Detailed abundances for M giants in two inner bulge fields from Infrared Spectroscopy

TL;DR

This study provides detailed infrared spectroscopic abundance measurements for 30 M giant stars in the inner Galactic bulge, confirming uniformity in metallicity and alpha-element enhancement within 1 kpc of the Galactic Center.

Contribution

First detailed infrared abundance analysis of M giants in two inner bulge fields, confirming the lack of vertical abundance gradients and consistent alpha enhancement.

Findings

Iron abundances around -0.16 to -0.21 dex

Enhanced [alpha/Fe] of +0.3 dex

No significant vertical abundance gradient

Abstract

We report abundance analysis for 30 M giant stars in two inner Galactic bulge fields at (l,b)=(0,-1.75) deg and at (l,b)=(1,-2.65) deg, based on R=25,000 infrared spectroscopy from 1.5-1.8um using NIRSPEC at the Keck II telescope. We find iron abundances of <[Fe/H]>=-0.16 +/- 0.03 dex with a 1-sigma dispersion of 0.12 +/- 0.02 and <[Fe/H]>=-0.21 +/- 0.02 dex, with a 1-sigma dispersion of 0.09+/- 0.016 for the (l,b)=(0,-1.75) and (l,b)=(1,-2.65) deg fields, respectively. In agreement with all prior studies, we find enhanced [alpha/Fe] of +0.3 dex. We confirm the lack of any major vertical abundance or composition gradient in the innermost ~600 pc between Baade's window and 150 pc from the Galactic plane. We also confirm that the known enhancement of alpha elements observed between 500 and 1000 pc from the nucleus is also present over the volume of the inner bulge and may therefore be presumed to be a general characteristic of bulge/bar stars within 1 kpc of the Galactic Center.