S + R Star Element Abundances And Neutron Capture Nucleosynthesis Parametric Study

In this paper we use the parametric model to fit the elemental abundances observed in the s- and r-rich, very metal-poor stars (hereafter s+r stars) and suggest a new concept of component coefficients to describe the contributions of the individual neutron-capture processes to s+r stars. The overlap factor in the AGB stars where the observed s-process elements were produced lies between 0.1 and 0.81. Taking into account the overlap factor-core mass law and the initial-final mass relations, this wide range of values could possibly be explained by a wide range of initial-mass values of AGB stars at low metallicity. The metal-poor AGB stars with the initial mass in excess of 3—4M(?) may cause the degenerate cores to reach the Chandresekhar mass, leading to Type-1.5 supernova, this process would explain the enhancement pattern of the s+r stars produced by small overlap factors r ~ 0.1 - 0.2 . Because the core mass of the AGB star is remarkably large at low metallicity. we propose that the binary systems with lower-mass AGB stars (M<3—4M(?)) may also cause the AIC, which can explain the enhancement pattern of the s+r stars produced by large overlap factor r ~ 0.3-0.8. The binary systems with low-mass AGB stars (M<3.0M(?)) and large initial orbital separation could not cause AIC because the white dwarf accretes matter insufficiently from the polluted star and the companion is polluted only by the former AGB star, which can explain the formation of s-stars. The neutron exposure per pulse deduced for the s+r stars lies between 0.45 and 0.88 mb-1. The formation of a Pb star ([Pb/Ba]>1.0) could be explained by a transfer of matter from the low-mass AGB star(M<3.0M(?)). The component coefficient of the r-process is strongly correlated with the component coefficient of the s-process for the s+r stars. This implies an increase of r-process matter accreted by the s+r starswith increasing s-process matter accreted from the AGB star. This provides strongsupport to the scenario that the r-process material in s+r stars comes from an AIC orType-1.5 supernova.