ON THE ABUNDANCE OF PRIMORDIAL HELIUM

We have used recent observations of helium-4, nitrogen, and oxygen fro m some four dozen, low-metallicity, extragalactic H II regions to defi ne mean N vs. O, He-4 vs. N, and He-4 vs. O relations which are extrap olated to zero metallicity to determine the primordial He-4 mass fract ion Y-P. The data and various subsets of the data, selected on the bas is of nitrogen and oxygen, are all consistent with Y-P = 0.232 +/- 0.0 03. For the 2 sigma (statistical) upper bound we find Y(P)2(sigma) les s than or equal to 0.238. Estimating a 2% systematic uncertainty (sigm a(syst) = +/- 0.005) leads to a maximum upper bound to the primordial helium mass fraction: Y-P(MAX) = YP2sigma + sigma(syst) less than or e qual to 0.243. We compare these upper bounds to Y-P With recent calcul ations of the predicted yield from big bang nucleosynthesis to derive upper bounds to the nucleon-to-photon ratio eta(eta(10) = 10(10)eta) a nd the number of equivalent light (less than or similar to 10 MeV) neu trino species. For Y-P less than or equal to 0.238 (0.243), we find et a(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 e nhanced 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 observatio ns. Better chemical evolution models and more data-especially D-absorp tion in the QSO Ly-alpha clouds-will be crucial to resolve this potent ial 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 univer sal 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 le ss than or equal to Omega(BBN) less than or equal to 0.09 (for H-0 = 5 0 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).