Studies of stellar nucleosynthesis: stars in the Galactic halo

The halo of the Milky Way contains some of the oldest stars in our Galaxy. These stars can provide us with a wealth of information on the formation and evolution of the Galaxy, stellar structure and nucleosynthesis, and conditions in the early Universe. In this thesis we have investigated two groups of stars that belong to the Milky Way’s halo population. The first study is an investigation of warm halo dwarfs that show extreme depletions of lithium. A high resolution spectroscopic study of six lithium-poor stars has been undertaken in order to identify characteristics that distinguish these stars from the general halo population, and thus may provide clues to the origin of the lithium deficiency. The study includes a detailed abundance analysis and measurements of projected rotation velocities, v sin i. We find that two stars show a range of abundance anomalies, in agreement with previous studies: G186-26 shows enhancements of neutron capture elements, while G139-8 is deficient in Na, Mg, Al, Ca, Sr and Ba relative to Fe. The finding of extremely low Na in G139-8 is a new discovery. The remaining four stars show abundances typical of the general halo population. Four out of six stars exhibit rotational line broadening indicative of rotation rates that exceed that expected in old halo dwarfs. For G122-69, G66-30, G202-65 and HD97916 we infer v sin i of 4.7, 5.6, 8.6, 10.4 km s"1 respectively. The two stars showing abnormal abundances, G139-8 and G186-26, do not show rotational line broadening and we infer upper limits on v sin i of 1.9 and 2.5 km s_1. We discuss the lithium-poor stars possible origin via mass-transfer from a now evolved companion, a suggestion put forward by previous authors and supported by a high incidence of binarity. In such a scenario, the transfer of mass and angular momentum may result in the destruction/dilution of lithium and lead to an increase in rotation. We discuss possible reasons for the lack of abundance anomalies observed in the rotating lithium-poor stars, and note similarities between the lithium-poor stars and the Galaxy’s populations of field blue stragglers and blue metal-poor stars. The second study is an investigation of the large aluminium enhancements observed in many globular cluster red giants. We have undertaken high resolution spectroscopic observations of Na and Al in five M4 red horizontal branch stars to test the idea that production of the unstable isotope, 26Al, is responsible for the enhancements observed on the giant branch. We find that all five stars have low [Al/Fe] which, alone, is consistent with 26Al production. Our results are inconclusive however, as the stars also show low [Na/Fe], indicating that we have observed members of the ‘unprocessed’ M4 population. The finding of low [Na/Fe] in the red horizontal branch stars is discussed in light of the recent findings of two photometrically and spectroscopically distinct red giant branches in M4. Our findings are consistent with the idea put forward by previous authors that the unprocessed stars of M4 reside on the red HB before moving to the AGB phase, while the processed stars move to the blue HB and don’t evolve to the AGB.