Carbon Abundances in Metal-Poor Dwarf Emission-Line Galaxies from HST Observations
Session 108 -- Dwarf and Irregular Galaxies
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[108.11] Carbon Abundances in Metal-Poor Dwarf Emission-Line Galaxies from HST Observations

D. R. Garnett, E. D. Skillman (U. Minnesota), R. J. Dufour (Rice U.), M. Peimbert, S. Torres-Peimbert (UNAM), E. Terlevich, R. J. Terlevich (RGO), G. A. Shields (U. Texas)

We present observations of UV emission lines of C~III], O~III] and Si~III] from H~II regions in dwarf galaxies, using the Faint Object Spectrograph on HST. The observation of these ionic species together in the UV spectrum allows us to derive the C/O and Si/O abundance ratios with only small dependences on electron temperature and interstellar reddening, a significant improvement over IUE observations.

We find that the C/O ratio in the most metal-poor galaxies (with O/H less than one-tenth solar) is significantly smaller than the solar system ratio, by 0.5-0.6 dex. This is consistent with the C/O ratios found in high-redshift absorption line systems by Reimer et al. (1992, Nature, 360, 561). The Si/O abundance ratio appears to be constant in the dwarf galaxies but below the solar system ratio, consistent with greater depletion of silicon onto grains. The results from our small sample suggest that the C/O ratios in the most metal-poor galaxies are roughly consistent with Weaver and Woosley's (1992, Phys. Rep., 227, 65) best estimate of the $^{12}C(\alpha,\gamma)^{16}O$ nuclear reaction rate.