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Principal Component Abundance Analysis of Microlensed Bulge Dwarf and Subgiant Stars

Andrews B. H., Weinberg D. H., Johnson J. A., Bensby T., Feltzing S.

Acta Astronomica

2012

Vol. 62, No. 3

269--279

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 α-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 α-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 α-enrichment histories. However, the disk PC2 is dominated by a Y-Ba correlation, likely indicating a contribution of s-process enrichment from long-lived asymptotic giant branch stars that is absent from the bulge PC2 because of its rapid formation.

Protecting Life in the Milky Way: Metals Keep the GRBs Away

Stanek K. Z., Gnedin O. Y., Beacom J. F., Gould A. P., Johnson J. A., Kollmeier J. A., Modjaz M., Pinsonneault M. H., Pogge R., Weinberg D. H.

Acta Astronomica

2006

Vol. 56, No. 4

333--345

The host galaxies of the five local, z≤0.25, long-duration gamma-ray bursts (GRBs 980425, 020903, 030329, 031203 and 060218), each of which had a well-documented associated supernova, are all faint and metal-poor compared to the population of local star-forming galaxies. We quantify this statement by using a previous analysis of star-forming galaxies (0.005iso, steeply decreases with increasing host oxygen abundance. This might further indicate that (low) metallicity plays a fundamental physical role in the GRB phenomenon, and suggesting an upper metallicity limit for "cosmological" GRBs at ≈0.15 Z_solar.