CCC - The Cosmic Carbon Conundrum
We propose STIS G230L ultraviolet spectroscopy of a carefully chosen sample of starburst galaxies to measure the strength of the C III] 1909 and other diagnostic lines. The C III] line is a key indicator of the carbon abundance. Atomic physics and ionization conditions in galaxies preclude any determination of nebular carbon abundances in the optical. Only HST's ultraviolet capabilities allow us to document, explore, model, and interpret the behavior of the 4th most abundant element in the universe in these galaxies. Our sample of 18 galaxies exhibits the signatures of Wolf-Rayet stars, whose presence will greatly facilitate population synthesis modeling and at the same time will provide clues on possible short-comings in stellar evolution modeling. The sample is large enough for meaningful statistics and parameter space coverage but still allows us to reach our science goals in an economic way. Together with short but deep co-spatial optical spectroscopy we will use the data for comparison with Mappings III photoionization models to determine abundances of the major elements. The full star-formation history will be established using the Starburst99 code. We will investigate the behavior of, e.g., C/O vs. O/H in order to constrain the production mechanism of carbon. We will test the predictions of chemical evolution models of galaxies and address the importance of starburst durations, infall, and outflows. The proposed study will help us understand the puzzling carbon abundances in objects as diverse as low-mass Galactic halo stars or distant damped Lyman-alpha absorbers.
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