Difference in chemical composition between the bright and faint red clump stars in the Milky Way bulge

TL;DR

This study uses high-resolution spectroscopy to compare chemical compositions of stars in the double red clump of the Milky Way bulge, revealing differences that support a formation scenario involving dissolved globular clusters with multiple populations.

Contribution

It provides the first detailed chemical abundance comparison between bright and faint red clump stars, supporting the dissolved globular cluster origin of the double red clump.

Findings

Bright RC stars are more metal-rich than faint RC stars by 0.149 dex.

Bright RC stars show enhanced [Na/Fe] and possible depletion in [Al/Fe] and [O/Fe].

Presence of Na-rich stars suggests multiple stellar populations in the bulge.

Abstract

The double red clump (RC) observed in color-magnitude diagrams of the Milky Way bulge is at the heart of the current debate on the structure and formation origin of the bulge. This feature can be explained by the difference between the two RCs either in distance ("X-shaped scenario") or in chemical composition ("multiple-population scenario"). Here we report our high-resolution spectroscopy for the RC and red giant branch stars in a high-latitude field (b ~ -8.5$\deg$) of the bulge. We find a difference in [Fe/H] between the stars in the bright and faint RC regimes, in the sense that the bright stars are enhanced in [Fe/H] with respect to the faint stars by 0.149 $\pm$ 0.036 dex. The stars on the bright RC are also enhanced in [Na/Fe] but appear to be depleted in [Al/Fe] and [O/Fe], although more observations are required to confirm the significance of these differences. Interestingly, these chemical patterns are similar to those observed among multiple stellar populations in the metal-rich bulge globular cluster Terzan 5. In addition, we find a number of Na-rich stars, which would corroborate the presence of multiple populations in the bulge. Our results support an origin of the double RC from dissolved globular clusters that harbor multiple stellar populations. Thus, our study suggests that a substantial fraction of the outer bulge stars would have originated from the assembly of such stellar systems in the early phase of the Milky Way formation.