Comparison Of The Nucleosynthesis Parameters Of S+r Stars And Ba Stars

In this paper, we use a parametric model of the asymptotic giant branch (AGB) stars, to calculate the nucleosynthesis parameters in 29 very metal-poor stars and 26 barium stars (i.e. Ba stars), respectively. In our calculations, we use the observations of s+r stars and Ba stars as the constraints and synchronously consider the contributions of s-process and r-process. Through comparison and analysis on the corresponding parameters obtained for the above stars, we get the possible conditions which the s+r stars formed in. The calculation results show that there is great difference in the distribution range of neutron exposures for s+r stars and Ba stars, i.e., the value ofΔτfor Ba stars is almost between 0 to 0.5 mbarn-1 and that for s+r stars is almost between 0.5 and 0.9mbarn-1, furthermore the value of mean neutron exposure of most s+r stars is greater than that of Ba stars, it result in the low abundance ratio of [Pb/Ba] in Ba stars. The r-process-component coefficient ( C r) of Ba stars is between 0 and 10, while the s+r stars is between 0 and 90. The great differences of the distribution range of C r imply that they maybe formed in different sites. In the very metal-poor stars, the s-only stars should belong to the binary systems with large initial orbital separation, by comparing the s-process-component coefficient ( C s) values of s+r stars with those of Ba stars. For s+r stars, there is strong correlation between their C sand C r(r-process-component coefficient) but no correlation for Ba stars. This strongly confirms the possibility that the s+r stars should form through the accretion-induced collapse (AIC) or type 1.5 supernova mechanism. With the ascent of initial-mass of AGB stars, the core-mass value is on the increase, while the overlap factors are on the decrease. We also concluded the Pb-starts had been created from AGB stars with smaller initial mass. The mass fraction of 13C pocket q deduced for the Pb stars is comparable to the overlap factor r .Although the two fundamental parameters,Δτand q obtained for the Pb stars are very different from the AGB stellar model, the results of the larger value of q and the smaller value ofΔτcan also explain the abundance distribution of the Pb stars. Although the neutron number density N nin the larger initial mass AGB stars(m>3M☉) is high, the neutron irradiation timeΔtis shorter, so the neutron exposure per interpulse in the AGB stars should be smaller. It is noteworthy that the total amounts of 13C in metal poor condition are close to the ST case, which is consistent with the primary nature of the 13C neutron source.