Chemical Abundance Analysis of Various Populations in the Milky Way

Abstract

We perform chemical abundance analysis of two populations of stars to better understand the chemical evolution history the Milky Way. We take two approaches to answer this question, intending to add to the growing body of knowledge that comprises the field of galactic archaeology. In the first approach, we provide a means by which we can improve current methods to measure the metallicity of M-dwarf stars, useful tracers of the chemical evolution of the Milky Way. The measurement of M-dwarf metallicity relies on the use of empirical relationships calibrated with F/G/K+M binary pairs. We have measured the radial velocity of 77 F/G/K stars and 62 M-dwarfs previously identified by common proper motion to be potentially in F/G/K+M binaries. Of the 63 candidate pairs where we have observed both the F/G/K primary and the M-dwarf secondary, we have identified 47 F/G/K+M-dwarf binaries using a 2-sigma agreement in their measured radial velocities. In order to indirectly measure the metallicity of the M-dwarf secondaries, we have performed chemical abundance analysis of 58 F/G/K stars, 47 of which were identified by radial velocity as F/G/K+M-dwarf binaries. Our sample of confirmed binaries spans a metallicity range of -1.94 < [Fe/H] < +0.01. This allows the extension of these empirically calibrated relationships used to determine metallicity to more metal-poor M-dwarfs, enabling this tracer population to probe further back into the chemical history of the Milky Way. Our second approach is to study the chemical abundance of an ultra-faint dwarf galaxy, allowing us to study the chemical evolution of similar Milky Way progenitors and to probe early Universe nucleosynthesis. We have performed a detailed chemical abundance analysis of three stars in the ultra-faint dwarf Horologium I. We have found that despite its metal-poor nature, there is an unexpected lack of alpha-element enrichment. We discuss possible scenarios that could cause this abundance pattern and discuss the stochasticity of early nucleosynthesis that these scenarios suggest. By understanding the chemical abundance of these two populations, we can probe the chemical history of the Galaxy and the origin of the elements, learning how the Milky Way evolved into its present chemical state.