Heavy element abundances in a new sample of low-metallicity blue compact galaxies

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Abstract

We present high-quality spectrophotometric observations of 15 supergiant H II regions in 14 new low-metallicity blue compact galaxies (BCGs) selected mainly from the First and Second Byurakan Surveys and with oxygen abundance 12 + log O/H between 7.37 and 8.04 (Z_sun_5/35 <= Z <= Z_sun_/7). We use the data to determine abundances for the elements N, O, Ne, S, Ar, and, for the first time in BCGs, Fe; discuss their origin; and constrain current nucleosynthesis stellar models. The main result of the present study is that none of the heavy element-to-oxygen abundance ratios studied here (N/O, Ne/O, S/0, Ar/O, Fe/O) depend on oxygen abundance. We conclude that all these heavy elements have a primary origin and are produced by the same massive (M >=10 M_sun_) stars. The dispersion of the N/O ratio is found to be remarkably small, being only +/-0.08 in the log. This can only be understood if primary N is produced in massive stars, not in intermediate-mass (4 M_sun_ <= M <= 9 M_sun_) stars as commonly thought. BCGs show the same O/Fe overabundance with respect to the Sun (~0.34 in the log) as galactic halo stars, suggesting the same chemical enrichment history, and supporting the scenario of an early enrichment of the galactic halo by massive Population III stars. We have compared the observed heavy element abundance ratios with theoretical yields from current massive star nucleosynthesis models from Weaver & Woosley (1993). The small dispersion in the heavy element abundance ratios suggests that there is not a large IMF variation between BCGs with different metallicities.

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