Stellar Population Changes in Post-Core-Collapse Globular Clusters

Color gradients, in the sense of becoming bluer inwards, are found in postcore-collapse (PCC) clusters. No gradients are seen in clusters with King-model morphology. The gradients seem to be caused by the demise of red giants and/or subgiants, and possibly an increased number of blue stragglers or some other population of faint, blue objects. Practically all PCC clusters have blue horizontal branches, with faint blue tails. Bright red giants are clearly underabundant in the central regions of PCC clusters, whereas HB stars seem to be unaffected. At a. fixed metallicity, PCC clusters also show blue FUV colors, as seen in the archival data from the IUE and ANS (Figure 1). This is consistent with our observations of their HB morphologies, but a presence of an additional hot population cannot be excluded. The gradients may be a consequence of stellar interactions during and after the core collapse, and the mechanism may be important for the formation of millisecond and binary pulsars and LMXB's. These effects represent a strong evidence that dynamical evolution of star clusters can physically modify their stellar populations. It is possible that similar effects may operate in at least some galactic nuclei. More details and further references are given by Djorgovski et al. 1991, ApJ, 372, L41.