Theory of Recombination Lines in Astrophysical Sources

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

AST-0506889 Pradhan Determination of accurate element abundances is a fundamental problem in astrophysics. But abundance anomalies and inaccuracies abound in individual and entire classes of sources. Galactic and extragalactic H II regions are among the most extensively observed objects; yet, abundances from different methods are discrepant by several factors or even orders of magnitude, such as the large variations in the iron/nickel ratio deduced from emission line studies. Dr. Anil Pradhan's effort with this award will enable accurate determination of physical conditions and element abundances derived from emission lines by (A) the first calculations of recombination line intensities for a number of astrophysically abundant ions, and (B) including the heretofore neglected contribution of electron-ion recombination to prominent emission lines. The main theoretical issue (as opposed to observational uncertainties) is that accurate analysis of observed lines from abundant atomic ions depend on atomic parameters that are highly uncertain. The two primary mechanisms are collisional excitation and electron-ion recombination. A longstanding problem in astrophysics is the continuing discrepancy between abundances derived using these two methods. Whereas considerable theoretical work has been done on the calculation of collisional excitation cross sections and rate coefficients, little effort has been devoted to the theory of recombination lines except for hydrogen-like, helium-like and a few other light ions. This is due to the inherent difficulty in the treatment of the atomic physics of a large number of levels that contribute to the formation of recombination lines. Abundance inhomogeneities may also be related to structure and physical processes. Very detailed optical and near-infrared (O/NIR) spectra already exist, and will undergo a huge increase in quality and quantity with the advent of new 8-m class telescopes and high-resolution spectrographs. In principle, therefore, spectral studies should be able to ascertain element abundances precisely. Dr. Pradhan's work will help rule out uncertainties due to atomic data and neglected atomic processes. He will utilize recent extensions of the most advanced atomic physics techniques, developed under the Opacity Project and the ongoing Iron Project, using the powerful and state-of-the-art relativistic R-matrix method, and a new method for calculating total and level-specific recombination rate coefficients subsuming both the radiative and the dielectronic recombination processes. Detailed treatments of relativistic fine structure and resonance phenomena are essential for accuracy, and would be incorporated in an ab initio manner. The primary emphasis will be on some of the most important elements C,N,O,Ne,Si,S, Fe, and Ni in multiple ionization states. Several areas of physics and astrophysics should benefit from the study of radiative processes and data in the proposed research. Very accurate and large sets of transition probabilities will be calculated for use in the construction of recombination-cascade matrices that yield the effective rate coefficients. In addition, Dr. Pradhan's theoretical approach is based on a self-consistent treatment of photoionization and recombination in an ab initio quantum mechanical manner. A wide range of applications to laboratory and astrophysical plasma sources are envisaged. The results of this work should be broadly applicable to the understanding of elemental composition of stars, nebulae and HII regions, galaxies, supernovae, etc., and hence the chemical history of the Universe. This project is especially designed for training of graduate students as the next generation of astrophysicists capable of these complex calculations. The project relies heavily on high-performance computing. The computational tools and resources needed for this undertaking have been secured on a variety of vector and massively parallel platforms through a Special Allocation at the Ohio Supercomputer Center at no cost to NSF. ***