On the abundance of primordial helium

We have used recent observations of helium-4, nitrogen and oxygen from some four dozen, low metallicity, extra-galactic HII regions to define mean N versus O, ⁴He versus N and ⁴He versus O relations which are extrapolated to zero metallicity to determine the primordial ⁴He mass fraction YP. The data and various subsets of the data, selected on the basis of nitrogen and oxygen, are all consistent with YP = 0. 232 0. 003. For the 2 (statistical) upper bound we find YP^2 0. 238. Estimating a 2\% systematic uncertainty (ₒₘₒₓ = 0. 005) leads to a maximum upper bound to the primordial helium mass fraction: YP^MAX = YP^2 + ₒₘₒₓ 0. 243. We compare these upper bounds to YP with recent calculations of the predicted yield from big bang nucleosynthesis to derive upper bounds to the nucleon-to-photon ratio (₁₀ 10^10) and the number of equivalent light (10 MeV) neutrino species. For YP 0. 238 (0. 243), we find ₁₀ 2. 5 (3. 9) and N_ 2. 7 (3. 1). If indeed YP 0. 238, then BBN predicts enhanced production of deuterium and helium-3 which may be in conflict with the primordial abundances inferred from model dependent (chemical evolution) extrapolations of solar system and interstellar observations. Better chemical evolution models and more data - especially D-absorption in the QSO Ly- clouds - will be crucial to resolve this potential crisis for BBN. The larger upper bound, YP 0. 243 is completely