Metal-poor stars and the chemical enrichment of the universe

Metal-poor stars hold the key to our understanding of the origin of the elements and the chemical evolution of the Universe. This chapter describes the process of discovery of these rare stars, the manner in which their surface abundances (produced in supernovae and other evolved stars) are determined from the analysis of their spectra, and the interpretation of their abundance patterns to elucidate questions of origin and evolution. More generally, studies of these stars contribute to other fundamental areas that include nuclear astrophysics, conditions at the earliest times, the nature of the first stars, and the formation and evolution of galaxies including our own Milky Way. This is illustrated with results from studies of lithium formed during the Big Bang; of stars dated to within Gyr of that event; of the most metal-poor stars, with abundance signatures very different from all other stars; and of the buildup of the elements over the first several Gyr. The combination of abundance and kinematic signatures constrains how the Milky Way formed, while recent discoveries of extremely metal-poor stars in the Milky Way’s dwarf galaxy satellites constrain the hierarchical build-up of its stellar halo from small dark-matter dominated systems. Two areas needing priority consideration are discussed. The first is improvement of abundance analysis techniques. While one-dimensional, local thermodynamic equilibrium (1D/LTE) model atmospheres provide a mature and precise formalism, proponents of more physically realistic 3D/non-LTE techniques argue that 1D/LTE results are not accurate, with systematic errors often of order c0.5 dex or even more in some cases. Self-consistent 3D/non-LTE analysis as a standard tool is essential for meaningful comparison between the abundances of metal-poor stars and models of chemical enrichment. The second need is for larger samples of metal-poor stars, in particular those with [Fe/H] 4 and those at large distances (2050 kpc), including the Galaxy’s ultra-faint dwarf satellites. With future astronomical surveys and facilities, these endeavors will become possible. This will provide new insights into small-scale details of nucleosynthesis as well as large-scale issues such as galactic formation.