The Evolution of C/O in Dwarf Galaxies from HST Fos Observations

We present FOS observations of O III] 1666 A and C III] 1909 A emission in H II regions in dwarf irregular galaxies to measure C/O abundance ratios in those galaxies. We observe a continuous increase in C/O with increasing O/H over the range -4.7 < log(O/H) < -3.6. One interpretation is that the most metal-poor galaxies are the youngest and dominated by the products of early enrichment by massive stars, while more metal-rich galaxies show increasing, delayed contributions of C from intermediate mass stars. However, recent evolution models for massive stars including mass loss suggest that the yield of C relative to O may increase with metallicity; new chemical evolution models for the solar neighborhood which account for this predict a C/O abundance evolution similar to that observed in the metal-poor galaxies. The C/N ratio increases steadily with O/H in the irregular galaxies, but is much smaller in solar neighborhood stars and H II regions. This may indicate that the bulk of N production is decoupled from that of C in the Galaxy.