THE EVOLUTION OF C/O IN DWARF GALAXIES FROM HUBBLE-SPACE-TELESCOPE FOS OBSERVATIONS

We present UV observations of seven H II regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds obtained with the Faint Object Spectrograph on the Hubble Space Telescope (HST) in order to measure the C/O abundance ratio in the interstellar medium (ISM) of those galaxies. We measure both O III] 1666 Angstrom and C III] 1909 Angstrom in our spectra, enabling us to determine C+2/O+2 with relatively small uncertainties. The results from our HST observations show a continuous increase in C/O with increasing O/H, consistent with a power law having an index of 0.43 +/- 0.09 over the range -4.7 to -3.6 in log (O/H). One possible interpretation of this trend 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 carbon from intermediate-mass stars. However, recent evolution models for massive stars including mass loss suggest that the yield of carbon from massive stars may increase with metallicity relative to the yield of oxygen; new chemical evolution models for the solar neighborhood which include nucleosynthesis from these recent stellar evolution models predict a C/O abundance evolution similar to that observed in the metal-poor galaxies. The trend in the C/N ratio increases steadily with O/H in the irregular galaxies, but decreases suddenly for solar neighborhood stars and H II regions. This may indicate that the bulk of nitrogen production is decoupled from the synthesis of carbon in our Galaxy. Our results also suggest that it may not be appropriate to combine abundances in irregular galaxies with those in spiral galaxies to study the evolution of chemical abundances. Our measured C/O ratios in the most metal-poor galaxies are consistent with predictions of nucleosynthesis from massive stars for Weaver SZ Woosley's best estimate for the C-12(alpha, gamma)O-16 nuclear reaction rate, assuming negligible contamination from carbon produced in intermediate-mass stars in these galaxies. We detect a weak N III] 1750 Angstrom multiplet in SMC N88A and obtain interesting upper limits for two other objects. Our 2 sigma upper limits on the 1750 Angstrom feature indicate that the N+2/O+2 ratios in these objects are not significantly larger than the N+/O+ ratios measured from optical spectra. This behavior is consistent with predictions of photoionization models, although better detections of N III] are needed to confirm the results.