On the abundance of primordial helium

Olive, Keith A.; Steigman, Gary

doi:10.1086/192134

On the abundance of primordial helium

Keith A. Olive
(
- Minnesota U.
)
,
Gary Steigman
(
- Ohio State U. and
- Cambridge U.
)

Apr, 1994

27 pages

Published in:

Astrophys.J.Suppl. 97 (1995) 49-58

e-Print:

astro-ph/9405022 [astro-ph]

DOI:

10.1086/192134

Report number:

UMN-TH-1230-94,
OSU-TA-6-94

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Abstract: (arXiv)

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

,

~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 nucleon-to-photon 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 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