1. 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

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

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

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

5. 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.

6. 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

7. 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

8. 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

9. 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

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

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

15. 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.

16. 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

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

19. 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.

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

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

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

23. 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

24. 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.

25. 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

26. Thank you/Gracias