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Title:
Shaping the Globular Cluster Mass Function by Stellar-Dynamical Evaporation
Authors:
McLaughlin, Dean E.; Fall, S. Michael
Affiliation:
AA(Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK.; Permanent address: Astrophysics Group, Lennard-Jones Laboratories, Keele University, Keele, Staffordshire, ST5 5BG, UK; .), AB(Institute for Advanced Study, Einstein Drive, Princeton, NJ 08450.; Permanent address: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; .)
Publication:
The Astrophysical Journal, Volume 679, Issue 2, pp. 1272-1287. (ApJ Homepage)
Publication Date:
06/2008
Origin:
UCP
ApJ Keywords:
Galaxies: Star Clusters, Galaxy: Globular Clusters: General
DOI:
10.1086/533485
Bibliographic Code:
2008ApJ...679.1272M

Abstract

We show that the globular cluster mass function (GCMF) in the Milky Way depends on cluster half-mass density, ρh, in the sense that the turnover mass MTO increases with ρh while the width of the GCMF decreases. We argue that this is the expected signature of the slow erosion of a mass function that initially rose toward low masses, predominantly through cluster evaporation driven by internal two-body relaxation. We find excellent agreement between the observed GCMF-including its dependence on internal density ρh, central concentration c, and Galactocentric distance rgc-and a simple model in which the relaxation-driven mass-loss rates of clusters are approximated by -dM/dt=μev~ρ1/2h. In particular, we recover the well-known insensitivity of MTO to rgc. This feature does not derive from a literal ``universality'' of the GCMF turnover mass, but rather from a significant variation of MTO with ρh-the expected outcome of relaxation-driven cluster disruption-plus significant scatter in ρh as a function of rgc. Our conclusions are the same if the evaporation rates are assumed to depend instead on the mean volume or surface densities of clusters inside their tidal radii, as μev~ρ1/2t or μev~Σ3/4t-alternative prescriptions that are physically motivated but involve cluster properties (ρt and Σt) that are not as well defined or as readily observable as ρh. In all cases the normalization of μev required to fit the GCMF implies cluster lifetimes that are within the range of standard values (although falling toward the low end of this range). Our analysis does not depend on any assumptions or information about velocity anisotropy in the globular cluster system.
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