Analysis of two SMC HII Regions Considering Thermal Inhomogeneities: Implications for the Determinations of Extragalactic Chemical Abundances

Abstract

We present long slit spectrophotometry considering the presence of thermal inhomogeneities (t^2) of two HII regions in the Small Magellanic Cloud (SMC): NGC 456 and NGC 460. Physical conditions and chemical abundances were determined for three positions in NGC 456 and one position in NGC 460, first under the assumption of uniform temperature and then allowing for the possibility of thermal inhomogeneities. We determined t^2 values based on three different methods: i) by comparing the temperature derived using oxygen forbidden lines with the temperature derived using helium recombination lines, ii) by comparing the abundances derived from oxygen forbidden lines with those derived from oxygen recombination lines, and iii) by comparing the abundances derived from ultraviolet carbon forbidden lines with those derived from optical carbon recombination lines. The first two methods averaged t^2=0.067+-0.013 for NGC 456 and t^2=0.036+-0.027 for NGC 460. These values of t^2 imply that when gaseous abundances are determined with collisionally excited lines they are underestimated by a factor of nearly 2. From these objects and others in the literature, we find that in order to account for thermal inhomogeneities and dust depletion, the O/H ratio in low metallicity HII regions should be corrected by 0.25-0.45 dex depending on the thermal structure of the nebula, or by 0.35 dex if such information is not available.