Nuclear star clusters have been proposed as a byproduct of galaxy formation, analogous to supermassive black holes, because their mass scales with host galaxy properties in a similar way as the (M_bh-sigma) and (M_bh-M_bulge) relationships. While the nuclear star cluster at the center of the Milky Way offers us an opportunity to study such a system with a level of detail not possible in external galaxies, this potential has not been realized due to the impracticality in obtaining wide-field, high angular resolution, and high precision kinematic measurements from the ground. We propose to use WFC3 on HST to measure, for the first time, the proper motions of stars out beyond the half-light radius of the cluster (~7 pc). These observations will allow us to derive three fundamental properties of the cluster that have been poorly constrained in the past: (1) the total mass, (2) the distribution of mass, and (3) the rotational signature of the cluster. The underlying mass distribution and total mass are requirements for placing the Milky Way nucleus in context with galaxy scaling relationships. The rotational profile will provide constraints on the origin of the nuclear star cluster, offering perhaps a physical explanation for the link between supermassive black holes and nuclear star clusters. Our characterization of the nuclear star cluster will be conducted with both astrometric and three-filter photometric measurements. The Galactic center is optically obscured, therefore, all observations will be conducted with WFC3 in the near infrared. The specific requirements and exposure times for the astrometry and photometry are described below. Astrometry To achieve the science goals outlined in this proposal, we require a 3x3 mosaic with WFC3-IR to obtain complete azimuthal sampling out to 180" from the cluster center; this will enable a robust determination of the orientation of the cluster rotation as well as characterize the velocity anisotropy for mass modeling. As stated in the original proposal, we will include the central field in Cycle 20, which was not included in Cycle 18, and 19 because it was observed by another program; however, in Cycle 20, it is not observed by any other HST program, and the field is necessary to establish an astrometric reference frame. We will use the F153M medium band filter for astrometric measurements, because the broad-band H filter will saturate the bright stars (H < 11) before the first read. We estimate that with t_int=249 s using STEP50 readout pattern with 10 MULTIACCUM reads, we can efficiently achieve a SNR ~100 for H=18, which would be required for 1 mas astrometric precision (astrometric precision scales as 0.7 FWHM/SNR). This integration time and sampling mode requires 248 s to dump the WFC3 buffer, which can be done in parallel with the following exposure, thus minimizing readout overhead. Also importantly with this integration time, sources fainter than H=15 will be in no danger of saturation, ensuring that persistence will have minimal impact for most regions of the detector. Each field will be composed of 4 dithers in sub-pixel step sizes to sample the pixel phase of the PSF to account for the under-sampled PSF of WFC3. We require that the orientation of the mosaic be the same in each cycle in order to perform precise relative astrometry. The Orient special requirement should be 90 degrees to combine this data set with that from cycle 19. We would also like the entire mosaic in F153M to be completed in 1 visit (5 orbits) in order to minimize drifts in the astrometry. To combine all fields, a 10" overlap between each field is necessary to include enough common stars to derive a robust transformation. One additional fields adjacent to this mosaic (with a 10 arcsecond overlap) is observed to provide statistics on the number of bulge stars that may be present in our sample to gauge the effect of bulge contamination on our kinematic measurements of the cluster. The astrometric experiment will require 5 orbits in Cycle 20. RESOLVED STELLAR POPULATIONS 12 false false 0.0 0.0 false false false SUPPORTED STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.7, H=7.2, K=6.0 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.7, H=7.2, K=6.0 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.5, H=8.6, K=8.3 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.5, H=8.6, K=6.8 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.5, H=8.6, K=6.8 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.7, H=7.2, K=6.0 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.7, H=7.2, K=6.0 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.1, H=7.5, K=6.7 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=11.0, H=10.0, K=8.1 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=11.0, H=10.0, K=8.1 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=10.9, H=9.8, K=8.3 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.1, H=7.5, K=6.7 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=9.1, H=7.5, K=6.7 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 STELLAR CLUSTER NUCLEUS This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 WFC3/IR Imaging No No F139M WFC3/IR Imaging No No F127M WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M WFC3/IR Imaging No No F153M POS-TARG DITHER 0.572 LINE 0.365 false true 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1 false The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. Astrometric fields. The orientation should be 270 in order to optimally combine with observations from cycle 18 and 19 in order to measure proper motions. We request that the observations be made with the same guide stars as cycle 18 (GO-12182) if possible. One adjacent field to the north-east of the mosaic is used for calibration of the effect of bulge star contamination in the cluster kinematic measurements. This cycle will include the central field as it will not be observed by any other HST program and is required to establish an astrometric reference frame for the mosaic. We also request that the observations be made after August 1st, 2013 in order to obtain a sufficient timebaseline between Cycle 19 and Cycle 20 to achieve our astrometric goals. 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