The first chemical abundance analysis of K giants in the inner Galactic disc

Abstract

Aims. The elemental abundance structure of the Galactic disc has been extensively studied in the solar neighbourhood using long-lived stars such as F and G dwarfs or K and M giants. These are stars whose atmospheres preserve the chemical composition of their natal gas clouds, and are hence excellent tracers of the chemical evolution of the Galaxy. As far as we are aware, there are no such studies of the inner Galactic disc, which hampers our ability to constrain and trace the origin and evolution of the Milky Way. Therefore, we aim in this study to establish the elemental abundance trend(s) of the disc(s) in the inner regions of the Galaxy. Methods. Based on equivalent width measurements in high-resolution spectra obtained with the MIKE spectrograph on the Magellan II telescope on Las Campanas in Chile, we determine elemental abundances for 44 K-type red giant stars in the inner Galactic disc, located at Galactocentric distances of 4-7 kpc. The analysis method is identical to the one recently used on red giant stars in the Galactic bulge and in the nearby thin and thick discs, enabling us to perform a truly differential comparison of the different stellar populations. Results. We present the first detailed elemental abundance study of a significant number of red giant stars in the inner Galactic disc. We find that these inner disc stars show the same type of chemical and kinematical dichotomy as the thin and thick discs show in the solar neighbourhood. The abundance trends of the inner disc agree very well with those of the nearby thick disc, and also to those of the Bulge. The chemical similarities between the Bulge and the Galactic thick disc stellar populations indicate that they have similar chemical histories, and any model trying to understand the formation and evolution of either of the two should preferably incorporate both of them.