Neutron Capture Elements ins‐Process–Rich, Very Metal‐Poor Stars

We report abundance estimates for neutron-capture elements, including lead(Pb), and nucleosynthesis models for their origin, in two carbon-rich, verymetal-poor stars, LP625-44 and LP706-7. These stars are subgiants whose surfaceabundances are likely to have been strongly affected by mass transfer fromcompanion AGB stars that have since evolved to white dwarfs. The detections ofPb, which forms the final abundance peak of the s-process, enable a comparisonof the abundance patterns from Sr (Z=38) to Pb (Z=82) with predictions of AGBmodels. The derived chemical compositions provide strong constraints on the AGBstellar models, as well as on s-process nucleosynthesis at low metallicity. Thepresent paper reports details of the abundance analysis for 16 neutron-captureelements in LP 625-44, including the effects of hyperfine splitting and isotopeshifts of spectral lines for some elements. A Pb abundance is also derived forLP706-7 by a re-analysis of a previously observed spectrum. We investigate thecharacteristics of the nucleosynthesis pathway that produces the abundanceratios of these objects using a parametric model of the s-process withoutadopting any specific stellar model. The neutron exposure tau is estimated tobe about 0.7mb^-1, significantly larger than that which best fits solar-systemmaterial, but consistent with the values predicted by models of moderatelymetal-poor AGB stars. This value is strictly limited by the Pb abundance, inaddition to those of Sr and Ba. We also find that the observed abundancepattern can be explained by a few recurrent neutron exposures, and that theoverlap of the material that is processed in two subsequent exposures is small(the overlap factor r~0.1).Comment: 49 pages, Latex, AASTEX, 9 figure