RSAA Colloquia / Seminars / Feast-of-Facts: Friday, 17 May 2019, 14:00-14:30; Duffield Lecture Theatre

Dongwook Lim

"The origin of the double red clump in the Milky Way bulge"

At 2010, the presence of two red clumps (RCs) has been discovered in the high-latitude fields of the Milky Way bulge from 2MASS and OGLE surveys. This double RCs are widely accepted as evidence for a giant X-shaped structure originated from the bar instability. We suggested, however, a drastically different interpretation based on the multiple stellar population phenomenon as is observed in globular clusters (GCs), where the bright RC is from He and Na enhanced second-generation stars, while the faint RC is representing first-generation stars with normal abundances. Because these two scenarios imply very different pictures of the formation of the bulge, understanding the origin of the double RC is of crucial importance. As supporting evidence for multiple population scenario, here we report our discovery that the stars in the two RCs show a significant difference in CN-band strength. In addition, we found that the red giant branch stars in the outer bulge ( > 5.5° from the Galactic center) are divided into two groups according to Na abundance in the [Na/Fe] - [Fe/H] plane. Since these chemical patterns and characteristics are only explained by stars originated in GCs, this is evidence that the double RC is due to the multiple population phenomenon, and that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way.