Population synthesis of s-process element enhanced stars: Constraining the 13C efficiency

We study s-process element abundance ratios in stars by carrying out stellar population synthesis, using a rapid synthetic stellar evolution code which includes an up-to-date treatment of AGB nucleosynthesis and evolution. In contrast to other studies, we find that a large spread in the 13C efficiency parameter (13C_eff) is not needed to explain the observed spread in the ratios of heavy s-process to light s-process elements ([hs/ls]), but this comes naturally from the range of different initial stellar masses and their time evolution. As a result, the 13C efficiency needed for fitting most stars in the galactic disk is constrained to 1 < 13C_eff < 2.5. In the same fashion we also study the [Pb/Ce] ratios of lead stars and find out that for low metallicities 13C_eff is approximately 0.5.