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Because of strong and spatially highly variable interstellar extinction andextreme source crowding, the faint (K>15) stellar population in the Milky Way'snuclear cluster is still poorly studied. RR Lyrae stars provide us with a toolto estimate the mass of the oldest, relative dim stellar population. Recently,we analyzed HST/WFC3/IR observations of the central 2.3'x2.3' of the Milky Wayand found 21 variable stars with periods between 0.2 and 1d. Here, we present afurther comprehensive analysis of these stars. The period-luminosityrelationship of RR Lyrae is used to derive their extinctions and distances.Using multiple approaches, we classify our sample as four RRc, four RRab andthree candidates, ten binaries. Especially, the four RRabs show sawtooth lightcurves and fall exactly onto the Oosterhoff I division in the Bailey diagram.Compared to the RRabs reported by Minniti et al, 2016, our new RRabs havehigher extinction (A_K>1.8) and should be closer to the Galactic Centre. Theextinction and distance of one RRab match those for the nuclear star clustergiven in previous works. We perform simulations and find that after correctingfor incompleteness, there could be no more than 40 RRabs within the nuclearstar cluster and in our field-of-view. Through comparing with the knownglobular clusters of 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 starcluster on a scale of 5x5pc, then it contributes at most 4.7x10^5 solar mass,i.e., ~18% of the stellar mass.

monthly notices of the royal astronomical society

Near-Infrared Variability Study of the Central 2.3 arcmin x 2.3 arcmin  of the Galactic Centre II. Identification of RR Lyrae Stars in the Milky Way  Nuclear Star Cluster