AAS 204th Meeting, June 2004
Session 61 Structures in the Interstellar Medium
Poster, Wednesday, June 2, 2004, 10:00am-7:00pm, Ballroom

[61.02] Accurate Radiative Recombination Coefficients for Helium

R.P. Bauman, R.L. Porter, K.B. MacAdam, G.J. Ferland (Univ. of Kentucky)

Precision measurement of the primordial abundance of the light elements is one of the fundamental tests of the Big~Bang. The primordial abundance of helium, Y\rm p, the ratio of He to H by mass, is measured from emission lines that form during the recombination-decay process, {\rm He}+ + e- arrow {\rm He}0(nL) + h\nu followed by radiative cascade. A definitive test of the Big~Bang requires an accuracy of 1% or better. Y\rm p is measured from ratios of intensities of He{\sc I} and H{\sc I} recombination lines in H{\sc II}~Regions. The density ratio n({\rm He}+)/n({\rm H}+) is proportional to the observed line intensities.

We report the results of a recalculation of the He{\sc I} recombination process at a temperature of 10,000~K in the "Case~B" approximation. Our work builds on previous calculations by improving the physical treatment of radiative recombination and subsequent cascades by explicitly including fine structure in the helium transition rates and energies. Certain large-angular-momentum levels have strongly mixed spin multiplicities, leading to electric-dipole decays that act to mix the singlets and triplets. The transitions which are the result of this singlet-triplet mixing are included. However, changes of 1% or greater due to singlet-triplet mixing are not observed, as our earlier work suggested. Comparisons with previous calculations are presented along with an assessment of the remaining major uncertainties. This project is supported by the NSF and NASA through grants AST~0071180 and NAG5-8212.