Identification of RR Lyrae Stars in the Milky Way Nuclear Star Cluster

Dong, Hui; Schödel, Rainer; Williams, Benjamin; Do, Tuan; Ghez, Andrea; Morris, Mark; Rich, Michael; Nogueras-Lara, Francisco; Gallego-Cano, Eulalia; Gallego-Calvente, Teresa; Wang, Daniel; Li, Zhiyuan

June, 2018

We present our recent efforts to use RR Lyrae type ab (RRab) stars to constrain old metal-poor stellar populations in the Milky Way nuclear star cluster. From the four-year baseline HST/WFC3/IR observations of the central 2.3'×2.3' of the Milky Way, we found four RRab and three candidates. Specifically, they fall exactly onto the Oosterhoff I division in the Bailey diagram. The extinction and distance of one RRab match those of the nuclear star cluster given in previous works. We performed simulations and found that after correcting for incompleteness, there could be no more than 40 RRab stars within the nuclear star cluster and in our field-of-view. Through comparing with the known globular clusters of the Milky Way, we estimated that if there exists an old metal-poor (-1.5<{[Fe/H]}<-1) stellar population in the Milky Way nuclear star cluster, then it contributes at most ˜18% of the total stellar mass of the nuclear star cluster.