593 research outputs found
Star Cluster Survival in Star Cluster Complexes under Extreme Residual Gas Expulsion
After the stars of a new, embedded star cluster have formed they blow the
remaining gas out of the cluster. Especially winds of massive stars and
definitely the on-set of the first supernovae can remove the residual gas from
a cluster. This leads to a very violent mass-loss and leaves the cluster out of
dynamical equilibrium. Standard models predict that within the cluster volume
the star formation efficiency (SFE) has to be about 33 per cent for sudden
(within one crossing-time of the cluster) gas expulsion to retain some of the
stars in a bound cluster. If the efficiency is lower the stars of the cluster
disperse mostly. Recent observations reveal that in strong star bursts star
clusters do not form in isolation but in complexes containing dozens and up to
several hundred star clusters, i.e. in super-clusters. By carrying out
numerical experiments for such objects placed at distances >= 10 kpc from the
centre of the galaxy we demonstrate that under these conditions (i.e. the
deeper potential of the star cluster complex and the merging process of the
star clusters within these super-clusters) the SFEs can be as low as 20 per
cent and still leave a gravitationally bound stellar population. Such an object
resembles the outer Milky Way globular clusters and the faint fuzzy star
clusters recently discovered in NGC 1023.Comment: 21 pages, 8 figures, accepted by Ap
The dynamical state of the Globular Cluster M10 (NGC 6254)
Studying the radial variation of the stellar mass function in globular
clusters (GCs) has proved a valuable tool to explore the collisional dynamics
leading to mass segregation and core collapse. In order to study the radial
dependence of the luminosity and mass function of M 10, we used ACS/HST deep
high resolution archival images, reaching out to approximately the cluster's
half-mass radius (rhm), combined with deep WFPC2 images that extend our radial
coverage to more than 2 rhm. From our photometry, we derived a radial mass
segregation profile and a global mass function that we compared with those of
simulated clusters containing different energy sources (namely hard binaries
and/or an IMBH) able to halt core collapse and to quench mass segregation. A
set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of
the total cluster mass, comprising different initial mass functions (IMFs) and
primordial binary fractions, was used to predict the observed mass segregation
profile and mass function. The mass segregation profile of M 10 is not
compatible with cluster models without either an IMBH or primordial binaries,
as a source of energy appears to be moderately quenching mass segregation in
the cluster. Unfortunately, the present observational uncertainty on the binary
fraction in M10 does not allow us to confirm the presence of an IMBH in the
cluster, since an IMBH, a dynamically non-negligible binary fraction (~ 5%), or
both can equally well explain the radial dependence of the cluster mass
function.Comment: 15 pages, 8 figures, accepted for publication on Ap
Lifetimes of tidally limited star clusters with different radii
We study the escape rate, dN/dt, from clusters with different radii in a
tidal field using analytical predictions and direct N-body simulations. We find
that dN/dt depends on the ratio R=r_h/r_j, where r_h is the half-mass radius
and r_j the radius of the zero-velocity surface. For R>0.05, the "tidal
regime", there is almost no dependence of dN/dt on R. To first order this is
because the fraction of escapers per relaxation time, t_rh, scales
approximately as R^1.5, which cancels out the r_h^1.5 term in t_rh. For R<0.05,
the "isolated regime", dN/dt scales as R^-1.5. Clusters that start with their
initial R, Ri, in the tidal regime dissolve completely in this regime and their
t_dis is insensitive to the initial r_h. We predicts that clusters that start
with Ri<0.05 always expand to the tidal regime before final dissolution. Their
t_dis has a shallower dependence on Ri than what would be expected when t_dis
is a constant times t_rh. For realistic values of Ri, the lifetime varies by
less than a factor of 1.5 due to changes in Ri. This implies that the
"survival" diagram for globular clusters should allow for more small clusters
to survive. We note that with our result it is impossible to explain the
universal peaked mass function of globular cluster systems by dynamical
evolution from a power-law initial mass function, since the peak will be at
lower masses in the outer parts of galaxies. Our results finally show that in
the tidal regime t_dis scales as N^0.65/w, with w the angular frequency of the
cluster in the host galaxy. [ABRIDGED
Theoretical and Observational Agreement on Mass Dependence of Cluster Life Times
Observations and N-body simulations both support a simple relation for the
disruption time of a cluster as a function of its mass of the form: t_dis = t_4
* (M/10^4 Msun)^gamma. The scaling factor t_4 seems to depend strongly on the
environment. Predictions and observations show that gamma ~ 0.64 +/- 0.06.
Assuming that t_dis ~ M^0.64 is caused by evaporation and shocking implies a
relation between the radius and the mass of a cluster of the form: r_h ~
M^0.07, which has been observed in a few galaxies. The suggested relation for
the disruption time implies that the lower mass end of the cluster initial mass
function will be disrupted faster than the higher mass end, which is needed to
evolve a young power law shaped mass function into the log-normal mass function
of old (globular) clusters.Comment: 2 pages, to appear in "The Formation and Evolution of Massive Young
Star Clusters", 17-21 November 2003, Cancun (Mexico
Mass loss rates and the mass evolution of star clusters
We describe the interplay between stellar evolution and dynamical mass loss
of evolving star clusters, based on the principles of stellar evolution and
cluster dynamics and on a grid of N-body simulations of cluster models. The
cluster models have different initial masses, different orbits, including
elliptical ones, and different initial density profiles. We use two sets of
cluster models: initially Roche-lobe filling and Roche-lobe underfilling. We
identify four distinct mass loss effects: (1) mass loss by stellar evolution,
(2) loss of stars induced by stellar evolution and (3) relaxation-driven mass
loss before and (4) after core collapse. Both the evolution-induced loss of
stars and the relaxation-driven mass loss need time to build up. This is
described by a delay-function of a few crossing times for Roche-lobe filling
clusters and a few half mass relaxation times for Roche-lobe underfilling
clusters. The relaxation-driven mass loss can be described by a simple power
law dependence of the mass dM/dt =-M^{1-gamma}/t0, (with M in Msun) where t0
depends on the orbit and environment of the cluster. Gamma is 0.65 for clusters
with a King-parameter W0=5 and 0.80 for more concentrated clusters with W0=7.
For initially Roche-lobe underfilling clusters the dissolution is described by
the same gamma=0.80. The values of the constant t0 are described by simple
formulae that depend on the orbit of the cluster. The mass loss rate increases
by about a factor two at core collapse and the mass dependence of the
relaxation-driven mass loss changes to gamma=0.70 after core collapse. We also
present a simple recipe for predicting the mass evolution of individual star
clusters with various metallicities and in different environments, with an
accuracy of a few percent in most cases. This can be used to predict the mass
evolution of cluster systems.Comment: 25 pages, 17 figures, 4 tables, 2 appendices; accepted for
publication in MNRA
Monte Carlo Simulations of Globular Cluster Evolution. VI. The Influence of an Intermediate Mass Black Hole
We present results of a series of Monte Carlo simulations investigating the
imprint of a central intermediate-mass black hole (IMBH) on the structure of a
globular cluster. We investigate the three-dimensional and projected density
profiles, and stellar disruption rates for idealized as well as realistic
cluster models, taking into account a stellar mass spectrum and stellar
evolution, and allowing for a larger, more realistic, number of stars than was
previously possible with direct N-body methods. We compare our results to other
N-body and Fokker-Planck simulations published previously. We find, in general,
very good agreement for the overall cluster structure and dynamical evolution
between direct N-body simulations and our Monte Carlo simulations. Significant
differences exist in the number of stars that are tidally disrupted by the
IMBH, which is most likely an effect of the wandering motion of the IMBH, not
included in the Monte Carlo scheme. These differences, however, are negligible
for the final IMBH masses in realistic cluster models as the disruption rates
are generally much lower than for single-mass clusters. As a direct comparison
to observations we construct a detailed model for the cluster NGC 5694, which
is known to possess a central surface brightness cusp consistent with the
presence of an IMBH. We find that not only the inner slope but also the outer
part of the surface brightness profile agree well with observations. However,
there is only a slight preference for models harboring an IMBH compared to
models without.Comment: 37 pages, 10 figures, Accepted for publication in ApJ Supplement.
Substantial additions on modeling NGC 5694 since original versio
The Globular Cluster Luminosity Function and Specific Frequency in Dwarf Elliptical Galaxies
The globular cluster luminosity function, specific globular cluster
frequency, S_N, specific globular cluster mass, T_MP, and globular cluster mass
fraction in dwarf elliptical galaxies are explored using the full 69 galaxy
sample of the HST WFPC2 Dwarf Elliptical Galaxy Snapshot Survey. The GCLFs of
the dEs are well-represented with a t_5 function with a peak at
M_{V,Z}^0(dE,HST) = -7.3 +/- 0.1. This is ~0.3 magnitudes fainter than the GCLF
peaks in giant spiral and elliptical galaxies, but the results are consistent
within the uncertainties. The bright-end slope of the luminosity distribution
has a power-law form with slope alpha = -1.9 +/- 0.1. The trend of increasing
S_N or T_MP with decreasing host galaxy luminosity is confirmed. The mean value
for T_MP in dE,N galaxies is about a factor of two higher than the mean value
for non-nucleated galaxies and the distributions of T_MP in dE,N and dE,noN
galaxies are statistically different. These data are combined with results from
the literature for a wide range of galaxy types and environments. At low host
galaxy masses the distribution of T_MP for dE,noN and dI galaxies are similar.
This supports the idea that one pathway for forming dE,noN galaxies is by the
stripping of dIs. The formation of nuclei and the larger values of T_MP in dE,N
galaxies may be due to higher star formation rates and star cluster formation
efficiencies due to interactions in galaxy cluster environments.Comment: 53 pages, 13 figures, 12 tables, accepted by the Astrophysical
Journa
N-body simulations of star clusters
Two aspects of our recent N-body studies of star clusters are presented: (1)
What impact does mass segregation and selective mass loss have on integrated
photometry? (2) How well compare results from N-body simulations using NBODY4
and STARLAB/KIRA?Comment: 2 pages, 1 figure with 4 panels (in colour, not well visible in
black-and-white; figures screwed in PDF version, ok in postscript; to see
further details get the paper source). Conference proceedings for IAUS246
'Dynamical Evolution of Dense Stellar Systems', ed. E. Vesperini (Chief
Editor), M. Giersz, A. Sills, Capri, Sept. 2007; v2: references correcte
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