Hubble Space Telescope observations of collapsed-core globular clusters

Globular clusters are self-gravitating systems of roughly 105 --106 stars. Gravitational interactions between stars cause a long-term evolution of the central core region of the cluster to smaller size and higher density, in a process that has been termed "core collapse." It appears that 25% of Galactic globular clusters have already undergone core collapse (Lightman 1982; Cohn and Hut 1984; Djorgovski and King 1986). The observational predictions of this model for the collapsed state are a very small core surrounded by a power-law stellar density cusp, the radial segregation of stars by mass, and a core dominated by the most massive stellar population. Multi-mass Fokker-Planck simulations of evolving globular clusters also indicate that the core size becomes smaller, and cusp slope becomes steeper, as the stellar mass increases (Murphy and Cohn 1988). Cohn (1985) shows that a scaling relation can be used to place an upper limit on the mass of the dominant population in the core, based on the measured cusp slope.; The globular clusters NGC 6284 and NGC 6293 were determined in a ground-based survey by Lugger et al. (1995) to have collapsed cores with core radii of less than 1&inches;. For the present study, NGC 6284 and NGC 6293 were imaged with the Hubble Space Telescope WFPC2 in the equivalent of the UBV system. These images were analyzed to determine the radial structure and color-magnitude diagram morphologies of these clusters. Artificial star tests were performed to quantify the precision and completeness of the photometry. Small blue straggler populations were found in each cluster. The projected stellar density was parameterized as a modified power-law with a core. A maximum-likelihood analysis of the stellar positions was used to determine the "best-fit" core radius and power-law slope for different brightness groups. A comparison of the results to those of Lugger et al. show consistency with the earlier findings of very small (<1&inches;) core sizes. The best-fit, power-law cusp slopes of zero-core-radius models are approximately -0.9, consistent with a dominant core population of mheavy ≃ 1.0 M⊙ in each cluster. This result suggests that these clusters do not contain substantial neutron star populations.