Metal-poor stars: The fingerprints of the first stars and the early galaxy

The chemical composition of metal-poor stars are fossil records of the first generations of stars and of the early Galactic environment. In this thesis I cover two of the main challenges in studying metal-poor stars. The first is finding metal-poor candidates, and the second is the high-resolution spectral follow-up study of a sample of metal-poor stars.; In the first part of my thesis I explore one process to identify and study very metal-poor stars (VMP, [Fe/H] ≤ -2.0), using moderate-resolution spectra (R∼7000) from the Echellete Spectrograph and Imager (ESI) instrument at the Keck Observatory. With ESI it is possible to obtain high signal-to-noise spectra efficiently (V ∼ 13 stars in ∼ 15 minutes). Even at this moderate resolution I am able to derive abundances for Mg, Ca, Ti, Cr, and Ba. I also am able to derive C and Sr abundances through spectral synthesis. With the C abundances in hand, I also explore the phenomenon of carbon-enhanced metal-poor (CEMP) stars. A surprising discovery in the study of metal-poor stars is the number which are enhanced in carbon to levels of [C/Fe] ≥ 1.0. I explore how the metallicity distribution function of any particular sample is an important aspect in determining this fraction.; The second part of this thesis is a detailed study of a sample of VMP stars using high-resolution spectral analysis. Most of the stars in the sample came from the ESI study. These observations were carried out with the High Resolution Echelle Spectrometer (HIRES) instrument at Keck, with spectral resolution of R∼45,000. From these HIRES spectra, I have measured the abundances of up to 25 elements from C through Eu in 28 VMP stars. In this sample I have discovered a new star that is highly enhanced by r-process material, CS 31078-018, and a new [Fe/H]= -4.0 star, CS 30336-049. The abundance ratios in the stars in this sample may be the products of nucleosynthesis in the very first stars. With this in mind I have compared the abundance ratios of the sample in aggregate, as well as the abundance ratios of CS 30336-049, with the zero-metallicity supernova type II nucleosynthesis models of Heger & Woosley (2008). From this comparison I show that metal-free progenitor with masses ∼ 10 to 20 M⊙ can match our abundances very well.; In the last part of this dissertation I discuss a future direction in the study of metal-poor stars. The Sloan Digital Sky Survey, and in particular by its extension, the Sloan Extension for Galactic Understanding and Exploration (SEGUE), has greatly increased the number of metal-poor candidates to date. Through a well calibrated pipeline, accurate stellar parameters can be estimated and make the selection of metal-poor stars even more efficient. Coupled with the powerful capabilities of ESI, I will be able to study the relatively unexplored regions of the outer Halo of the Galaxy to search for signs of Galaxy formation processes and for further insights into the various classes of VMP discussed in this thesis.