Chemical abundances as population tracers

Abstract

A discussion of elemental abundance ratios as tracers of stellar populations is presented. The emphasis is on F, G, and K stars because they represent a wide range of ages and have atmospheres providing a fossi record of the chemical evolution of the Galaxy. Instrumentation and methods to determine chemical abundances in stellar atmospheres are discussed in Sect High-resolution (R > 20, 000) spectra are required to derive precise abundance ratios, but lower resolution spectra may be useful in connection with large statistical studies of populations. Most abundance analyses are based on homogeneous 1D model atmospheres and the assumption of local thermodynamic equilibrium (LTE), but recent works have shown that 3D non-LTE corrections can change the derived trends of abundance ratios as a function of stellar metallicity significantly. However, when comparing stars having similar effective temperatures, surface gravities, and metallicities, 3D non-LTE corrections tend to cancel out. Such a differential approach is the best way to disentangle stellar populations on the basis of chemical abundances. Abundance ratios particularly useful as population tracers are discussed in Sect, including C/O, Na/Fe, Ni/Fe, Ba/Y, Eu/Ba, and α/Fe, where α is the average abundance of Mg, Si, Ca, and Ti. The nucleosynthesis of the elements involved occurs on different timescales in stars and supernovae with different masses. This is the main reason that these abundance ratios can be used as population tracers. The following sections deal with a discussion of populations in the Galactic disk, the bulge, and the halo. Based on abundance ratios, there is clear evidence for two main populations in the disk: an old, thick disk formed on a timescale of 9 years and a younger, thin disk formed over a more extended period. For the bulge, interesting new abundance results have been obtained in recent years, including data from microlensed dwarfs, but it is too early to draw any robust conclusions about how and when the bulge formed. For the halo, there is evidence for the existence of two discrete populations with low and high values of α/Fe, respectively. Globular clusters stand out from the halo field stars by showing Na-O and Al-Mg anticorrelations; there is increasing evidence that they consist of multiple stellar populations.