The Milky Way without X: An alternative interpretation of the double red clump in the Galactic bulge

TL;DR

This paper proposes that the double red clump in the Milky Way bulge is due to multiple stellar populations with helium enhancement, challenging the common interpretation of an X-shaped structure.

Contribution

It introduces an alternative explanation for the double red clump phenomenon based on multiple populations, particularly helium-enhanced stars, rather than the X-shaped bulge structure.

Findings

Double RC can be explained by helium-enhanced second-generation stars.

Models reproduce the metallicity and latitude dependence of the double RC.

If confirmed, the Milky Way's bulge may lack a prominent X-shaped structure.

Abstract

The presence of two red clumps (RCs) in high latitude fields of the Milky Way bulge is interpreted as evidence for an X-shaped structure originated from the bar instability. Here we show, however, that this double RC phenomenon is more likely to be another manifestation of multiple populations observed in globular clusters (GCs) in the metal-rich regime. As in the bulge GC Terzan 5, the helium enhanced second generation stars (G2) in the classical bulge component of the Milky Way are placed on the bright RC, which is about 0.5 mag brighter than the normal RC originated from the first generation stars (G1), producing the observed double RC. In a composite bulge, where a classical bulge can coexist with a boxy pseudo bulge, our models can also reproduce key observations, such as the dependence of the double RC feature on metallicity and Galactic latitude and longitude. If confirmed by Gaia trigonometric parallax distances, this would indicate that the Milky Way bar is not sufficiently buckled to form the X-shaped structure in the bulge, and suggest that the early-type galaxies would be similarly prevailed by super-helium-rich subpopulation.