J‐Resolved HeiEmission Predictions in the Low‐Density Limit

Determinations of the primordial helium abundance are used in precisioncosmological tests. These require highly accurate He I recombination ratecoefficients. Here we reconsider the formation of He I recombination lines inthe low-density limit. This is the simplest case and it forms the basis for themore complex situation where collisions are important. The formation of arecombination line is a two-step process, beginning with the capture of acontinuum electron into a bound state and followed by radiative cascade toground. The rate coefficient for capture from the continuum is obtained fromphotoionization cross sections and detailed balancing, while radiativetransition probabilities determine the cascades. We have made every effort touse today's best atomic data. Radiative decay rates are from Drake'svariational calculations, which include QED, fine structure, andsinglet-triplet mixing. Certain high-$L$ fine-structure levels do not have asinglet-triplet distinction and the singlets and triplets are free to mix indipole-allowed radiative decays. We use quantum defect or hydrogenicapproximations to include levels higher than those treated in the variationalcalculations. [Abstract clipped]Comment: 31 pages, 2 figures, accepted by ApJ, tentatively scheduled for 20 July 2005, v628