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