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Register a new userRecombination Line vs. Forbidden Line Abundances in Planetary Nebulae
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Mark Robertson-Tessi
Publication date
2004
fields
Physics
and research's language is
English
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Recombination lines (RLs) of C II, N II, and O II in planetary nebulae (PNs) have been found to give abundances that are much larger in some cases than abundances from collisionally-excited forbidden lines (CELs). The origins of this abundance discrepancy are highly debated. We present new spectroscopic observations of O II and C II recombination lines for six planetary nebulae. With these data we compare the abundances derived from the optical recombination lines with those determined from collisionally-excited lines. Combining our new data with published results on RLs in other PNs, we examine the discrepancy in abundances derived from RLs and CELs. We find that there is a wide range in the measured abundance discrepancy Delta(O+2) = log O+2(RL) - log O+2(CEL), ranging from approximately 0.1 dex up to 1.4 dex. Most RLs yield similar abundances, with the notable exception of O II multiplet V15, known to arise primarily from dielectronic recombination, which gives abundances averaging 0.6 dex higher than other O II RLs. We compare Delta(O+2) against a variety of physical properties of the PNs to look for clues as to the mechanism responsible for the abundance discrepancy. The strongest correlations are found with the nebula diameter and the Balmer surface brightness. An inverse correlation of Delta(O+2) with nebular density is also seen. Similar results are found for carbon in comparing C II RL abundances with ultraviolet measurements of C III].
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We have obtained deep optical, long-slit spectrophotometry of the Galactic HII regions M 17, NGC 3576 and of the Magellanic Cloud HII regions 30 Doradus, LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of CII, NII and OII. Temperature-insensitive ORL C2+/O2+ and N2+/O2 ratios are obtained for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable agreement within each galactic system, while also being in agreement with the corresponding CEL ratios. For all five nebulae, the O2+/H+ abundance derived from multiple OII ORLs is found to be higher than the corresponding value derived from the strong [OIII] 4959, 5007A CELs, by factors of 1.8--2.7 for four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy factor for the O2+ ion, ~5. Thus these HII regions exhibit ORL/CEL abundance discrepancy factors that are similar to those previously encountered amongst planetary nebulae. Our optical CEL O2+/H+ abundances agree to within 20-30 per cent with published O2+/H+ abundances that were obtained from observations of infrared fine-structure lines. Since the low excitation energies of the latter make them insensitive to variations about typical nebular temperatures, fluctuations in temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance discrepancies. We present evidence that the observed OII ORLs from these HII regions originate from gas of very similar density (<3500 cm-3) to that emitting the observed heavy-element optical and infrared CELs, ruling out models that employ high-density ionized inclusions in order to explain the abundance discrepancy. We consider a scenario whereby much of the heavy-element ORL emission originates from cold (<=500 K) metal-rich ionized regions.
We present new observations of O II recombination lines in ten bright planetary nebulae, along with spatially-resolved measurements of O II and [O III] in the Ring nebula NGC 6720, to study the discrepancy between abundances derived from O II recombination lines and those derived from collisionally-excited [O III]. We see a large range in the difference between O II- and [O III] derived abundances, from no difference up to a factor six difference. The size of this discrepancy is anti-correlated with nebular surface brightness; compact, high-surface-brightness nebulae have the smallest discrepancies. O II levels that are populated mainly by dielectronic recombination give larger abundances than other levels. Finally, our long-slit observation of the Ring nebula shows that the O II emission peaks interior to the bright shell where [O III] and H-beta are strongest. Based on the observed correlations, we propose that the strong recombination line emission in planetary nebulae is a result of enhanced dielectronic recombination in hot gas in the nebular interior, perhaps driven by a hot stellar wind.
We present spectrophotometry of 12 Galactic and 3 Magellanic Cloud planetary nebulae (PNe). Nine of the Galactic PNe were observed by scanning the slit across the PN. We use the fluxes of collisionally excited lines (CELs) to derive electron densities (Ds) and temperatures (Ts), and ionic abundances. We find that the Ds derived from optical CEL ratios are systematically higher than those derived from the ratios of the IR fine-structure (FS) lines of [OIII], indicating the presence of significant density variations within the PNe. We also compare Ts obtained from the ratio of optical nebular to auroral [OIII] lines with those obtained from the ratio of [OIII] optical to IR FS lines. We find that when the latter are derived using Ds based on the [OIII] 52um/88um ratio, they yield values that are significantly higher than the optical [OIII] Ts. Contrasting this, [OIII] optical/IR Ts derived using the higher Ds obtained from [ClIII] 5517A/5537A ratios show much closer agreement with optical [OIII] Ts, implying that the observed [OIII] optical/IR ratios are significantly weighted by Ds in excess of the critical densities of both [OIII] FS lines. Consistent with this, ionic abundances derived from [OIII] and [NIII] FS lines using Ds from optical CELs show much better agreement with abundances derived for the same ions from optical and UV CELs than do abundances derived from the FS lines using the lower Ds obtained from the 52um/88um ratios. The behaviour of Ts obtained making use of the T-insensitive IR FS lines provides no support for significant T-fluctuations within the PNe that could be responsible for derived Balmer jump Ts being lower than those obtained from the much more T-sensitive [OIII] optical lines.
[Abridged] Deep optical observations of the spectra of 12 Galactic planetary nebulae (PNe) and 3 Magellanic Cloud PNe were presented in Paper I by Tsamis et al. (2003b), who carried out an abundance analysis using the collisionally excited forbidden lines. Here, the relative intensities of faint optical recombination lines (ORLs) from ions of carbon, nitrogen and oxygen are analysed in order to derive the abundances of these ions relative to hydrogen. We define an abundance discrepancy factor (ADF) as the ratio of the abundance derived for a heavy element ion from its recombination lines to that derived for the same ion from its ultraviolet, optical or infrared collisionally excited lines (CELs). All of the PNe in our sample are found to have ADFs that exceed unity. There is no dependence of the magnitude of the ADF upon the excitation energy of the UV, optical or IR CEL transition used, indicating that classical nebular temperature fluctuations--i.e. in a chemically homogeneous medium--are not the cause of the observed abundance discrepancies. Instead, we conclude that the main cause of the discrepancy is enhanced ORL emission from cold ionized gas located in hydrogen-deficient clumps inside the main body of the nebulae. We have developed a new electron temperature diagnostic, based upon the relative intensities of the OII 4f-3d 4089A and 3p-3s 4649A recombination transitions. For six out of eight PNe for which both transitions are detected, we derive O2+ ORL electron temperatures of <300 K, very much less than the O2+ forbidden-line and Balmer jump temperatures derived for the same nebulae. These results provide direct observational evidence for the presence of H-deficient, cold plasma regions within the nebulae, consistent with gas cooled largely by infrared fine structure and recombination transitions.
The CHAOS project is building a large database of LBT H II region spectra in nearby spiral galaxies to use direct abundances to better determine the dispersion in metallicity as a function of galactic radius. Here, we present CHAOS LBT observations of C II $lambda$4267 emission detected in 10 H II regions in M 101, and, using a new photoionization model based ionization correction factor, we convert these measurements into total carbon abundances. A comparison with M101 C II recombination line observations from the literature shows excellent agreement, and we measure a relatively steep gradient in log(C/H) of -0.37 +/- 0.06 dex/R_e. The C/N observations are consistent with a constant value of log(C/N) = 0.84 with a dispersion of only 0.09 dex, which, given the different nucleosynthetic sources of C and N, is challenging to understand. We also note that when plotting N/O versus O/H, all of the H II regions with detections of CII $lambda$4267 present N/O abundances at the minimum of the scatter in N/O at a given value of O/H. If the high surface brightness necessary for the detection of the faint recombination lines is interpreted as an indicator of H II region youth, then this may point to a lack of nitrogen pollution in the youngest H II regions. In the future, we anticipate that the CHAOS project will significantly increase the total number of C II $lambda$4267 measurements in extragalactic H II regions.
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