The Primordial Helium Abundance: Systematic Effects and a New Determination

Abstract

We present new high-quality spectrophotometric observations of 27 supergiant H II regions in 23 low-metallicity blue compact galaxies (BCGs) with oxygen abundance 12 + log (O/H) between 7.22 and 8.17 ( Z ☉ /50 ≤ Z ≤ Z ☉ /6), most of which were selected from the first and second Byurakan objective-prism surveys. Combining this new data with our previous sample of 10 low-metallicity BCGs, we extract a subsample of 27 H II regions in 24 BCGs most appropriate for the determination of the primordial helium abundance Y p . We find that the most metal-deficient BCG known, I Zw 18 ( Z ~ Z ☉ /50), cannot be used for this purpose because of its abnormally low He I line intensities. We have examined critically the systematic effects that may influence the determination of Y p . We find that the effects of the corrections for neutral helium and for underlying stellar absorption in the He I lines, possible deviations from case B recombination theory, fluorescent enhancement of the He I line intensities, temperature fluctuations in H II regions, Wolf-Rayet stellar winds, and supernova shock waves to be small. The main effect comes from the particular set of atomic data used. The best set of atomic data for use in the determination of Y p is composed of Smits's He I emissivities and Kingdon & Ferland's collisional enhancement correction factors. This set gives the smallest dispersion of the data in the Y -O/H and Y -N/H planes and corrects best the data for collisional enhancement effects. By extrapolating the Y versus O/H and Y versus N/H linear regressions to O/H = N/H = 0, we obtain Y p = 0.243 ± 0.003 for both regressions, considerably larger than the values derived before. In the framework of the standard hot big bang nucleosynthesis model with a number of neutrino families N ν = 3 and a neutron half-lifetime τ n = 887 s, our new Y p gives a baryon-to-photon number ratio η=3.5 1.0 −0.7 × 10 -10 , or a baryonic mass fraction Ω h 2 50 =0.05±0.01. Our Y p determination is fully consistent with measurements of the primordial 7 Li in Galactic halo stars and of (D + 3 He) in the local interstellar medium and the solar system, and consistent at the 2 σ level with the primordial D abundance derived in a quasar absorption system by Tytler, Fan, & Burles. We derive a slope dY / dZ = 1.7 ± 0.9, considerably smaller than the slopes obtained before and consistent with chemical evolution models for star-forming dwarf galaxies with an outflow of well-mixed material. Extrapolation to solar metallicity with such a slope gives the correct solar helium mass fraction within the errors.