| Description |
Abundance surveys of a large sample of Galactic planetary nebulae (PNe) have led to the discovery of a group of super-metal-rich nebulae whose spectra show prominent optical recombination lines (ORLs) from C, N, O, and Ne ions. The heavy element abundances derived from ORLs for several PNe are a factor >10higher than those derived from the traditional method based on collisionally excited lines (CELs). This ratio is called the abundance discrepancy factor (adf).A promising proposition to explain the nebular abundance problem posits that these nebulaecontain (at least) two distinct regions - one of normal electron temperature, Te (virgul10000 K) and chemical composition (virgulsolar) and another of very low Te (< 1000) that is H-deficient, thus having high metal abundances relative to H. The latter component emits strong heavyelement ORLs and IR fine-structure (FS) CELs but essentially no optical/UV CELs.Efforts to directly detect these inclusions in PNe have been unsuccessful to date.However, there is mounting circumstantial evidence for their existence, such as presented inour recent paper that modeled the high-adf PN NGC 6153 using a 3-D photoionization code.The models that included the low Te, H-deficient knots fit most observations far betterthan did those models without the clumps. With the launch of Herschel, there is finallythe capability to perform a test we.ve been dreaming of. Measurements have shown that theadf varies with position in a PN and is highest close to the central star. The very low Teinclusions must be cooled via FS IR lines. We propose to use Herschel to map the FS IR linesin 5 bright PNe on the largest adf list, to find if these lines peak where the adf peaks. These spectra will also provide a feast for our other team expertise/interests: a legacydataset of molecular lines to explore PDRs, how the central star interacts with the AGBejecta and shapes the PN, how the shocks are produced, what comprises the chemistry of themolecular ejecta, and how do PNe evolve. |
