The first detailed abundance analysis of Galactic bulge K giants in Baade's window

Abstract

We report analysis of echelle spectra (R = 17,000, S/N approximately equal to 50) of 12 Galactic bulge K giants in Baade's window from the sample of Rich (1988). We perform an abundance analysis for 11 stars ranging from (Fe/H) = -1 to (Fe/H) = 0.45. The accuracy of our abundance scale is confirmed relative to the disk super-metal-rich stars via the metal-rich giant Mu Leonis, and to the Galactic globular clusters using a star in NGC 5927. We investigate the effects of CN blends on abundance analysis for stars more metal-rich than (Fe/H) = -0.3, through spectrum synthesis of the region from 6140 to 7600 A, including in excess of 8000 (12)CN and (13)CN lines. We confirm Peterson's assertion that blends of atomic lines with CN vibration-rotation lines can affect abundance results at R = 17,000; failure to reject affected Fe lines would have resulted in a spurious 0.3 dex increase in the derived Fe abundances of our most metal-rich stars. We find that one of the most metal-rich Baade's window stars (BW IV-67) has approximately the same metallicity as the super-metal-rich disk giant Mu Leo. The abundance scale of Rich (1988), based on measurement of strong Fe and Mg lines, is approximately equal to 0.3 dex high but may agree with our results if Rich's (M/H) actually measures ((Fe + Mg)/H). Using our abundances to transform Rich's (M/H) distribution, we find that the mean (Fe/H) is -0.25 for the bulge, slightly less than, but not appreciably different from, the mean (Fe/H) for solar neighborhood G and K giants. Surprisingly, this is not in conflict with the Galactic metallicity gradient found for the solar neighborhood. The extant data on (Fe/H) with Galactocentric radius are completely consistent with a zero gradient within the solar circle. Other noteworthy results include enhanced Al abundances similar to those seen in globular cluster giants, and (tentatively) enhancement of the r-process element Eu in the metal-rich bulge stars. The iron-peak and s-process elements appear normal relative to Fe. We find one super-Li-rich giant, BW I-194. We also identify BW I-155 as a spectroscopic double.