METAL-POOR STARS OBSERVED WITH THE MAGELLAN TELESCOPE. I. CONSTRAINTS ON PROGENITOR MASS AND METALLICITY OF AGB STARS UNDERGOING s-

We present a comprehensive abundance analysis of two newly-discovered carbon-enhanced metalpoor (CEMP) stars. HE 2138−3336 is a s-process-rich star with [Fe/H] = −2.79, and has the highest [Pb/Fe] abundance ratio measured thus far, if NLTE corrections are included ([Pb/Fe] = +3.84). HE 2258−6358, with [Fe/H] = −2.67, exhibits enrichments in both sand r-process elements. These stars were selected from a sample of candidate metal-poor stars from the Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R ∼ 2, 000) spectroscopy with GEMINI/GMOS. We report here on derived abundances (or limits) for a total of 34 elements in each star, based on high-resolution (R ∼ 30, 000) spectroscopy obtained with Magellan-Clay/MIKE. Our results are compared to predictions from new theoretical AGB nucleosynthesis models of 1.3 M⊙ with [Fe/H] = −2.5 and −2.8, as well as to a set of AGB models of 1.0 to 6.0 M⊙ at [Fe/H] = −2.3. The agreement with the model predictions suggests that the neutron-capture material in HE 2138−3336 originated from mass transfer from a binary companion star that previously went through the AGB phase, whereas for HE 2258−6358, an additional process has to be taken into account to explain its abundance pattern. We find that a narrow range of progenitor masses (1.0≤ M(M⊙)≤1.3) and metallicities (−2.8 ≤ [Fe/H] ≤ −2.5) yield the best agreement with our observed elemental abundance patterns. Subject headings: Galaxy: halo—methods: spectroscopy—stars: abundances—stars: atmospheres— stars: Population II