The First Detailed Abundances for M Giants in the Inner Bulge from Infrared Spectroscopy

Abstract

We report the first abundance analysis of 17 M giant stars in the inner Galactic bulge at (l, b) = (0°, -1°), based on R=25,000 infrared spectroscopy (1.5-1.8 μm) using NIRSPEC at the Keck telescope. Based on their luminosities and radial velocities, we identify these stars with a stellar population older than 1 Gyr. We find the iron abundance =-0.22+/-0.03, with a 1 σ dispersion of 0.14+/-0.024. We also find the bulge stars have an enhanced [α/Fe] abundance ratio at the level of +0.3 dex relative to solar stars, and low <12C/13C> ~ 6.5 +/- 0.3. The derived iron abundance and composition for this inner bulge sample is indistinguishable from that of a sample of M giants our team has previously studied in Baade's window (l, b) = (0.9, -4). We find no evidence of any major iron abundance or abundance ratio gradient between this inner field and Baade's window. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.