Near-infrared variability study of the central 2.3 × 2.3 arcmin(2) of the Galactic Centre - II. Identification of RR Lyrae stars in the Milky Way nuclear star cluster

Because of strong and spatially highly variable interstellar extinction and extreme source crowding, the faint (K>15) stellar population in the Milky Way's nuclear cluster is still poorly studied. RR Lyrae stars provide us with a tool to estimate the mass of the dim oldest stellar population. Recently, we analyzed HST WFC3/IR observations of the central 2.3 arcminx2.3 arcmin (~5x5 pc^2) of the Milky Way and found 21 variable stars with 0.2 d 1.8) and should be closer to the Galactic Nuclear Bulge. The extinction and distance of one RRab match those for the nuclear star cluster given in the previous works. We perform simulations and find that after correcting for incompleteness, there could be no more than 40 RRabs within the nuclear star cluster and in our field-of-view. Through comparing with the number of RRabs in the known globular clusters in the Milky Way, we estimate 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 4.4x10^5 M, i.e. ~17% of the stellar mass.