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, He versus N and He versus O relations which are extrapolated to zero metallicity to determine the primordial He 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 Y 2σ P ≤ 0.238. Estimating a 2% systematic uncertainty (σsyst = ±0.005) leads to a maximum upper bound to the primordial helium mass fraction: Y MAX P = Y 2σ P +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 ≡ 10 η) and the number of equivalent light (< ∼ 10 MeV) neutrino species. For YP ≤ 0.238 (0.243), we find η10 ≤ 2.5(3.9) and Nν ≤ 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 consistent with BBN which, now, bounds the universal density of nucleons (for Hubble parameter 40 ≤ Ho ≤ 100 kms Mpc and cosmic background radiation temperature T = 2.726 ± 0.010) to lie in the range 0.01 ≤ ΩBBN ≤ 0.09 (for Ho = 50h50kms Mpc, 0.04 ≤ ΩBBNh 2