The chemical composition of HII regions in blue compact dwarf galaxies

Abstract

A set of empirical relationships for the ionization correction factors used by various authors to determine the chemical compositions of the gas in nebular objects is tested. New expressions for the ionization correction factors are used to find the nebular-gas compositions in HII regions in blue compact dwarf galaxies. The abundances of He, N, O, Ne, S, and Ar in 41 HII regions are determined. The derived elemental abundances are compared with the results of other studies. The Y -O/H, Y -N/H, and Y -Z dependences are analyzed in detail. The primordial helium abundance Yp and its enrichment dY/d Z are also determined. c � 2002 MAIK "Nauka/Interperiodica". Blue compact dwarf galaxies (BCD galaxies) are characterized by active star-formation processes, with giant HII regions observed around the sites of star formation. These galaxies have low metallicities, and evolve from material that has not yet been pro- cessed by stellar evolution. Therefore, BCD galaxies are important objects for studies of the primordial helium abundance Yp and the rate of its enrichment dY/d Z during the chemical evolution of matter in our Metagalaxy. The determination of accurate elemental abun- dances in HII regions in BCD galaxies is a large task. Only a small number of ion lines of a single element are observed in real nebulae, so that the numbers of ions in most ionization stages cannot be derived directly from observations. So-called ionization cor- rection factors (ICFs) are usually used to determine the abundance of an element based on its ion abun- dance. Some of these factors (which are empirical expressions) are based only on the proximity of the ionization potentials of the corresponding ions and do not take into account differences in their effective ionization cross sections. Others are based on calcu- lations using grids of photoionization models (PhM) for HII regions (see, for example, (1)). However, cal- culated total relative ion abundances are usually used to find ICFs. It is obvious that using ICFs derived from PhM is a more accurate method for determining * E-mail: