AAS 202nd Meeting, May 2003
Session 29 Molecular Clouds and the ISM
Poster, Tuesday, May 27, 2003, 10:00am-6:30pm, West Exhbit Hall

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[29.13] He/H ratio for Cosmology: Accurate He recombination coefficients including fine structure and singlet-triplet mixing

R.P. Bauman, G.J. Ferland, K.B. MacAdam (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. All previous studies of He~{\sc I} recombination coefficients have assumed that the triplets and singlets are distinct radiative cascade systems that communicate only through electron exchange collisions. Actually, certain large-angular-momentum levels have strongly mixed spin multiplicities, leading to conventional electric-dipole decays that act to mix the singlets and triplets. We present calculations of He~{\sc I} recombination coefficients at a temperature 10,000 K in "Case B" explicitly including fine structure in the helium transition rates and energies. Changes 1% or greater are observed in several important emission line intensities, and these changes may effect the deduced Y\rm p by cosmologically significant amounts. This project is supported by the NSF and NASA through grants AST 0071180 and NAG5-8212.

The author(s) of this abstract have provided an email address for comments about the abstract: bauman@pa.uky.edu

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