Carbon in spiral galaxies from Hubble Space Telescope spectroscopy

We present measurements of the gas-phase abundance ratio C/O in six H II re gions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectr oscopy using the Faint Object Spectrograph on the Hubble Space Telescope, T he ratios of C to O increase systematically with O/H in both galaxies, from log C/O approximate to -0.8 at log O/H = -4.0 to log C/O approximate to -0 .1 at log O/H = -3.4. C/N shows no correlation with O/H. The rate of increa se of C/O is somewhat uncertain because of uncertainty as to the appropriat e UV reddening law and uncertainty in the metallicity dependence on grain d epletions. However, the trend of increasing C/O with O/H is clear, confirmi ng and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H acr oss spiral galaxies are steeper than the gradients in O/H. Comparing the da ta to chemical-evolution models for spiral galaxies shows that models in wh ich the massive star yields do not vary with metallicity predict radial C/O gradients that are much Batter than the observed gradients. The most likel y hypothesis at present is that stellar winds in massive stars have an impo rtant effect on the yields and thus on the evolution of carbon and oxygen a bundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-populatio n ages much younger than those of the inner disks.