Comparison of chemical compositions between the bright and faint red clumps for the metal-poor and metal-rich populations in the Milky Way bulge

TL;DR

This study investigates the chemical composition differences between the bright and faint red clumps in the Milky Way bulge, revealing that metal-poor populations may be linked to globular cluster-like multiple populations, while metal-rich populations are likely due to structural effects.

Contribution

It provides the first spectroscopic evidence of chemical differences between RC groups in the bulge, supporting the dual-origin scenario involving both multiple populations and structural features.

Findings

No significant chemical difference in CN index for metal-poor RCs.

Na enhancement observed in metal-poor bright RCs suggests multiple populations.

Metal-rich RCs show no chemical differences, indicating different origins.

Abstract

We examined the double red clump (RC) observed in the Galactic bulge, interpreted as a difference in distance ("X-shaped bulge scenario") or in chemical composition ("multiple population scenario"). To verify chemical differences between the RC groups, we performed low-resolution spectroscopy for RC and red giant branch (RGB) stars using Gemini-South/GMOS in three fields of the bulge, and collected diverse data from literature. We divided our sample stars not only into bright and faint RC groups, but also into bluer ([Fe/H] < -0.1) and redder ([Fe/H] > -0.1) groups following the recent u-band photometric studies. For the metal-poor stars, no statistically significant difference in CN index was detected between the bright and faint RC groups for all observed fields. However, we found, from cross-matching with high-resolution spectroscopic data, a sign of Na enhancement in the "metal-poor and bright" RC group compared to the "metal-poor and faint" group at (l,b)=(-1 deg,-8.5 deg). When the contributions of the RGB stars on the RC regimes are taken into account, the Na abundance difference between genuine RCs would correspond to approximately 0.23 dex, similar to globular cluster (GCs) with multiple populations. In contrast, the metal-rich stars do not show chemical differences between the bright and faint RCs. It implies that the double RC observed in the metal-poor component of the bulge might be linked to the multiple populations originated from GC-like subsystem, whereas that of the metal-rich component would have produced by the X-shaped structure. Our results support the previous studies suggesting composite nature of the Milky Way bulge.