Mass Table 2. Mass Functions

Using the Faint Object Camera on the repaired Hubble Space Telescope, we have observed two fields in the globular cluster M15: the central density cusp, and a field at r = 20. These are the highest-resolution images ever taken of this cluster’s dense core, and are the first to probe the distribution of stars well below the main-sequence turnoff. After correction for incompleteness, we measure a logarithmic cusp slope (d log σ/d log r) of −0.70 ± 0.05 (1-sigma) for turnoff (∼ 0.8M⊙) stars over the radial range from 0. 3 to 10; this slope is consistent with previous measurements. We also set an approximate upper limit of ∼ 1. 5 (90% confidence limit) on the size of any possible constant-surface-density core, but discuss uncertainties in this limit that arise from crowding corrections. We find that fainter stars in the cusp also have power-law density profiles: a mass group near 0.7M⊙ has a logarithmic slope of −0.56 ± 0.05 (1-sigma) over the radial range from 2 to 10. Taken together, the two slopes are not well matched by the simplest core-collapse or black-hole models. We also measure a mass function at r = 20, outside of the central cusp. Both of the FOC fields show substantial mass segregation, when compared with a mass function measured with the WFPC2 at r = 5. In comparing the overall mass functions of the two FOC fields and the r = 5 field, we find that the radial variation of the mass function is somewhat less than that predicted by a King–Michie model of the cluster, but greater than that predicted by a Fokker–Planck model taken from the literature. Subject headings: globular clusters—stellar systems (kinematics, dynamics) Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.