Principal Component Abundance Analysis of Microlensed Bulge Dwarf and   Subgiant Stars
  Elemental abundance patterns can provide vital clues to the formation and enrichment history of a stellar population. Here we present an investigation of the Galactic bulge, where we apply principal component abundance analysis (PCAA)---a principal component decomposition of relative abundances [X/Fe]---to a sample of 35 microlensed bulge dwarf and subgiant stars, characterizing their distribution in the 12-dimensional space defined by their measured elemental abundances. The first principal component PC1, which suffices to describe the abundance patterns of most stars in the sample, shows a strong contribution from alpha-elements, reflecting the relative contributions of Type II and Type Ia supernovae. The second principal component PC2 is characterized by a Na--Ni correlation, the likely product of metallicity-dependent Type II supernova yields. The distribution in PC1 is bimodal, showing that the bimodality previously found in the [Fe/H] values of these stars is robustly and independently recovered by looking at only their relative abundance patterns. The two metal-rich stars that are alpha-enhanced have outlier values of PC2 and PC3, respectively, further evidence that they have distinctive enrichment histories. Applying PCAA to a sample of local thin and thick disk dwarfs yields a nearly identical PC1; in PC1, the metal-rich and metal-poor bulge dwarfs track kinematically selected thin and thick disk dwarfs, respectively, suggesting broadly similar alpha-enrichment histories. However, the disk PC2 is dominated by a Y--Ba correlation, likely indicating a greater contribution of s-process enrichment from long-lived asymptotic giant branch stars that is absent from the bulge PC2 because of its rapid formation. 
Brett H. Andrews, David H. Weinberg, Jennifer A. Johnson, Thomas   Bensby, Sofia Feltzing
astro-ph.GA astro-ph.SR