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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.