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 2σ (statistical) upper bound we find YP2σ ≤ 0.238. Estimating a 2% systematic uncertainty (σ syst = ± 0.005) leads to a maximum upper bound to the primordial helium mass fraction: YPMAX = YP2σ + σsyst ≤ 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 η (η₁₀ ≡ 10¹⁰η) and the number of equivalent light ( 10 MeV) neutrino species. For YP ≤ 0.238 ($0.243$), we find η₁₀ ≤ 2.5 (3.9) and N_ν ≤ 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-α clouds - will be crucial to resolve this potential crisis for BBN. The larger upper bound, YP ≤ 0.243 is completely