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 ⁴Helium mass fraction Y_p=0.2443±0.0015 with dY/dZ=2.4±1.0. This result is in excellent agreement with the average Y_p=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 (1) 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 Hei absorption, and conclude that combining all systematic effects, our Y_p may be underestimated by 2–4%. Taken at face value, our Y_p implies a baryon-to-photon number ratio =(4.7^+1.0_–0.8)×10^–10 and a baryon mass fraction _bh²₁₀₀=0.017±0.005 (2), consistent with the values obtained from deuterium and Cosmic Microwave Background measurements. Correcting Y_p upward by 2–4% would make the agreement even better.