Systematic uncertainties in the determination of the primordial 4 He abundance

The primordial helium abundance Yp is commonly inferred from abundance determinations in lowmetallicity extragalactic H ii-regions. Such determinations may be subject to systematic uncertainties that are investigated here. Particular attention is paid to two effects: icf -corrections for “imperfect” ionization structure leading to significant amounts of (unobservable) neutral helium or hydrogen and “tcf ”-corrections due to nonuniform temperature. Model H ii-regions with a large number of parameters are constructed and it is shown that required corrections are almost exclusively functions of two physical parameters: the number of heliumto hydrogenionizing photons in the illuminating continuum Q(He)/Q(H), and the ratio of width to radius δrS/rS of the Strömgren sphere. For clouds of sufficient helium-ionizing photons Q(He )/Q(H) & 0.15 and nonnegligible width of the Strömgren sphere a significant overestimate of helium abundances may result. Such clouds show radiation softness parameters in the range −0.4 . log η . 0.3 coincident with the range of η in observed H ii-regions. Existing data of H ii-regions indeed seem to display a correlation which is consistent with a typical ∼ 2 − 4% overestimate of helium abundances due to these effects. In case such an interpretation prevails, and in the absence of other compensating effects, a significant downward revision of Yp may result. It is argued that caution should be exercised regarding the validity of commonly quoted error bars on Yp.