Metal-Poor Stars Observed with the Magellan Telescope I. Constraints on
  Progenitor Mass and Metallicity of AGB Stars Undergoing s-Process
  Nucleosynthesis
  We present a comprehensive abundance analysis of two newly-discovered
carbon-enhanced metal-poor (CEMP) stars. HE2138-3336 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). HE2258-6358, with
[Fe/H] = -2.67, exhibits enrichments in both s- and 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 Mo with
[Fe/H] = -2.5 and -2.8, as well as to a set of AGB models of 1.0 to 6.0 Mo at
[Fe/H] = -2.3. The agreement with the model predictions suggests that the
neutron-capture material in HE2138-3336 originated from mass transfer from a
binary companion star that previously went through the AGB phase, whereas for
HE2258-6358, 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(Mo) < 1.3) and metallicities (-2.8 < [Fe/H] < -2.5) yield the best agreement
with our observed elemental abundance patterns.
