Deep HST/FOC Imaging of the Central Density Cusp of the Globular Cluster M15

Abstract

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 (~ 0.8 \Msun) 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 0.7 \Msun 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.

40 pages single-spaced, including 12 figures; to appear in AJ, April 1997

Cited by in corpus (26)

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• The discovery of two extremely low luminosity Milky Way globular clusters
• CFHT Adaptive Optics Observations of the Central Kinematics in M15
• On the disruption of pulsar and X-ray binaries in globular clusters
• Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: I. STIS Spectroscopy and WFPC2 Photometry
• The puzzling dynamical status of the core of the globular cluster NGC 6752
• The Global Kinematics of the Globular Cluster M15
• HST-NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core
• The Globular Cluster M15. I. Identification, Discovery, and Period Determination of Variable Stars
• A Stringent Limit on the Accretion Luminosity of the Possible Central Black Hole in the Globular Cluster M15
• The Discovery of a Very Faint X-ray Transient in the Globular Cluster M15
• The Global Kinematics of the Globular Cluster M92
• Open-cluster density profiles derived using a kernel estimator
• Discovery of a double Blue Straggler sequence in M15: new insight into the core-collapse process
• The global mass function of M15
• Fokker-Planck Models for M15 without a Central Black Hole: The Role of the Mass Function
• The X-ray source population of the globular cluster M15: Chandra high resolution imaging
• A Simple Dynamical Model for Omega Cen
• An Inverse Look at the Center of M15
• Radial Color Gradient and Main Sequence Mass Segregation in M30 (NGC 7099)