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Session 84 - Dwarf Galaxies.
Oral session, Wednesday, January 15
Harbour B,

[84.07] High C/O in I Zw 18 from New HST Spectra: Not a "Primordial" Galaxy After All?

D. R. Garnett, E. D. Skillman (U. Minnesota), R. J. Dufour (Rice U.), G. A. Shields (U. Texas)

We have obtained new measurements of the C/O abundance ratio in both the NW and SE H II regions in the extreme dwarf H II galaxy I Zw 18, based on UV/optical spectroscopy with the Faint Object Spectrograph on the Hubble Space Telescope. From analysis of the emission lines, we have determined values of log C/O = -0.63\pm0.10 for the NW region and log C/O = -0.56\pm0.09 for the SE region. In comparison, log C/O = -0.85\pm0.07 in the three most metal-poor H II galaxies measured previously by Garnett et al. (1995, ApJ, 443, 64). The average value of log C/O = -0.60\pm0.09 in I Zw 18 is significantly higher than in other comparably metal-poor galaxies. This result and the fact that both components have very similar abundances argue against the H II region self-pollution hypothesis of Kunth amp; Sargent (1986, ApJ, 300, 496). Instead, comparison with chemical evolution models for starbursting dwarf galaxies suggest that the current abundances in I Zw 18 reflect the nucleosynthesis products from a star formation event that occurred several hundred Myr ago.

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