Investigation on the Central Density Profile of NGC 6652 Using Hubble Space Telescope Data

According to dynamical models, the surface brightness profile or stellar density profile of a globular cluster can trace its evolutionary history. Among the various stages of evolution, core collapse occurring at the cluster center is one of the most dramatic and important processes in the late stages. After core collapse, the stellar density in the central region increases sharply, and the central density profile transitions from a flat core to a steep, power-law-like “density cusp”. Detecting whether a density cusp exists in the center of a globular cluster can therefore indicate whether it has undergone core collapse, providing key clues to its dynamical evolution. In early studies, NGC 6652 was generally classified as a non-core-collapsed cluster. However, later studies detected a steep density cusp in its central surface brightness distribution, leading to debate over whether it has in fact undergone core collapse. This study combines high-resolution data from the Hubble Space Telescope (HST) with Gaia Space Telescope data to investigate whether a density cusp exists at the center of NGC 6652. By selecting stars in post-main-sequence evolutionary stages, we ensured that the HST and Gaia samples cover a similar mass range. We then used Gaia Early Data Release 3 (Gaia EDR3) astrometry to remove some non-member stars. However, by comparing with the color-magnitude diagram (CMD) of Galactic field stars simulated using TRIdimensional modeL of thE GALaxy (TRILEGAL), it became clear that field star contamination could not be fully removed using Gaia EDR3 data alone. We therefore need to further clean the sample using CMD, ultimately obtaining a relatively clean member star sample. The density profile constructed from this cleaned sample shows a flat central region with no evidence of a cusp. We therefore conclude that NGC 6652 may not have undergone core collapse. We also found that the density profile constructed without removing field stars via the CMD does show a prominent central density cusp. This suggests that previous classifications of NGC 6652 as a core-collapsed cluster may have been influenced by insufficient subtraction of Galactic field star contamination. In addition, our results indicate the limitations of Gaia EDR3 data in removing field stars from the central region of NGC 6652. Future observations with the China Space Station Telescope may help to further address this issue.