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G.J. Ferland, R.P. Bauman, K.B. MacAdam (Univ of Kentucky)
Hydrogen and helium recombination emission lines are used to determine chemical abundances and physical conditions in a variety of objects, and form the foundation for much of our understanding of the chemical evolution of the Universe. We are developing the basic software tools needed to predict recombination line intensities, atomic transition probabilities, photoionization cross-sections, and radiative recombination rate coefficients, for H-like and He-like ions. This work is an improvement over previous calculations since we explicitly take fine structure into account by basing our transition rates on the results of Drake [Phys. Rev. A 46, 2378 (1992)]. We will discuss implications for HeI line formation with application to the primordial helium abundance.
Although the needed radiative rates are known to high precision, the collisional rate coefficients involving higher levels of H and He are uncertain by factors of 2 to 3. These introduce 5-20 percent uncertainties in optical and infrared lines for some conditions. Our codes will be made publicly available so that anyone can check the effects of other collisional rates on observed lines, with the hope of promoting further work. This project is supported by the NSF and NASA through grants AST 0071180 and NAG5-8212.
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