1,920 research outputs found
Black hole mergers in the universe
Mergers of black-hole binaries are expected to release large amounts of
energy in the form of gravitational radiation. However, binary evolution models
predict merger rates too low to be of observational interest. In this paper we
explore the possibility that black holes become members of close binaries via
dynamical interactions with other stars in dense stellar systems. In star
clusters, black holes become the most massive objects within a few tens of
millions of years; dynamical relaxation then causes them to sink to the cluster
core, where they form binaries. These black-hole binaries become more tightly
bound by superelastic encounters with other cluster members, and are ultimately
ejected from the cluster. The majority of escaping black-hole binaries have
orbital periods short enough and eccentricities high enough that the emission
of gravitational radiation causes them to coalesce within a few billion years.
We predict a black-hole merger rate of about per year per
cubic megaparsec, implying gravity wave detection rates substantially greater
than the corresponding rates from neutron star mergers. For the first
generation Laser Interferometer Gravitational-Wave Observatory (LIGO-I), we
expect about one detection during the first two years of operation. For its
successor LIGO-II, the rate rises to roughly one detection per day. The
uncertainties in these numbers are large. Event rates may drop by about an
order of magnitude if the most massive clusters eject their black hole binaries
early in their evolution.Comment: 12 pages, ApJL in pres
The Relation Between the Globular Cluster Mass and Luminosity Functions
The relation between the globular cluster luminosity function (GCLF,
dN/dlogL) and globular cluster mass function (GCMF, dN/dlogM) is considered.
Due to low-mass star depletion, dissolving GCs have mass-to-light (M/L) ratios
that are lower than expected from their metallicities. This has been shown to
lead to an M/L ratio that increases with GC mass and luminosity. We model the
GCLF and GCMF and show that the power law slopes inherently differ (1.0 versus
0.7, respectively) when accounting for the variability of M/L. The observed
GCLF is found to be consistent with a Schechter-type initial cluster mass
function and a mass-dependent mass-loss rate.Comment: 4 pages, 2 figures. To appear in the proceedings of "Galaxy Wars:
Stellar Populations and Star Formation in Interacting Galaxies" (Tennessee,
July 2009
On the Interpretation of the Age Distribution of Star Clusters in the Small Magellanic Cloud
We re-analyze the age distribution (dN/dt) of star clusters in the Small
Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud
Photometric Survey. For ages younger than 3x10^9 yr the dN/dt distribution can
be approximated by a power-law distribution, dN/dt propto t^-beta, with
-beta=-0.70+/-0.05 or -beta=-0.84+/-0.04, depending on the model used to derive
the ages. Predictions for a cluster population without dissolution limited by a
V-band detection result in a power-law dN/dt distribution with an index of
~-0.7. This is because the limiting cluster mass increases with age, due to
evolutionary fading of clusters, reducing the number of observed clusters at
old ages. When a mass cut well above the limiting cluster mass is applied, the
dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is
of small importance in shaping the dN/dt distribution and incompleteness causes
dN/dt to decline. The reason that no (mass independent) infant mortality of
star clusters in the first ~10-20 Myr is found is explained by a detection bias
towards clusters without nebular emission, i.e. cluster that have survived the
infant mortality phase. The reason we find no evidence for tidal (mass
dependent) cluster dissolution in the first Gyr is explained by the weak tidal
field of the SMC. Our results are in sharp contrast to the interpretation of
Chandar et al. (2006), who interpret the declining dN/dt distribution as rapid
cluster dissolution. This is due to their erroneous assumption that the sample
is limited by cluster mass, rather than luminosity.Comment: 8 pages, 4 figures, accepted for publication in Ap
How many young star clusters exist in the Galactic center?
We study the evolution and observability of young compact star clusters
within about 200pc of the Galactic center. Calculations are performed using
direct N-body integration on the GRAPE-4, including the effects of both stellar
and binary evolution and the external influence of the Galaxy. The results of
these detailed calculations are used to calibrate a simplified model applicable
over a wider range of cluster initial conditions. We find that clusters within
200 pc from the Galactic center dissolve within about 70 Myr. However, their
projected densities drop below the background density in the direction of the
Galactic center within 20 Myr, effectively making these clusters undetectable
after that time. Clusters farther from the Galactic center but at the same
projected distance are more strongly affected by this selection effect, and may
go undetected for their entire lifetimes. Based on these findings, we conclude
that the region within 200 pc of the Galactic center could easily harbor some
50 clusters with properties similar to those of the Arches or the Quintuplet
systems.Comment: ApJ Letters in pres
The influence of initial mass segregation on the runaway merging of stars
We have investigated the effect of initial mass segregation on the runaway
merging of stars. The evolution of multi-mass, dense star clusters was followed
by means of direct N-body simulations of up to 131.072 stars. All clusters
started from King models with dimensionless central potentials of 3.0 <= W_0 <=
9.0. Initial mass segregation was realized by varying the minimum mass of a
certain fraction of stars whose either (1) distances were closest to the
cluster center or (2) total energies were lowest. The second case is more
favorable to promote the runaway merging of stars by creating a high-mass core
of massive, low-energy stars. Initial mass segregation could decrease the
central relaxation time and thus help the formation of a high-mass core.
However, we found that initial mass segregation does not help the runaway
stellar merger to happen if the overall mass density profile is kept constant.
This is due to the fact that the collision rate of stars is not increased due
to initial mass segregation. Our simulations show that initial mass segregation
is not sufficient to allow runaway merging of stars to occur in clusters with
central densities typical for star clusters in the Milky Way.Comment: 25 pages, 9 figures, 3 tables, accepted for publication in Ap
Stability of Multiplanetary Systems in Star Clusters
Most stars form in star clusters and stellar associated. To understand the
roles of star cluster environments in shaping the dynamical evolution of
planetary systems, we carry out direct -body simulations of four planetary
systems models in three different star cluster environments with respectively
N=2k, 8k and 32k stars. In each cluster, an ensemble of initially identical
planetary systems are assigned to solar-type stars with and
evolved for 50~Myr. We found that following the depletion of protoplanetary
disks, external perturbations and planet-planet interactions are two driving
mechanisms responsible for the destabilization of planetary systems. The planet
survival rate varies from in the N=2k cluster to in the
N=32k cluster, which suggests that most planetary systems can indeed survive in
low-mass clusters, except in the central regions. We also find that planet
ejections through stellar encounters are cumulative processes, as only of encounters are strong enough to excite the eccentricity by . Short-period planets can be perturbed through orbit crossings with
long-period planets. When taking into account planet-planet interactions, the
planet ejection rate nearly doubles, and therefore multiplicity contributes to
the vulnerability of planetary systems. In each ensemble, of
planetary orbits become retrograde due to random directions of stellar
encounters. Our results predict that young low-mass star clusters are promising
sites for next-generation planet surveys, yet low planet detection rates are
expected in dense globular clusters such as 47 Tuc. Nevertheless, planets in
denser stellar environments are likely to have shorter orbital periods, which
enhances their detectability.Comment: 19 pages, 13 figures, 4 tables, accepted for publication in MNRA
The Evolution of Globular Clusters in the Galaxy
We investigate the evolution of globular clusters using N-body calculations
and anisotropic Fokker-Planck (FP) calculations. The models include a mass
spectrum, mass loss due to stellar evolution, and the tidal field of the parent
galaxy. Recent N-body calculations have revealed a serious discrepancy between
the results of N-body calculations and isotropic FP calculations. The main
reason for the discrepancy is an oversimplified treatment of the tidal field
employed in the isotropic FP models. In this paper we perform a series of
calculations with anisotropic FP models with a better treatment of the tidal
boundary and compare these with N-body calculations. The new tidal boundary
condition in our FP model includes one free parameter. We find that a single
value of this parameter gives satisfactory agreement between the N-body and FP
models over a wide range of initial conditions.
Using the improved FP model, we carry out an extensive survey of the
evolution of globular clusters over a wide range of initial conditions varying
the slope of the mass function, the central concentration, and the relaxation
time. The evolution of clusters is followed up to the moment of core collapse
or the disruption of the clusters in the tidal field of the parent galaxy. In
general, our model clusters, calculated with the anisotropic FP model with the
improved treatment for the tidal boundary, live longer than isotropic models.
The difference in the lifetime between the isotropic and anisotropic models is
particularly large when the effect of mass loss via stellar evolution is rather
significant. On the other hand the difference is small for relaxation-
dominated clusters which initially have steep mass functions and high central
concentrations.Comment: 36 pages, 11 figures, LaTeX; added figures and tables; accepted by
Ap
Gravitational Radiation From Globular Clusters
Space-based gravitational wave detectors will have the ability to observe
continuous low frequency gravitational radiation from binary star systems. They
can determine the direction to continuous sources with an angular resolution
approaching tens of arcminutes. This resolution should be sufficient to
identify binary sources as members of some nearby globular clusters. Thus,
gravitational radiation can be used to determine the population of hard
binaries in globular clusters. For particularly hard binaries, the orbital
period may change as a result of gravitational wave emission. If one of these
binaries can be identified with a globular cluster, then the distance to that
cluster can be determined. Thus, gravitational radiation may provide
reddening-independent distance measurements to globular clusters.Comment: 26 pages, 1 figure, LaTeX, uses aasms4.sty, submitted to Ap.
Dynamical Interactions and the Black Hole Merger Rate of the Universe
Binary black holes can form efficiently in dense young stellar clusters, such
as the progenitors of globular clusters, via a combination of gravitational
segregation and cluster evaporation. We use simple analytic arguments supported
by detailed N-body simulations to determine how frequently black holes born in
a single stellar cluster should form binaries, be ejected from the cluster, and
merge through the emission of gravitational radiation. We then convolve this
``transfer function'' relating cluster formation to black hole mergers with (i)
the distribution of observed cluster masses and (ii) the star formation history
of the universe, assuming that a significant fraction gcl of star formation
occurs in clusters and that a significant fraction gcand of clusters undergo
this segregation and evaporation process. We predict future ground--based
gravitational wave (GW) detectors could observe ~500 (gcl/0.5) (gcand/0.1)
double black hole mergers per year, and the presently operating LIGO
interferometer would have a chance (50%) at detecting a merger during its first
full year of science data. More realistically, advanced LIGO and similar
next-generation gravitational wave observatories provide unique opportunities
to constrain otherwise inaccessible properties of clusters formed in the early
universe.Comment: 4 pages, 2 figures. To appear in PRD Rapid Communication
The effect of spiral arm passages on the evolution of stellar clusters
We study the effect of spiral arm passages on the evolution of star clusters on planar and circular orbits around the centres of galaxies. Individual passages with different relative velocity (V_drift) and arm width are studied using N-body simulations. When the ratio of the time it takes the cluster to cross the density wave to the crossing time of stars in the cluster is much smaller than one, the energy gain of stars can be predicted accurately in the impulsive approximation. When this ratio is much larger than one, the cluster is heated adiabatically and the net effect of heating is largely damped. For a given duration of the perturbation, this ratio is smaller for stars in the outer parts of the cluster compared to stars in the inner part. The cluster energy gain due to perturbations of various duration as obtained from our N-body simulations is in good agreement with theoretical predictions taking into account the effect of adiabatic damping. Perturbations by the broad stellar component of the spiral arms on a cluster are in the adiabatic regime and, therefore, hardly contribute to the energy gain and mass loss of the cluster. We consider the effect of crossings through the high density shocked gas in the spiral arms, which result in a more impulsive compression of the cluster. The time scale of disruption is shortest at ~0.8-0.9 R_CR since there V_drift is low. This location can be applicable to the solar neighbourhood. In addition, the four-armed spiral pattern of the Milky Way makes spiral arms contribute more to the disruption of clusters than in a similar but two-armed galaxy. Still, the disruption time due to spiral arm perturbations there is about an order of magnitude higher than what is observed for the solar neighbourhood.[ABRIDGED
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