The Primordial Helium Abundance: Systematic Effects and a New Determination

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 Yp. 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.