Abstract
We have made a parametric study of s-process nucleosynthesis in the metal poor ([Fe/H] = –2.7) stars LP625-44 and LP706-7. We find that a high neutron exposure and a small overlap factor are necessary to fit the abundance pattern observed in these two metal-deficient stars, particularly the abundance ratios, Pb/Ba ≈ 1 and Ba/Sr ≈ 10. We have also constructed stellar models to better understand how such s-process conditions could arise. We have calculated a 2M ⊙ model star with metallicity [Fe/H] = –2.7 from the ZAMS up to AGB phase. We find that for such low-metallicity stars the He convective shell reaches the bottom of the overlying H-rich envelope at the second thermal pulse. Protons are then carried into the hotter He burning layers and 13C is formed as protons mix into the He shell. Subsequently, material in the H-flash driven convective zone experiences a high neutron exposure due to the 13C(α, n) reaction. This results in a new neutron-capture s-process paradigm in which the abundances are characterized by only one neutron exposure. We suggest that this new s-process site may be a significant contributor to the s-process abundances in low-metallicity ([Fe/H] ≤ –2.5) stars.
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