ϯ15%. emitting layers. A difficulty has been to obtain N and T for the e e The [Cl III] nebular emission line ratio same ion, as different ions may be located in strata of differing parameters. For example, the intensity ratio of the auroral͞ ϭ 2 34 2 32 2 34 Ϫ 2 32 R1 I(3s 3p S–3s 3p D5/2)I(3s 3p S 3s 3p D3/2) nebular lines of [O III] have long been used to derive Te for nebular plasmas, while the ratio of nebular lines of [O II] (3729 ϭ I(5517.7 Å)͞I(5537.9 Å) Å͞3726 Å) and [S II] (6717 Å͞6730 Å) have provided densities. 2ϩ ϩ ϩ Thus we obtain Te in the O zone and Ne in the O or S zone. is a well known density diagnostic for planetary nebulae (12). 2ϩ To get Ne in the O zone from [O III] lines we would need not However, Czyzak et al. (13) have pointed out that ratios in P-like only the usual optical region transitions, but also fine-structure ions involving both the nebular lines and the auroral 3s23p3 2 2 32 infrared lines, which require observations from above the Earth’s D–3s 3p P transitions should allow both Te and Ne to be atmosphere. derived, and illustrated this by plotting theoretical results for [S 3 ͞ The lines of [O II], [Ne IV], [S II], and [Ar IV] all arise from p II]. In Figs. 1 and 2 we therefore plot R1 against the auroral configurations and fall in the optical or near-UV part of the nebular line ratios spectrum. By comparing the auroral or transauroral type tran- ϭ 2 32 2 32 ͞ 2 34 2 32 sitions with those of nebular type (for example, the 4068 Å R2 I(3s 3p D3/2–3s 3p P3/2 I(3s 3p S–3s 3p D3/2,5/2) transauroral with the 6717, 6730 Å nebular transitions of [S II]), ASTRONOMY ϭ I(8433.9 Å)͞I(5517.7 ϩ 5537.9 Å) we can derive both Te and Ne in the layers responsible for the emission. Accurate calculations for all relevant transitions have now been obtained for these ions (1–4). Abbreviation: HES, Hamilton Echelle Spectrograph. Another ion of this group is Cl2ϩ, for which Ramsbottom et al. ‡To whom reprint requests should be addressed. E-mail: [email protected]. (5) have very recently calculated electron impact excitation rates The publication costs of this article were defrayed in part by page charge payment. This by using the R-matrix method as adapted for the Opacity Project article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. (6, 7). In this paper, we use the Ramsbottom et al. (5) results to §1734 solely to indicate this fact. derive emission line ratios applicable to planetary nebulae, and Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073͞pnas.070590597. we compare these with high spectral resolution optical obser- Article and publication date are at www.pnas.org͞cgi͞doi͞10.1073͞pnas.070590597 PNAS ͉ April 25, 2000 ͉ vol. 97 ͉ no. 9 ͉ 4551–4555 Downloaded by guest on September 25, 2021 2 32 Fig. 2. Same as Fig. 1, except for R1 against R3 ϭ I(3s 3p D3/2– Fig. 1. Plot of the theoretical [Cl III] nebular emission line ratio R1 ϭ 2 32 2 34 2 32 3s 3p P1/2)͞I(3s 3p S–3s 3p D3/2,5/2) ϭ I(8500 Å)͞I(5518 ϩ 5538 Å). 2 34 2 32 2 34 2 32 I(3s 3p S–3s 3p D5/2)͞I(3s 3p S–3s 3p D3/2) ϭ I(5518 Å)͞I(5538 Å) against 2 32 2 32 2 34 2 3 the auroral͞nebular ratio R2 ϭ I(3s 3p D3/2–3s 3p P3/2)͞I(3s 3p S–3s 3p 2 ϭ ͞ ϩ D3/2,5/2) I(8434 Å) I(5518 5538 Å), where I is in energy units, for a range brightness. For nebular observations, a slit width of 640 ␮m (1.16 of electron temperatures (Te ϭ 5,000–20,000 K in steps of 2,500 K) and ϭ Ϫ3 arcsec) and a slit length of 4 arcsec were adopted, giving a logarithmic electron densities (log Ne 1.5–5.5 in steps of 0.5 dex; Ne in cm ). ϳ Points of constant Te are connected by solid lines, whereas those of constant spectral resolution of 0.2 Å (full width at half maximum). Ne are joined by dashed lines. These choices were imposed by constraints on spectral purity (for slit width) and overlapping orders, especially in the red (for slit length). The total area accepted by the slit is generally much and smaller than the whole nebular image. In addition to the usual exposures on laboratory arcs and appropriate comparison stars, ϭ 2 32 2 32 ͞ 2 34 2 32 R3 I(3s 3p D3/2–3s 3p P1/2 I(3s 3p S–3s 3p D3/2,5/2) one also needs a diffuse continuous source to allow for pixel- ϭ I(8500.0 Å)͞I(5517.7 ϩ 5537.9 Å), to-pixel sensitivity variations. Our basic observing and reduction procedures are described in Keyes et al. (14) and Hyung (15). respectively, for a grid of (Te, log Ne) values. Clearly, measure- In Table 1 we list the planetary nebulae considered in the ments of R1 and R2,orR1 and R3, should in principle allow the present analysis, their date of observation, and the measured simultaneous determination of Te and Ne for the [Cl III] emitting intensities (I) of the [Cl III] 5518, 5538, 8434, 8481, and 8500 Å ␤ ϭ 2 3 2 32 region of the nebula.