Abundances of Stars in the Galactic Bulge Obtained Using the Keck Telescope

Abstract

We report on detailed abundances of giants in the Galactic bulge, measured with the HIRES echelle spectrograph on the 10-m Keck telescope. We also review other work on the bulge field population and globular clusters using Keck/HIRES. Our new spectra have 3 times the resolution and higher S/N than previous spectra obtained with 4m telescopes. We are able to derive $\log g$ from Fe II lines and excitation temperature from Fe I lines, and do not rely on photometric estimates for these parameters. We confirm that the iron abundance range extends from -1.6 to +0.55 dex. The improved resolution and S/N of the Keck spectra give [Fe/H] typically 0.1 to 0.2 dex higher than previous studies of bulge stars more metal rich than the Sun. Alpha elements are enhanced even for stars at the Solar metallicity (as is the case for bulge globular clusters). We confirm our earlier abundance ananlysis of bulge giants and find that Mg and Ti are enhanced relative to Ca and Si even up to [Fe/H]=+0.55. We also resport the first reliable estimates of the bulge oxygen abundance. Our element ratios confirm that bulge giants have a clearly identifiable chemical signature and suggest a rapid formation timescale for the bulge.