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