Planetary Nebulae

 Planetary nebulae (PNe) occupy a pivotal position in several branches of modern astrophysics. They are the chief source in the Universe of carbon, the basic constituent of life, and of s-process elements, which in the solar system, dominate the abundances of elements heavier than the iron-group. In terms of stellar evolution, PNe provide a link between the last ascent of the red giant branch (the asymptotic giant branch, or AGB) and the white dwarf end-point to the life of low- and intermediate-mass stars. PNe enrich the surrounding interstellar medium and directly influence the next generation of star formation, thus having important implications for galactic evolution.

Figure 1. The various forms and shapes of Planetary Nebulae.

PNe have played a crucial role in the development of the nebular diagnostic techniques that are applied to all galactic and extragalactic nebulae, This is because they have well-resolved structures, usually posses's a high degree of symmetry, and are ionized by a single, centrally located star. For this reason they provide probably the best astrophysical laboratories for the diagnosis of physical processes operating within ionized nebulae. They provide excellent test beds for atomic data and for stellar and nebular models as well as testing important stellar evolution predictions.

Abundance determinations - discrepancy & opportunity

Emission line nebulae are ionized and heated by the strong UV radiation fields from stars (or the central engines in the case of AGNs) and glow by emitting strong emission lines. The spectra of ionized nebulae are dominated by strong H and He recombination lines, emitted following capture of free electrons by ionized H and He atoms, and by collisional lines excited by inelastic electron impacts with heavy elements ions. Until recently, heavy element abundances of ionized nebulae have been based on bright optical and UV collisionally excited lines (CELs, e.g. [O III] 5007 Å, C III] 1908 Å).

Figure 2. A typical planetary nebula spectrum.

The advent of high quantum efficiency large-area CCD detectors on medium and large telescopes opened up a new era of very deep spectroscopy of nebulae, with simultaneous access to reliable fluxes for extremely weak and relatively strong emission lines. For the first time, the recombination spectra of the elements heavier than helium could be analyzed, precisely at the time when extensive and reliable atomic data on photoionization and recombination became available. Accurate ionic abundances from optical recombination lines (ORLs) are now possible. Our recent abundance analyses of PNe using this new technique strongly indicate that the traditional method based on CELs may have significantly underestimated metal abundances for many nebulae. Being excited by the same mechanism as H recombination lines, ionic abundances derived from ORLs are almost independent of the thermal and density structure of the objects under study and are therefore intrinsically more reliable than CEL abundances, which have an exponential (Boltzmann factor) dependence on the adopted electron temperature. For CELs with low critical densities, the results are also sensitive to the adopted electron density. Using high quality measurements, we found that the ORL CNO abundances deduced for NGC 7009 were all about a factor of 5 higher than the corresponding CEL values. These large discrepancies between ORL and CEL abundances are not an isolated phenomenon -- analyses of a large sample of Galactic PNe for which we have obtained deep optical spectra show that the ORL to CEL abundance ratios vary from source to source and span a wide range from 1-80.

The large discrepancies between the CEL and ORL abundances observed in many PNe point to the possibility that there are fundamental flaws in our current understanding of the nebular thermal and ionization structure. Without a better understanding of when, why and how the discrepancy occurs, nebular abundances cannot be taken as intrinsically secure. Since the discrepancy varies from object to object, the cause is likely to be found in the nebular physical conditions, rather than in the basic atomic physics. Given the strong temperature dependence of CEL abundances, the lower CEL abundances have often been attributed to the presence of large temperature fluctuations in nebulae. The large temperature fluctuations required to reconcile ORL and CEL abundances are not predicted by photoionization models. Proposed mechanisms include shocks, density inhomogeneities and abundance gradients. Direct observational evidence in favour of such interpretations still remains to be found however.

Figure 3. Multi-wavelength imaging of NGC 6153.

Our multi-waveband abundance analysis of NGC 6153 provided fresh insight into this fundamental problem. Deep high resolution spectroscopy of this unusual nebula unveiled a forest of strong ORLs from CNO and Ne ions. Utilizing data from the UV to the far-IR, we found that its ORL CNO and Ne abundances relative to H are all consistently higher, by a factor of ten, than the corresponding values deduced from optical, UV or infrared CELs, regardless of the excitation energies or critical densities of the latter. While the He abundance is constant across the nebula, the C and O ORL abundances decrease by a factor of two over a radius of 15 arcsec from the centre, pointing to abundance gradients.

Figure 4. Abell 30

The ORL O/H abundance of NGC 6153, six times solar, is three times that of NGC7009, and is amongst the highest of the 80 PNe surveyed by us, only surpassed by M1-42 and Hf 2-2. The high metallicity of nebulae such as NGC 6153, together with their very large ORL to CEL abundance discrepancy factors, offers a unique opportunity to test possible explanations for this fundamental abundance determination problem observed in many PNe. In the case of NGC 6153, temperature fluctuations alone were rejected because the IR fine structure lines, which have no significant temperature sensitivity, yield abundances very similar to those given by the UV/optical CELs. Similarly, density inhomogeneities alone failed to explain all the observed patterns, in particular the low electron density deduced from the high-n Balmer line decrement. Instead, we found that empirical nebular models containing two components with very different densities and temperatures were able to account for many of the observed patterns, but only if one of the components is significantly H-deficient and very cool, suggesting that NGC 6153 may have experienced a recent ejection of H-deficient knots, similar to those observed in the `born-again' PN A30.

Thanks to Xiaowei Liu for this article.