Identification of RR Lyrae Stars in the Milky Way Nuclear Star Cluster Hui Dong(IAA-CSIC) , Rainer Schoedel (IAA-CSIC), Benjamin Williams (Univ. of Washington, Seattle), Tuan Do (UCLA), Andrea Ghez (UCLA), Mark Morris (UCLA), Michael Rich (UCLA), Francisco Nogueras (IAA-CSIC), Eulalia Gallego (IAA-CSIC), Teresa Gallego (IAA-CSIC), Daniel Wang (Univ. of Mass., Amherst), Zhiyuan Li (Univ. of Nanjing) Sep 18th, 2017 RR Lyrae 2017 Conference, Niepolomice, Poland

Introduction Nuclear Star Clusters in 75% of local late-type spiral galaxies

Part of the sample of Boker et al. 2002, including 59 galaxies. HST/WFPC2 F814W image.

Part of the sample of Boker et al. 2002, including 59 galaxies. HST/WFPC2 F814W image. Solid lines: inward-extrapolation

Introduction Nuclear Star Clusters in 75% of local late-type spiral galaxies Several pc, 10^{6-7} solar mass Densest stellar systems in the present Universe Multiple-age stellar components Their masses show correlation with those of the embedded massive black holes. Could play an important role in the construction of the galactic nuclei, even, galactic bulges and galaxies themselves.

Introduction Nuclear Star Clusters Origin of the Nuclear Star Clusters In-situ star formation Materials from the disk Large metallicity and age ranges Infall globular clusters Dynamic friction Old and metal poor stellar populations The key point: a census of stellar populations in the Nuclear Star Clusters

Introduction Nuclear Star Clusters Origin of the Nuclear Star Clusters Milky Way Nuclear Star Cluster (MWNSC) Half light radius 4.2 pc Total mass, 2.5x107 M Distance 8.4 kpc away Sgr A*, massive black hole, 4x106 M Obscured in optical Credit: UCLA GC group

Introduction Nuclear Star Clusters Origin of the Nuclear Star Clusters Milky Way Nuclear Star Cluster (MWNSC) Young stellar population (~6 Myr) with top-heavy initial mass function (IMF, Genzel+2010, Lu+13) Extended star formation history (Pfuhl+2014) 6% metal poor stars [Fe/H]<-0.5 (Do+2015) However, the results are limited by the sample size; near-IR IFU on >8m telescopes, time consuming. Solution: RR Lyrae Stars

Instrument Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) IR camera Large field-of-view (2.3’ x 2’) Stable PSF Accurate photometric zero-point (~0.01 mag) High angular resolution (~0.14”, still suffers from confusion limit in the central few arc second) High sensitivity

Dataset Feb 28, Mar 10, Apr 2, 2014, Long duration, good for RRL (0.2 to 1d), δ Scuti (<0.2 d) 2010-2014, four-year baseline, good for identifying variables with long period, but is hard to constrain their period, due to the poor temporal sampling.

Source Catalog 33070 sources F127M - F153M < 1.7, foreground Galactic Bulge >2.5, MWNSC/ Background

Source Catalog 33070 sources Artificial star test Least chi square method, 3903 variables 3854 vary among 4 year baseline 76 vary within one day in Feb 28, Mar 10 and/or Apr 2, 2014 Used Lomb-Scargle periodogram analysis to determine the periods of 36 sources, 21 of which are between 0.2 to 1 d.

Results RRL stars 4 RRab stars: fundamental mode, typical sawtooth light curves 3 RRab candidates: sine shape with 0.4 < P < 1 d 4 RRc stars: sine shape with 0.2 < P < 0.4 d

RRc Stars 0.2 d < P < 0.4 d RRab candidates, 0.4 d < P < 1 d

Bailey Diagram: red and blue filled stars: RRab and RRc stars from VVV survey (Gran et al., 2015, 2016). Open stars, black filled triangles, open triangles, black filled circles and open squares are 4 RRc, 4 RRab, 3 RRab candidates, 8 eclipsing binaries and 2 candidates of eclipsing binaries. Solid and dashed lines: Oosterhoff type I and II. Globular clusters with Oo I and Oo II type with -1.5<[Fe/H]<-1 and -2.5<[Fe/H]<-1.5.

Results RRL stars 4 RRab stars: fundamental mode 2 are foreground, 1 in MWNSC, 1 background Average A_ks of MWNSC is 2.54+/-0.12. Schoedel et al. 2010. The distance to Sgr A* is 7.8+/-0.14 kpc (Boehle et al. 2014)

Results RRL stars 4 RRab stars: fundamental mode 2 are foreground, 1 in MWNSC, 1 background Not complete

Blue plus: Sgr A*, Green: RRab, Cyan: RRc, Magenta: RRab candidates, White: eclipsing binaries.

Results RRL stars 4 RRab stars: fundamental mode 2 are foreground, 1 in MWNSC, 1 background Not complete Simulation ~ 10, at most 40 in MWNSC

The correlation between the number of RRab stars and the total mass of globular clusters in the Milky Way. The diamonds and squares are for the OoI (-1.5<[Fe/H]<-1) and OoII (-2.5<[Fe/H]<-1) clusters respectively.

Results RRL stars 4 RRab stars: fundamental mode 2 are foreground, 1 in MWNSC, 1 background Not complete Simulation ~ 10, at most 40 in MWNSC Total mass of old stellar population with (- 1.5<[Fe/H]<-1), < 5 x 10^5 solar mass, i.e. < 18% None detection of OoII RRab put a constrain in old stellar population with -2.5<[Fe/H]<-1.5 is <13%. The infall globular clusters play a small role in MWNSC

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