Fokker-Planck Models for M15 without a Central Black Hole: The Role of the Mass Function

The Astrophysical Journal (Impact Factor: 6.73). 05/2012; 732. DOI: 10.1088/0004-637X/732/2/67
Source: arXiv

ABSTRACT We have developed a set of dynamically evolving Fokker-Planck models for the
collapsed-core globular star cluster M15, which directly address the issue of
whether a central black hole is required to fit Hubble Space Telescope (HST)
observations of the stellar spatial distribution and kinematics. As in our
previous work reported by Dull et al., we find that a central black hole is not
needed. Using local mass-function data from HST studies, we have also inferred
the global initial stellar mass function. As a consequence of extreme mass
segregation, the local mass functions differs from the global mass function at
every location. In addition to reproducing the observed mass functions, the
models also provide good fits to the star-count and velocity-dispersion
profiles, and to the millisecond pulsar accelerations. We address concerns
about the large neutron star populations adopted in our previous Fokker-Planck
models for M15. We find that good model fits can be obtained with as few as
1600 neutron stars; this corresponds to a retention fraction of 5% of the
initial population for our best fit initial mass function. The models contain a
substantial population of massive white dwarfs, that range in mass up to 1.2
solar masses. The combined contribution by the massive white dwarfs and neutron
stars provides the gravitational potential needed to reproduce HST measurements
of the central velocity dispersion profile.

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    ABSTRACT: The high stellar densities in the cores of globular clusters cause significant stellar interactions. These stellar interactions can produce close binary mass-transferring systems involving compact objects and their progeny, such as X-ray binaries and radio millisecond pulsars. Comparing the numbers of these systems and interaction rates in different clusters drives our understanding of how cluster parameters affect the production of close binaries. In this paper we estimate stellar encounter rates (Gamma) for 124 Galactic globular clusters based on observational data as opposed to the methods previously employed, which assumed "King-model" profiles for all clusters. By deprojecting cluster surface brightness profiles to estimate luminosity density profiles, we treat "King-model" and "core-collapsed" clusters in the same way. In addition, we use Monte-Carlo simulations to investigate the effects of uncertainties in various observational parameters (distance, reddening, surface brightness) on Gamma, producing the first catalog of GC stellar encounter rates with estimated errors. Comparing our results with published observations of likely products of stellar interactions (numbers of X-ray binaries, numbers of radio millisecond pulsars, and gamma-ray luminosity) we find both clear correlations and some differences with published results.
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    The Astrophysical Journal 05/2013; 772(1). · 6.73 Impact Factor
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    Physical review D: Particles and fields 12/2012; 87(4).


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