2001ApJ...562..727I

We present high-quality Keck telescope spectroscopic observations of the two metal-deficient blue compact dwarf (BCD) galaxies Tol 1214-277 and Tol 65. These data are used to derive the heavy-element and helium abundances. We find that the oxygen abundances in Tol 1214-277 and Tol 65 are the same, 12+logO/H=7.54±0.01, or Z_☉/24, despite the different ionization conditions in these galaxies. The nitrogen-to-oxygen abundance ratio in both galaxies is logN/O=-1.64±0.02 and lies in the narrow range found for the other most metal-deficient BCDs. We use the five strongest He I emission lines λλ3889, 4471, 5876, 6678, and 7065 to self-consistently correct their intensities for collisional and fluorescent enhancement mechanisms and to derive the ⁴He abundance. Underlying stellar absorption is found to be important for the He I λ4471 emission line in both galaxies, being larger in Tol 65. The weighted ⁴He mass fractions in Tol 1214-277 and Tol 65 are, respectively, Y=0.2458±0.0039 and 0.2410±0.0050 when the three He I emission lines λλ4471, 5876, and 6678 are used and are, respectively, Y=0.2466±0.0043 and 0.2463±0.0057 when the He I λ4471 emission line is excluded. These values are in very good agreement with recent measurements of the ⁴He mass fraction in others of the most metal-deficient BCDs by Izotov et al. We find that the combined effect of the systematic uncertainties due to the underlying He I stellar absorption lines, ionization, and temperature structure of the H II region and collisional excitation of the hydrogen emission lines is likely small, not exceeding ∼2% (the error is 2 σ). Our results support the validity of the standard big bang model of nucleosynthesis.