Stellar archaeology:: A Keck Pilot Program on extremely metal-poor stars from the Hamburg/ESO survey.: III.: The lead (Pb) star He 0024-2523

We present a detailed abundance analysis, including spectral syntheses, of a very metal-poor ([Fe/H] similar or equal to -2.7) peculiar main-sequence star (HE 0024-2523) detected during the course of the Keck Pilot Program. Radial velocities of this star were obtained during four different observing runs over a time span of 1.1 yr and demonstrate that it is clearly a short-period spectroscopic binary. An orbital solution was obtained, and orbital parameters were determined with high precision. The rotational velocity was also measured (v(rot) sin i = 9.7 +/- 1.5 km s(-1)); rotation appears likely to be synchronous with the orbit. The abundance analysis and spectral syntheses indicate that the object is a CH star characterized by extreme s-process enrichment, likely due to mass accretion from an evolved companion that has now probably become a white dwarf. The lead (Pb) abundance of (HE 0024 2523) is very high, the same as that of the recently discovered lead-rich, metal-poor star CS 29526-110, [Pb/Fe] = +3.3. The abundance ratio of the heavy to light s-elements, as characterized by Pb and Ba, [Pb/Ba] = +1.9, is the highest yet found for any metal-poor star and is about 0.7 dex higher than that of CS 29526-110. On the basis of the measured isotopic ratio of carbon (C-12/C-13 similar to 6) we argue that the mass donor must have had an original mass of at least similar to3 M.. The unusually short period of this CH star suggests that it underwent a past common-envelope phase with its evolved companion. Our results are compared with the latest available models for asymptotic giant branch yields and s-process nucleosynthesis. We also discuss the possible connection between HE 0024-2523, the lithium depletion of halo stars, and halo blue straggler formation.