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 >10 higher 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 nebulae contain (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 heavy element 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 in our 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 better than did those models without the clumps. With the launch of Herschel, there is finally the capability to perform a test we.ve been dreaming of. Measurements have shown that the adf varies with position in a PN and is highest close to the central star. The very low Te inclusions must be cooled via FS IR lines. We propose to use Herschel to map the FS IR lines in 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 legacy dataset of molecular lines to expl...ore PDRs, how the central star interacts with the AGB ejecta and shapes the PN, how the shocks are produced, what comprises the chemistry of the molecular ejecta, and how do PNe evolve.
Publication
The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder - II. Estimating radial velocity of SPIRE spectral observation sources . Scott Jeremy P. et al. . Monthly Notices of the Royal Astronomical Society . null . null . 2020MNRAS.496.4894S , The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder - III. Line identification and off-axis spectra . Benson Chris S. et al. . Monthly Notices of the Royal Astronomical Society . null . null . 2020MNRAS.496.4906B , The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder I. The Spectral Feature Finder and Catalogue . Hopwood R. et al. . Monthly Notices of the Royal Astronomical Society . null . null . 2020MNRAS.496.4874H ,
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
European Space Agency, 2013, Herschel.S Opportunity To Solve The Nebular Abundance Problem While Creating A Legacy Planetary Nebulae Dataset, SPG v14.2.0, European Space Agency, https://doi.org/10.5270/esa-k0qduf2
