The Primordial Helium-4 Abundance Determination: Systematic Effects

By extrapolating to O/H = N/H = 0 the empirical correlations Y –O/H and Y –N/H defined by a relatively large sample of ∼ 45 Blue Compact Dwarfs (BCDs), we have obtained a primordial Helium mass fraction Yp= 0.2443±0.0015 with dY /dZ = 2.4±1.0. This result is in excellent agreement with the average Yp= 0.2452±0.0015 determined in the two most metal-deficient BCDs known, I Zw 18 (Z⊙/50) and SBS 0335–052 (Z⊙/41), where the correction for He production is smallest. The quoted error (1σ) of ∼ < 1% is statistical and does not include systematic effects. We examine various systematic effects including collisional excitation of Hydrogen lines, ionization structure and temperature fluctuation effects, and underlying stellar He I absorption, and conclude that combining all systematic effects, our Yp may be underestimated by ∼ 2–4%. Taken at face value, our Yp implies a baryon-to-photon number ratio η = (4.7 −0.8)×10 −10 and a baryon mass fraction Ωbh100 = 0.017±0.005 (2σ), consistent with the values obtained from deuterium and Cosmic Microwave Background measurements. Correcting Yp upward by 2–4% would make the agreement even better.