HST Proper Motions of Stars within Globular Clusters

TL;DR

This paper discusses the use of Hubble Space Telescope data to measure proper motions of stars in globular clusters, enabling studies of black holes, anisotropy, and stellar dynamics with high precision.

Contribution

It introduces advanced astrometric techniques to analyze multi-epoch HST data, achieving high-precision proper motions for stars in globular clusters.

Findings

Proper motion errors of ~0.02 mas/yr achieved

Potential to detect intermediate-mass black holes

Constraints on cluster anisotropy and dynamics

Abstract

The stable environment of space makes HST an excellent astrometric tool. Its diffraction-limited resolution allows it to distinguish and measure positions and fluxes for stars all the way to the center of most globular clusters. Apart from small changes due to breathing, its PSFs and geometric distortion have been extremely stable over its 20-year lifetime. There are now over 20 globular clusters for which there exist two or more well-separated epochs in the archive, spanning up to 10+ years. Our photometric and astrometric techniques have allowed us to measure tens of thousands of stars per cluster within one arcmin from the center, with typical proper-motion errors of ~0.02 mas/yr, which translates to ~0.8 km/s for a typical cluster. These high-quality measurements can be used to detect the possible presence of a central intermediate-mass black hole, and put constraints on its mass. In addition, they will provide a direct measurement of the cluster anisotropy and equipartition. We present preliminary results from this project, and discuss them in the context of what is already known from other techniques