ON THE ABUNDANCE OF PRIMORDIAL HELIUM

We have used recent observations of helium-4, nitrogen, and oxygen from some four dozen, low-metallicity, extragalactic H II regions to define mean N vs. O, He-4 vs. N, and He-4 vs. O relations which are extrapolated to zero metallicity to determine the primordial He-4 mass fraction Y-P. The data and various subsets of the data, selected on the basis of nitrogen and oxygen, are all consistent with Y-P = 0.232 +/- 0.003. For the 2 sigma (statistical) upper bound we find Y(P)2(sigma) less than or equal to 0.238. Estimating a 2% systematic uncertainty (sigma(syst) = +/- 0.005) leads to a maximum upper bound to the primordial helium mass fraction: Y-P(MAX) = YP2sigma + sigma(syst) less than or equal to 0.243. We compare these upper bounds to Y-P With recent calculations of the predicted yield from big bang nucleosynthesis to derive upper bounds to the nucleon-to-photon ratio eta(eta(10) = 10(10)eta) and the number of equivalent light (less than or similar to 10 MeV) neutrino species. For Y-P less than or equal to 0.238 (0.243), we find eta(10) less than or equal to 2.5(3.9) and N-nu less than or equal to 2.7(3.1). If indeed Y-P less than or equal to 0.238, then BBN predicts enhanced production of deuterium and helium-3 which may be in condlict 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-alpha clouds-will be crucial to resolve this potential crisis for BBN. The larger upper bound, Y-P less than or equal to 0.243, is completely consistent with BBN which, now, bounds the universal density of nucleons (for Hubble parameter 40 less than or equal to H-0 less than or equal to 100 km s(-1) Mpc(-1) and cosmic background radiation temperature T = 2.726 +/- 0.010) to lie in the range 0.01 less than or equal to Omega(BBN) less than or equal to 0.09 (for H-0 = 50 h(50) km s(-1) Mpc(-1), 0.04 less than or equal to Omega(BBN) h(50)(2) less than or equal to 0.06).