Nucleosynthesis And Elemental Abundances In Metal-poor Stars

A comprehensive understanding of the formation and distribution of elements involving stars of many evolutionary phases, especially near the end of their evolution, is of vital importance in nuclear astrophysics. In contrast to s-process element formation occurring in Low- and intermediate- mass AGB stars, the nature of r-process requiring high neutron density is still elude us, which might be believed to take place during the explosive stage of stellar evolution. Although experimental investigation of r-process nuclei is still in infancy, theoretical investigations have build several sophisticated nuclear-physics-based and site-free r-process models. In this thesis, based on a large number of element abundances observed in metal-poor stars (including neutron-capture elements) with comprehensive classification, holistic study and comparison, we have obtained some the physical information intimately related to the s-process and the r-process nucleosynthesis.Since the chemical abundances of metal-poor stars are excellent sources of information for setting new constraints on models of Galactic chemical evolution at low-metallicity, it has been tried to fit the elemental abundances observed in the bright, metal-poor giant HD 175305, and derive isotopic fractions using a parametric model. The observed abundances can be well matched by the combined contributions from s- and r-process material. The component coefficients of the r- and s-processes are C₁ = 3.220 and C₃ = 1.134, respectively. The Sm isotopic fraction in this star where the observed neutron-capture elements are produced is predicted to be f_152+154 =0.582, which suggests that, even though the r-process is predominantly responsible for the synthesis of the neutron-capture elements in the early Galaxy, the onset of the s-process had already occurred at this metallicity of [Fe/H]=-1.6.It is noteworthy that r-rich stars, such as HD 221170, show overabundance of the heavier neutron-capture elements and excesses of lighter neutron-capture elements, which provides us a chance to better understand the weak r- and main r-process nucleosynthesis at low-metallicity. Utilizing a parametric model, it is found that the abundance pattern of light elements for most sample stars is close to the pattern of the weak r-process star, and the pattern of heavier neutron-capture elements is very similar to main r-process star, while the lighter neutron-capture elements can be fitted by the mixure of weak r- and main r-process material. The production of the weak r-process elements appears to be associated with the light elements, and the production of main r-process elements are almost not produced in conjunction with the light elements. We compare our results with the observed data at low metallicities, showing that the predicted trends are in good agreement with the observed trends, at least for the metallicity range [Fe/H]< -2.1. For most of sample stars, the abundance pattern of both neutron-capture elements and light elements could be best explained by a star formed in a molecular cloud that had been polluted by both weak r- and main r-process material.The chemical abundances of the very metal-poor double-enhanced stars, known as s+r stars, provide a lot of important information of neutron-capture processes at low metallicity. A new model is proposed that the double enhancements of r-process and s-process elements originate from a former intermediate-mass AGB companion in a wide binary system. Since AGB superwind only occurs at the final stage of AGB stars, it may allow the degenerate core of intermediate-mass AGB star to reach the Chandresekhar mass before the AGB superwind. In the new scenario, similar to physical conditions of a core-collapse supernova, the degenerate C-O core may collapse and explode, in which r-processing is expected to be significant, due to existence of an outer envelope. Both s-process elements enriched in the AGB envelope and r-process elements formed in the subsequent explosion would be ejected simultaneously and accreted by its companion. Furthermore the yield of Eu per AGB supernova event can be estimated, and using the yield of Eu, the overabundance of r-process elements in s+r stars can be accounted for, whilst the high efficiency of wind pollution from the AGB supernova means that the enhanced factor is much larger than unity as the effect of gravity of primary star and the result of gravitational focusing of secondary star.In summary , the holistic analysis, which combines various observation-based neutron-capture process corresponding the overall elemental abundance distribution with stellar evolution and the neutron-capture nucleosynthesis calculations, identifies the physical conditions and sites of the main r-process and the weak r-process nucleosynthesis. Taking observational elemental abundances data of s + r stars as a constraint to compare with elemental abundances of the s+r star, it can be determined that r-process nucleosynthesis and the matter accompanying r-process corresponding overall abundance distribution rules. The existing theory of r-process and stellar nucleosynthesis calculations are carefully compared with that of s + r stars in order to determine the r-process nucleosynthesis to the physical environment and sites. For further study of the relative contribution of the neutron-capture process on the neutron-capture elements in the early Galaxy and the major source of r-process material, some important issues are suggested relating to the neutron-capture nucleosynthesis and galactic chemical evolution.