s-Process nucleosynthesis in low-metallicity stars

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.