Mass Segregation and Equipartition of Energy in Two Globular Clusters with Central Density Cusps

Abstract

We begin by presenting the analysis of a set of deep B- and V-band images of the central density cusp of the globular cluster M30 (NGC 7099), taken with the Faint Object Camera aboard the Hubble Space Telescope. These images are the first to resolve lower-mass main-sequence stars in the cluster's central 10''. From the positions of individual stars, we measure an improved position for the cluster center; this new position is 2.6'' from the previously known position. We find no evidence of a ``flat'', constant-surface-density core; however, the data do not rule out the presence of a core of radius up to 1.9'' (95% confidence level). We measure a logarithmic cusp slope (d \log σ/ d \log r$) of -0.76 +- 0.07 (1-sigma) for stars with masses between 0.69 and 0.76 \Msun, and -0.82 +- 0.11 for stars with masses between 0.57 and 0.69 \Msun. We also compare the overall mass function (MF) of the cluster cusp with the MF of a field at r = 4.6' (near the cluster half-mass radius). The observed degree of mass segregation is well matched by the predictions of an isotropic, multimass King model. We then use the Jeans equation to compare the structure of M30 with that of M15, another cusped cluster, using data from this and a previous paper. We find that M30 is very close to achieving equipartition of energy between stellar species, at least over the observed range in mass and radius, while M15 is not. This difference may be a result of the longer relaxation time in the observed field in M15. The data also suggest that the degree of mass segregation within the two cluster cusps is smaller than one would expect from the measurements at larger radius. If so, this phenomenon might be the result of gravothermal oscillations, of centrally-concentrated populations of binaries, or of a ~10^3 \Msun black hole in one or more clusters.