196 research outputs found
Central energy equipartition in multi-mass models of globular clusters
In the construction of multi-mass King-Michie models of globular clusters, an
approximated central energy equipartition between stars of different masses is
usually imposed by scaling the velocity parameter of each mass class inversely
with the stellar mass, as if the distribution function were isothermal. In this
paper, this 'isothermal approximation' (IA) has been checked and its
consequences on the model parameters studied by a comparison with models
including central energy equipartition correctly. It is found that, under the
IA, the 'temperatures' of a pair of components can differ to a non-negligible
amount for low concentration distributions. It is also found that, in general,
this approximation leads to a significantly reduced mass segregation in
comparison with that given under the exact energy equipartition at the centre.
As a representative example, an isotropic 3-component model fitting a given
projected surface brightness and line-of-sight velocity dispersion profiles is
discussed. In this example, the IA gives a cluster envelope much more
concentrated (central dimensionless potential W=3.3) than under the true
equipartition (W=0.059), as well as a higher logarithmic mass function slope.
As a consequence, the inferred total mass (and then the global mass-to-light
ratio) results a factor 1.4 times lower than the correct value and the amount
of mass in heavy dark remnants is 3.3 times smaller. Under energy
equipartition, the fate of stars having a mass below a certain limit is to
escape from the system. This limit is derived as a function of the mass and W
of the giants and turn-off stars component.Comment: LaTeX 2e, 9 pages with 7 figures. Accepted for publication in MNRA
Binary Stars and Globular Cluster Dynamics
In this brief proceedings article I summarize the review talk I gave at the
IAU 246 meeting in Capri, Italy, glossing over the well-known results from the
literature, but paying particular attention to new, previously unpublished
material. This new material includes a careful comparison of the apparently
contradictory results of two independent methods used to simulate the evolution
of binary populations in dense stellar systems (the direct N-body method of
Hurley, et al. 2007 and the approximate Monte Carlo method of Ivanova, et al.
2005), that shows that the two methods may not actually yield contradictory
results, and suggests future work to more directly compare the two methods.Comment: 7 pages, 1 figure, to appear in "Dynamical Evolution of Dense Stellar
Systems", IAUS 246, ed. E. Vesperin
Parallelization of a Code for the Simulation of Self-gravitating Systems in Astrophysics. Preliminary Speed-up Results
We have preliminary results on the parallelization of a Tree-Code for
evaluating gravitational forces in N-body astrophysical systems. For our Cray
T3D/CRAFT implementation, we have obtained an encouraging speed-up behavior,
which reaches a value of 37 with 64 processor elements (PEs). According to the
Amdahl'law, this means that about 99% of the code is actually parallelized. The
speed-up tests regarded the evaluation of the forces among N = 130,369
particles distributed scaling the actual distribution of a sample of galaxies
seen in the Northern sky hemisphere. Parallelization of the time integration of
the trajectories, which has not yet been taken into account, is both easier to
implement and not as fundamental.Comment: 14 pages LaTeX + 1 EPS figure + 2 EPS colour figures, epsf.sty and
aasms4.sty included; to be published in Science & Supercomputing at CINECA,
Report 1997 (Bologna, Italy
An efficient parallel tree-code for the simulation of self-gravitating systems
We describe a parallel version of our tree-code for the simulation of
self-gravitating systems in Astrophysics. It is based on a dynamic and adaptive
method for the domain decomposition, which exploits the hierarchical data
arrangement used by the tree-code. It shows low computational costs for the
parallelization overhead -- less than 4% of the total CPU-time in the tests
done -- because the domain decomposition is performed 'on the fly' during the
tree setting and the portion of the tree that is local to each processor
'enriches' itself of remote data only when they are actually needed.
The performances of an implementation of the parallel code on a Cray T3E are
presented and discussed. They exhibit a very good behaviour of the speedup (=15
with 16 processors and 10^5 particles) and a rather low load unbalancing (< 10%
using up to 16 processors), achieving a high computation speed in the forces
evaluation (>10^4 particles/sec with 8 processors).Comment: 10 pages, 8 figures, LaTeX2e, A&A class file needed (included),
submitted to A&A; corrected abstract word wrappin
A mass estimate of an intermediate-mass black hole in omega Centauri
Context. The problem of the existence of intermediate-mass black holes
(IMBHs) at the centre of globular clusters is a hot and controversial topic in
current astrophysical research with important implications in stellar and
galaxy formation.
Aims. In this paper, we aim at giving further support to the presence of an
IMBH in omega Centauri and at providing an independent estimate of its mass.
Methods. We employed a self-consistent spherical model with anisotropic
velocity distribution. It consists in a generalisation of the King model by
including the Bahcall-Wolf distribution function in the IMBH vicinity.
Results. By the parametric fitting of the model to recent HST/ACS data for
the surface brightness profile, we found an IMBH to cluster total mass ratio of
M_BH/M = 5.8(+0.9-1.2) x 10^(-3). It is also found that the model yields a fit
of the line-of-sight velocity dispersion profile that is better without mass
segregation than in the segregated case. This confirms the current thought of a
non-relaxed status for this peculiar cluster. The best fit model to the
kinematic data leads, moreover, to a cluster total mass estimate of M = (3.1
+/- 0.3) x 10^6 Msol, thus giving an IMBH mass in the range 13,000 < M_BH <
23,000 Msol (at 1-sigma confidence level). A slight degree of radial velocity
anisotropy in the outer region (r > 12') is required to match the outer surface
brightness profile.Comment: LateX, 5 pages, 5 figures. Accepted for publication by Astronomy &
Astrophysic
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