An analysis of the distribution of globular clusters with postcollapse cores in the Galaxy

Abstract

We present a new compilation of structural parameters for Galactic globular clusters, based on the data from the complete survey published by Djorgovski, King, and collaborators in 1984, 1986, and 1987 and the literature. We find that the distribution of the post-core-collapsed (PCC) clusters is much more concentrated about the Galactic center than the distribution of the King model (KM) clusters. Within the KM family, a similar trend exists: centrally condensed KM clusters are found, on average, at smaller galactocentric radii. To deal with the problem of Galactic obscuration, we used a distance-independent analysis, similar to one developed and published by Frenk and White in 1982. We analyzed the shapes of the KM and PCC cluster systems; they are each consistent with a symmetrical distribution of the clusters about the Galactic center. At fixed distance from the center, the clusters at smaller heights above the plane (and thus the less inclined orbits) are marginally more concentrated. The data indicate that the more concentrated KM clusters tend to have higher luminosities. On the other hand, the PCC clusters are less luminous than the highest concentration KM clusters. The PCC clusters show signs of tidal distortions in their envelopes: their major axes tend to point toward the Galactic center; the KM clusters show no such effect. There is a very slight difference in the average metallicities of the two families, in the sense that PCC clusters have slightly lower metal abundances, but there is no significant trend with the concentration within the KM family. The fact that some internal properties of clusters correlate well with global variables, such as the galactocentric radius, suggests that some external effects are important in cluster evolution. These include the average cluster density, set by the mean tidal field of the Galaxy, and, perhaps, the strength of tidal shocks.