An observational study of metal-poor stars

The outer atmosphere of the first generation of low-mass stars retain to a great extent the original composition of the interstellar medium at the time and place of their birth. Hence the earliest phases of Galactical chemical evolution and nucleosynthesis can be investigated by means of studying the old, metal-poor stars. A minority of these stars exhibit dramatic enhancements in their abundances of heavy neutron-capture elements and/or of carbon. The key question for Galactic chemical evolution models is whether these peculiarities reflect the composition of the natal clouds, or if they are due to later (post-birth) mass transfer of chemically processed material from a binary companion. If the latter is the case, these stars should all be members of binary systems. This thesis presents high-resolution elemental-abundance analysis for a sample of 23 very metal-poor (VMP; [Fe=H] < -2.0) stars, 12 of which are extremely metal-poor (EMP; [Fe/H] < -3.0), and 4 of which are ultra metal-poor (UMP; [Fe/H] < -4.0). The results of radial velocity monitoring of 17 r-process enhanced stars (r-I and r-II stars), 24 CEMP-no stars, 18 CEMP-s and four CEMP-r/s stars, are also presented. The stars, for which the abundance analysis were performed, were targeted to explore differences in the abundance ratios of Li, C, N, O, the alpha-elements, the iron-peak elements, and a number of neutron-capture elements. These are elements that constrain the possible astrophysical sites of element production. This sample has substantially increased the number of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced metal-poor (NEMP) stars. The sample of stars include eight that are considered "normal" metal-poor stars, six CEMP-no stars, �ve CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. Lithium is detected for three of the six CEMP-no stars, all of which are Li-depleted with respect to the Li plateau for metal-poor dwarfs found by Spite & Spite. This suggests that whatever site(s) produced C either do not completely destroy lithium, or that Li has been astrated by early-generation stars and mixed with primordial Li in the gas that formed the stars observed at present. Carbon and nitrogen abundances for the CEMP stars reveal, for the majority, that a small degree of mixing has happened in their progenitor stars ([C/N] > 0). However, signs of a larger degree of mixing ([C/N] < 0) is found in some CEMP-no stars, but these stars are only found at the lowest metalicities ([Fe/H] < -3:4). CEMP-no stars with large enhancements in Na, Mg, and Al are also only found below this metallicity. This sample confirms the existence of two separate bands in the absolute carbon abundances of CEMP stars, as suggested by Spite et al. The derived abundances for the alpha-elements and iron-peak elements of the stars are similar to those found in previous large samples of metal-poor stars. Finally evidence for a 'floor' in the absolute Ba abundances of CEMP-no stars at A(Ba) ~ -2.0 is also presented. Binary frequencies of 18%, 17%, and 75% are found for the r-process enhanced, CEMPno and CEMP-s stars, respectively. These results show that the nucleosynthetic processes, responsible for the strong carbon excess in the CEMP-no stars, and the r-process element enhancement in the r-I and r-II stars, are unrelated to their binary population. Instead, the element excess was imprinted on the natal molecular clouds of these stars by an external, distant source. The high frequency of binary stars found for the CEMP-s stars however, demonstrate that the peculiar abundance pattern of these stars is coupled to the binary nature of the stars