Book Description

The chemical compositions of stellar atmospheres reflect the interstellar gas from which they formed, each one containing a snapshot of the Milky Way Galaxy’s chemical evolution. By comparing abundances of stars across the disk of our Galaxy, we can explore how different parts of the Galaxy were enriched, where star formation proceeded most rapidly, and whether there has been significant mixing of stars and gas through the disk. In general, older stars formed from material that had fewer metals and younger stars formed after more metals had been deposited into the interstellar medium. However, the relative dating of stars based on metallicity cannot easily be mapped to an absolute age, and the local age-metallicity relation cannot be used in different Galactic locations. The rate of metal enrichment depends strongly on the star formation rate. To use the full power of studying stellar atmosphere compositions across the Galaxy, we also need to determine absolute stellar ages. In this dissertation, I examine the ages and chemical abundances of stars in the solar neighborhood with the goal of extending this analysis to the larger APOGEE sample, which spans the full Milky Way disk. Through an independent analysis of optical spectra of APOGEE stars, I validate the individual chemical abundances measured by the near-infrared spectroscopic survey. The main stellar targets of APOGEE are red giant stars, for which the atmospheric parameters are degenerate with age. I develop a method of constraining the age distribution of a sample of giants, and determining individual ages to 0.1 dex, using a sample of local red giant stars with well measured distance. For the solar neighborhood, I find that the mean age of the sample is closely correlated with the abundance of [alpha]-elements in the stars, even at young ages. Using a sample of local subgiant stars, for which ages can be more precisely measured, I confirm this result for the solar neighborhood. I conclude that the method presented here for finding ages of red giant stars with known distances can robustly be applied to the full APOGEE sample for which distances will soon be available from the Gaia mission.