## Abstract

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, ^{4}He vs. N, and ^{4}He vs. O relations which are extrapolated to zero metallicity to determine the primordial ^{4}He 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 σ (statistical) upper bound we find Y_{P}^{2 σ} ≤ 0.238. Estimating a 2% systematic uncertainty (σ_{syst} = ± 0.005) leads to a maximum upper bound to the primordial helium mass fraction: Y_{P}^{MAX} = Y_{P}^{2 σ} + σ_{syst} ≤ 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 η(η_{10} ≡ 10^{10}_{η}) and the number of equivalent light (≲10 MeV) neutrino species. For Y_{p} ≤ 0.238 (0.243), we find η_{10} ≤ 2.5(3.9) and N_{v} ≤ 2.7(3.1). If indeed Y_{P} ≤ 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, Y_{P} ≤ 0.243, is completely consistent with BBN which, now, bounds the universal density of nucleons ( for Hubble parameter 40 ≤ H_{o} ≤ 100 km s^{-1} Mpc^{-1} and cosmic background radiation temperature T = 2.726 ± 0.010) to lie in the range 0.01 ≤ Ω_{BBN} ≤ 0.09 (for H_{o} = 50 h_{50} km s^{-1} Mpc^{-1}, 0.04 ≤ Ω_{BBN} h_{50}^{2} ≤ 0.06).

Original language | English (US) |
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Pages (from-to) | 49-58 |

Number of pages | 10 |

Journal | Astrophysical Journal, Supplement Series |

Volume | 97 |

Issue number | 1 |

DOIs | |

State | Published - 1995 |

## Keywords

- Cosmology: theory
- Nuclear reactions, nucleosynthesis, abundances