The First Detailed Abundances for M giants in the inner Bulge from Infrared Spectroscopy

TL;DR

This study presents the first detailed infrared spectroscopic abundance analysis of 17 M giant stars in the inner Galactic bulge, revealing their metallicity, alpha-element enhancement, and similarity to stars in Baade's Window, with no significant gradient detected.

Contribution

It provides the first detailed chemical abundances for M giants in the inner bulge using infrared spectroscopy, expanding understanding of bulge composition.

Findings

Iron abundance <[Fe/H]> = -0.22 with low dispersion.

Enhanced alpha/Fe ratio at +0.3 dex.

No significant abundance gradient between inner bulge and Baade's Window.

Abstract

We report the first abundance analysis of 17 M giant stars in the inner Galactic bulge at $(l,b)=(0\deg,-1\deg)$, based on $R=25,000$ infrared spectroscopy $(1.5-1.8 ~\mu \rm m)$ 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 $\rm < [Fe/H] > = -0.22 \pm 0.03$, with a $1\sigma$ dispersion of $0.14\pm 0.024$. We also find the bulge stars have enhanced [$\rm alpha$/Fe] abundance ratio at the level of +0.3 dex relative to the Solar stars, and low $\rm <^{12}C/^{13}C>\approx 6.5\pm 0.3$. 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.