The chemistry of stars in the bar of the Milky Way

Abstract

We use a sample of 938 red clump giant stars located in the direction of the Galactic long bar to study the chemistry of Milky Way bar stars. Kinematically separating stars on bar orbits from stars with inner disc orbits, we find that stars on bar-like orbits are more metal rich with a mean iron abundance of ⟨[Fe/H]⟩ = +0.30 compared to ⟨[Fe/H]⟩ = +0.03 for the inner disc. Spatially selecting bar stars is complicated by a strong vertical metallicity gradient of −1.1 dex kpc−1, but we find the metallicity distribution varies in a manner consistent with our orbital selection. Our results have two possible interpretations. The first is that the most metal rich stars in the inner Galaxy pre-existed the bar, but were kinematically cold at the time of bar formation and therefore more easily captured onto bar orbits when the bar formed. The second is that the most metal rich stars formed after the bar, either directly onto the bar following orbits or were captured by the bar after their formation.