Abstract
We have computed the ionization structure corresponding to H II regions and ionizing stars with y = N(ITe) /N(H) in the 0.07 to 0.30 range and T* in the 30 000 to 45 000 K range. We have adopted a pregalactic value of y = 0.07, Z = 0 and an evolutionary dependence of AY = 3AZ. An increase in the photospheric y value for a given effective temperature produces a decrease in the ratio of helium-to-hydrogen ionizing photons, P (He) /P (H). Consequently, it is found that for a moderate range of stellar temperatures the N (He+) /N ( H+) ratio in the nebula decreases as y increases. From stellar structure considerations it is found that an increase in y, for a star of a given mass, produces a significant increase in the stellar luminosity and effective temperatures; therefore for a higher value of y, and for a constant mass function, on the average the H II regions would have a higher degree of ionization of helium. These effects are very important in the study of emission line gradients in external galaxies; and in particular, in the study of the lack of ionized helium in the nucleus of our galaxy. Several explanations for the lack of ionized helium in the center of our galaxy are reviewed.
