On the Abundance of Primordial Helium
Abstract
We have used recent observations of helium4, nitrogen and oxygen from some four dozen, low metallicity, extragalactic 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 $Y_P$. The data and various subsets of the data, selected on the basis of nitrogen and oxygen, are all consistent with $Y_P = 0.232 \pm 0.003$. For the 2$\sigma$ (statistical) upper bound we find $Y_P^{2\sigma} \le 0.238$. Estimating a 2\% systematic uncertainty $(\sigma _{syst} = \pm 0.005)$ leads to a maximum upper bound to the primordial helium mass fraction: $Y_P^{MAX} = Y_P^{2\sigma} + \sigma_{syst} \le 0.243$. 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 nucleontophoton ratio $\eta$ ($\eta_{10} \equiv 10^{10}\eta$) and the number of equivalent light (\lsim 10 MeV) neutrino species. For $Y_P \le 0.238$ ($0.243$), we find $\eta_{10} \le 2.5 (3.9)$ and $N_\nu \leq 2.7 (3.1)$. If indeed $Y_P \le 0.238$, then BBN predicts enhanced production of deuterium and helium3 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$\alpha$ clouds  will be crucial to resolve this potential crisis for BBN. The larger upper bound, $Y_P \leq 0.243$ is completely
 Publication:

The Astrophysical Journal Supplement Series
 Pub Date:
 March 1995
 DOI:
 10.1086/192134
 arXiv:
 arXiv:astroph/9405022
 Bibcode:
 1995ApJS...97...49O
 Keywords:

 Abundance;
 Big Bang Cosmology;
 H Ii Regions;
 Helium;
 Metallicity;
 Nitrogen;
 Nuclear Fusion;
 Oxygen;
 Chemical Evolution;
 Lyman Alpha Radiation;
 Statistical Correlation;
 Astrophysics;
 COSMOLOGY: THEORY;
 NUCLEAR REACTIONS;
 NUCLEOSYNTHESIS;
 ABUNDANCES;
 Astrophysics
 EPrint:
 21 pages, LaTeX, 6 postscript figures available upon request, UMNTH1230