Inhomogeneous H II Regions and the Helium Abundance

Abstract

When calculating the helium (4He) abundance in low metallicity H ii regions, the ionization correction factor (icf) for unseen neutral helium (and hydrogen) is usually assumed to be unity. In this paper, we explore this factor for H ii regions ionized by young stellar clusters. Our main result is that icf < 1 for homogeneous H ii regions and, that the effect of density condensations in the H ii regions is to further reduce the icf. For icf < 1, the primordial helium abundance inferred from observations of low-metallicity, extragalactic H ii regions is decreased.