194 research outputs found
Eccentric double white dwarfs as LISA sources in globular clusters
We consider the formation of double white dwarfs (DWDs) through dynamical
interactions in globular clusters. Such interactions can give rise to eccentric
DWDs, in contrast to the exclusively circular population expected to form in
the Galactic disk. We show that for a 5-year Laser Interferometer Space Antenna
(LISA) mission and distances as far as the Large Magellanic Cloud, multiple
harmonics from eccentric DWDs can be detected at a signal-to-noise ratio higher
than 8 for at least a handful of eccentric DWDs, given their formation rate and
typical lifetimes estimated from current cluster simulations. Consequently the
association of eccentricity with stellar-mass LISA sources does not uniquely
involve neutron stars, as is usually assumed. Due to the difficulty of
detecting (eccentric) DWDs with present and planned electromagnetic
observatories, LISA could provide unique dynamical identifications of these
systems in globular clusters.Comment: Published in ApJ 665, L5
Mass Segregation in Globular Clusters
We present the results of a new study of mass segregation in two-component
star clusters, based on a large number of numerical N-body simulations using
our recently developed dynamical Monte Carlo code. Specifically, we follow the
dynamical evolution of clusters containing stars with individual masses m_1 as
well as a tracer population of objects with individual masses m_2=\mu m_1,
using N=10^5 total stars. For heavy tracers, which could represent stellar
remnants such as neutron stars or black holes in a globular cluster, we
characterize in a variety of ways the tendency for these objects to concentrate
in or near the cluster core. In agreement with simple theoretical arguments, we
find that the characteristic time for this mass segregation process varies as
1/\mu. For models with very light tracers (\mu <~ 10^-2), which could represent
free-floating planets or brown dwarfs, we find the expected depletion of light
objects in the cluster core, but also sometimes a significant enhancement in
the halo. Using these results we estimate the optical depth to gravitational
microlensing by planetary mass objects or brown dwarfs in typical globular
clusters. For some initial conditions, the optical depth in the halo due to
very low-mass objects could be much greater than that of luminous stars. If we
apply our results to M22, using the recent null detection of Sahu, Anderson, &
King (2001), we find an upper limit of ~25% at the 63% confidence level for the
current mass fraction of M22 in the form of very low-mass objects.Comment: Accepted for publication in ApJ. Minor revisions reflecting the new
results of Sahu et al. on M22. 13 pages in emulateapj style, including 9
figures and 3 table
First use of single-crystal diamonds as fission-fragment detector
Single crystal chemical vapor deposited diamond (sCVD) was investigated for its ability to act as Fission fragment detector. In particular we investigated timing and energy resolution for application in a simultaneous time and energy measurement to determine the mass of the detected fission fragment. Previous tests have shown that poly crystalline chemical vapor deposited (pCVD) diamonds provide sufficient timing resolution, but their poor energy resolution did not allow complete separation between very low energy fission fragments, alpha-particles and noise. Our present investigations prove artificial sCVD diamonds to show similar timing resolution as pCVD diamonds close to 100 ps. Improved pulse height resolution allows the unequivocal separation of fission fragments, and the detection efficiency reaches 100%, but remains with about a few percent behind requirements for fragment mass identification. With high-speed digital electronics a timing resolution well below 100 ps is possible. However, the strongly varying quality of the presently available diamond material does not allow application on a sufficiently large scale within reasonable investments
Monte Carlo Simulations of Globular Cluster Evolution. IV. Direct Integration of Strong Interactions
We study the dynamical evolution of globular clusters containing populations
of primordial binaries, using our newly updated Monte Carlo cluster evolution
code with the inclusion of direct integration of binary scattering
interactions. We describe the modifications we have made to the code, as well
as improvements we have made to the core Monte Carlo method. We present several
test calculations to verify the validity of the new code, and perform many
comparisons with previous analytical and numerical work in the literature. We
simulate the evolution of a large grid of models, with a wide range of initial
cluster profiles, and with binary fractions ranging from 0 to 1, and compare
with observations of Galactic globular clusters. We find that our code yields
very good agreement with direct N-body simulations of clusters with primordial
binaries, but yields some results that differ significantly from other
approximate methods. Notably, the direct integration of binary interactions
reduces their energy generation rate relative to the simple recipes used in
Paper III, and yields smaller core radii. Our results for the structural
parameters of clusters during the binary-burning phase are now in the tail of
the range of parameters for observed clusters, implying that either clusters
are born significantly more or less centrally concentrated than has been
previously considered, or that there are additional physical processes beyond
two-body relaxation and binary interactions that affect the structural
characteristics of clusters.Comment: Accepted for publication in ApJ; 17 pages, 19 figures; changes to
reflect accepted versio
The Candidate Intermediate-Mass Black Hole in the Globular Cluster M54
Ibata et al. reported evidence for density and kinematic cusps in the
Galactic globular cluster M54, possibly due to the presence of a 9400
solar-mass black hole. Radiative signatures of accretion onto M54's candidate
intermediate-mass black hole (IMBH) could bolster the case for its existence.
Analysis of new Chandra and recent Hubble Space Telescope astrometry rules out
the X-ray counterpart to the candidate IMBH suggested by Ibata et al. If an
IMBH exists in M54, then it has an Eddington ratio of L(0.3-8 keV) / L(Edd) <
1.4 x 10^(-10), more similar to that of the candidate IMBH in M15 than that in
G1. From new imaging with the NRAO Very Large Array, the luminosity of the
candidate IMBH is L(8.5 GHz) < 3.6 x 10^29 ergs/s (3 sigma). Two background
active galaxies discovered toward M54 could serve as probes of its intracluster
medium.Comment: 4 pages; 2 figures; emulateapj.cls; to appear in A
Predictions for Triple Stars with and without a Pulsar in Star Clusters
Though about 80 pulsar binaries have been detected in globular clusters so
far, no pulsar has been found in a triple system in which all three objects are
of comparable mass. Here we present predictions for the abundance of such
triple systems, and for the most likely characteristics of these systems. Our
predictions are based on an extensive set of more than 500 direct simulations
of star clusters with primordial binaries, and a number of additional runs
containing primordial triples. Our simulations employ a number N_{tot} of equal
mass stars from N_{tot}=512 to N_{tot}=19661 and a primordial binary fraction
from 0-50%. In addition, we validate our results against simulations with
N=19661 that include a mass spectrum with a turn-off mass at 0.8 M_{sun},
appropriate to describe the old stellar populations of galactic globular
clusters. Based on our simulations, we expect that typical triple abundances in
the core of a dense cluster are two orders of magnitude lower than the binary
abundances, which in itself already suggests that we don't have to wait too
long for the first comparable-mass triple with a pulsar to be detected.Comment: 11 pages, minor changes to match MNRAS accepted versio
Dynamical Processes in Globular Clusters
Globular clusters are among the most congested stellar systems in the
Universe. Internal dynamical evolution drives them toward states of high
central density, while simultaneously concentrating the most massive stars and
binary systems in their cores. As a result, these clusters are expected to be
sites of frequent close encounters and physical collisions between stars and
binaries, making them efficient factories for the production of interesting and
observable astrophysical exotica. I describe some elements of the competition
among stellar dynamics, stellar evolution, and other processes that control
globular cluster dynamics, with particular emphasis on pathways that may lead
to the formation of blue stragglers.Comment: Chapter 10, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Binary-induced collapse of a compact, collisionless cluster
We improve and extend Shapiro's model of a relativistic, compact object which
is stable in isolation but is driven dynamically unstable by the tidal field of
a binary companion. Our compact object consists of a dense swarm of test
particles moving in randomly-oriented, initially circular, relativistic orbits
about a nonrotating black hole. The binary companion is a distant, slowly
inspiraling point mass. The tidal field of the companion is treated as a small
perturbation on the background Schwarzschild geometry near the hole; the
resulting metric is determined by solving the perturbation equations of Regge
and Wheeler and Zerilli in the quasi-static limit. The perturbed spacetime
supports Bekenstein's conjecture that the horizon area of a near-equilibrium
black hole is an adiabatic invariant. We follow the evolution of the system and
confirm that gravitational collapse can be induced in a compact collisionless
cluster by the tidal field of a binary companion.Comment: 9 Latex pages, 14 postscript figure
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