  Several stars at the low-metallicity extreme of the Galactic halo show large
spreads of [Pb/hs]. Theoretically, a s-process pattern should be obtained from
an AGB star with fixed metallicity and initial mass. For the third dredge-up
and the s-process model, several important properties depend primarily on the
core mass of AGB stars. Zijlstra (2004) reported that the initial-final-mass
relation steepens at low metallicity, due to low mass-loss efficiency. This
perhaps affects the model parameters of the AGB stars, e.g. the overlap factor
and the neutron irradiation time, in particular at low metallicity. The
calculated results show indeed that the overlap factor and the neutron
irradiation time are significantly small at low metallicities, especially for
3.0Msun AGB stars. The scatter of [Pb/hs] found in low metallicities can
therefore be explained naturally when varying the initial mass of the low-mass
AGB stars.
