Temperature bias and the primordial helium abundance determination

We study the effect that the temperature structure has on the determination of the primordial helium abundance, Y-p. We provide an equation linking T (O III), the temperature derived from the [O III] lines, and T (He II), the temperature of the He I lines, both for H II regions with O++ only and for H II regions where a fraction of O+ is present. By means of T (He II), which is always smaller than T (O III), we derive the helium abundances of five objects with low and very low metallicity (NGC 346, NGC 2363, Haro 29, SBS 0335-052, and I Zw 18); these objects were selected from the literature because they include the three low-metallicity objects with the best line determinations and the two objects with the lowest metallicity. From these abundances we obtain that Y-p (nHc) = 0.2356+/-0.0020, a value 0.0088 lower than that derived by using T (O III). We call this determination Y-p(nHc) because the collisional contribution to the Balmer-line intensities has not been taken into account. All the recent Y-p determinations in the literature have not taken into account the collisional contribution to the Balmer-line intensities. By considering the collisional contribution to the Balmer-line intensities of these five objects, we derive that Y-p(+Hc) = 0.2384+/-0.0025.