Solving the Nebular Abundances Anomaly: New Features in Photoionization and Recombination

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This project aims to resolve longstanding discrepancies in the abundances of common elements such as carbon, nitrogen, oxygen, and neon in photoionizaed diffuse nebulae, particularly planetary nebulae. The ionization structure in optically thin environments is governed by photoionization and recombination. The physical processes underlying spectral line formation are electron impact excitation (EIE) or electron-ion recombination (RC), and radiative decay rates. A perplexing discrepancy in abundances, of up to an order of magnitude of more, is found between those derived from emission lines excited due to EIE and those from electron-ion recombination lines of the same atomic species. This not only calls into question the basic physics of photoionization and recombination, but also the interpretation of the physical structure and distribution of elements within gaseous nebulae. This project will examine the former in order to address the latter. Specifically, calculations will be based on: i) the unified theory of electron-ion recombination that subsumes radiative and dielectronic recombination processes, and provides a self-consistent set of atomic data for photoionization, recombination and excitation processes, (ii) recent extensions in the state-of-the-art R-matrix method including relativistic effects not heretofore considered, and (iii) pilot calculations for photoionization and recombination demonstrating the role of low-energy resonances that may be crucial to resolving the abundance(s) anomaly problem. The project will involve large-scale and high-accuracy calculations of the critical atomic parameters for a number of elements. High precision is imperative since astrophysical models need to rule out atomic physics as any significant source of uncertainty. The focus will be on some of the most prominent elements in low ionization stages: carbon, nitrogen, oxygen, neon, magnesium, silicon and sulfur. The project will result in extensive and self-consistent sets of high-accuracy radiative and collisional atomic parameters.Through analysis of their optical and infrared spectra, astronomers attempt to understand the abundances of the elements in diffuse insterstellar clouds or "nebulae" in which hydrogen is ionized. They do this by measuring the strengths of the characteristic emission from ions of different elements and use these to determine the physical properties within the nebula including the abundances of the constituent elements. These emission lines can be excited via several processes and there is a long standing discrepancy between the abundances measured using spectral lines excited by collisions versus those due to electron capture. This project aims to remove one level of uncertainty by calculating the theoretical values for the atomic parameters that relate the excitation to the strengths of the emission. The results of this research will be applicable across a number of sub-fields within astrophysics ranging from studies of our own galaxy to abundances in the most distant quasars.