Chemical abundances and kinematics of low-metallicity stars as tracers of early galactic formation, evolution and mergers

Abstract

In studies of chemical abundances among a large sample of giant stars in the similarly-aged globular clusters M4 and M5, I find that the clusters do not share universal abundance patterns in spite of the similarity of their overall chemical enrichment as described by their metallicity. M4 possesses overabundances of aluminum, silicon, barium, lanthanum and europium, enhancements not accounted for in the current theoretical understanding of stellar or globular cluster evolution but, instead present clear evidence for differing nucleosynthetic histories between cluster populations. Abundance “families” also exist among halo field stars. Most exhibit very similar abundance enhancements in α-elements such as magnesium, silicon, calcium, and titanium with respect to iron and thus track the massive-star and supernovae Type II nucleosynthesis yields that dominated the early Galactic halo ISM. However, recent studies have identified stars with low-α abundances. I present evidence of further chemical substructure within the low-α star population: some low-α stars possess titanium and iron-peak overabundances of up to a factor of ten. Understanding the nucleosynthetic origins of the overabundances does not lie in current Galactic chemical evolution models. Ratios of supernovae II/Ia events I have derived using chemical abundance patterns suggest that these stars may have witnessed early Galactic supernovae Type Ia events. The results provide constraints on current theories of supernovae Type Ia modeling and nucleosynthetic clues to the progenitor masses and explosion parameters. Also explored are the chemical abundances of stars which may have been born outside the Galactic halo and are possibly the stellar debris of merger events. The europium abundances and isotopic ratios I derive are consistent with those found for the general halo population, supporting the idea of the “universality” of the production process. However, I also find slight underabundances of the α- elements which need to be confirmed with better data and better modeling. Underabundances in the α-elements lend support to the idea that satellite systems, which began with more limited supplies of star formation material, may have undergone brief initial bursts of star formation followed by long periods of quiescent stellar evolution before supernovae Type Ia’s began contributing to the interstellar medium.