Establishing the presence or absence of intermediate-mass black holes (IMBH) in globular clusters is crucial for understanding the evolution of dense stellar systems. Observationally, this search has been hampered by the low number of stars with known velocities in the central few arcseconds. This limits our knowledge of the velocity dispersion in the region where the gravitational influence of any IMBH would be felt. In Cycle 13, we successfully obtained ACS/HRC images of the centers of five carefully chosen Galactic globular clusters (GO-10401) for a new proper motion study. Although the science case was approved and the first epoch images obtained, the requested future cycle observations were not granted (due to a general policy decision based on the strong uncertainties at the time concerning the immediate future of HST). We have now assessed the quality of the first epoch observations. The HRC resolution reveals many isolated stars in to the very center of each cluster that remained blended or unresolved in previous WFPC2 data. Given a two year baseline, we are confident that we can achieve the proper motion precision required to place strict limits on the presence of an IMBH. Therefore, we request the second-epoch, follow-up observations to GO-10401 in order to measure the proper motions of stars in our target clusters. These velocity measurements will allow us to: (i) place constraints on the mass of a central black hole in each cluster; (ii) derive the internal velocity dispersion as a function of cluster radius; (iii) verify or reject previous reports of cluster rotation; and (iv) directly measure velocity anisotropy as a function of radius. If no second epoch data are obtained then the observing time already invested in the first epoch will have been wasted.

The success of this project hinges on accurate astrometric measurements for a large number of stars in the centers of five crowded Galactic globular clusters. This program was originally scheduled for execution with ACS/HRC, and has been approved for WFPC2 with the loss of ACS/HRC. In order to suppress bright giants, we will take measurements in the F450W filter using the WFPC2, which provides the best compromise between crowding issues and reasonable exposure times. The first epoch observations were taken as part of GO-10401, and we use those to scale the exposure times here. Using the ACS and WFPC2 ETCs, we find a difference in sensitivity between ACS/HRC F435W and WFPC2 F450W of a factor of 2, and account for this in the exposure times. The exposure time estimates have also been checked independently based on WFPC2 observations available in the archive. In order to balance our need for longer exposure times with not saturating too many bright stars, we will use gain=15. Each cluster will be observed in two separate visits, taken at different orientations. This will allow us to test, and if needed account for, the effects of CTE. We have designed 4 and 5 point dither patterns to best sample the face of each PC pixel. For NGC7078, we also include 2 short exposures, which will give an additional handle on the CTE and possibly some measurements for stars which saturate on the long exposures (this cluster is expected to have the highest number of saturated stars in the central regions). The object coordinates are now on the ICRS system. We used the available web-based tool to convert the coordinates from proposal 10401 to this system. 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