The Chemical Composition of the Small Magellanic Cloud H ii Region NGC 346 and the Primordial Helium Abundance

Abstract

Spectrophotometry in the λλ3400-7400 range is presented for 13 areas of the brightest H II region in the SMC: NGC 346. The observations were obtained at CTIO with the 4 m telescope. Based on these observations, its chemical composition is derived. The helium and oxygen abundances by mass are given by Y(SMC)=0.2405+/-0.0018 and O(SMC)=0.00171+/-0.00025. From models and observations of irregular and blue compact galaxies it is found that ΔY/ΔO=3.5+/-0.9 and, consequently, that the primordial helium abundance by mass is given by Yp=0.2345+/-0.0026 (1 σ). This result is compared with values derived from big bang nucleosynthesis and with other determinations of Yp.