Program: GS-2007B-Q-237

Recent studies of metal-poor stars have revealed that a large percentage of stars with [Fe/H] < -2.0 exhibit large enhancements of their photospheric C abundances ([C/Fe] > 1.0). These carbon-enhanced metal-poor stars (CEMP) fall into two general categories: those with overabundances of s-process elements (CEMP-s) and those without such overabundances (CEMP-no). CEMP-s stars are thought to have been subjected to mass-transfer from companion AGB stars, but the chemical histories of the CEMP-no stars are much less certain. A potentially viable hypothesis is that CEMP-no stars formed from material that had been enriched by a primordial population of massive metal-poor stars. We propose an observational program that will allow us to constrain the nucleosynthetic origins of CEMP stars by deriving 19F abundances from the molecular HF (1-0)R9 line in high-resolution near-IR Gemini-S/Phoenix spectra of 13 CEMP stars. AGB stars are known to be producers of 19F, and current models predict that 19F will be overabundant in the envelopes of low-metallicity AGB stars. Conversely, recent models of massive, rapidly-rotating, metal-poor stars predict that 19F will be depleted in these objects, with a predicted difference of over four orders of magnitudes between the two sites. Measuring 19F abundances in CEMP stars will provide a unique diagnostic of their chemical histories and will greatly increase our understanding of the chemical evolution of the early Galaxy.