3,963 research outputs found
Formation of Protoplanets from Massive Planetesimals in Binary Systems
More than half of stars reside in binary or multiple star systems and many
planets have been found in binary systems. From theoretical point of view,
however, whether or not the planetary formation proceeds in a binary system is
a very complex problem, because secular perturbation from the companion star
can easily stir up the eccentricity of the planetesimals and cause
high-velocity, destructive collisions between planetesimals. Early stage of
planetary formation process in binary systems has been studied by restricted
three-body approach with gas drag and it is commonly accepted that accretion of
planetesimals can proceed due to orbital phasing by gas drag. However, the gas
drag becomes less effective as the planetesimals become massive. Therefore it
is still uncertain whether the collision velocity remains small and planetary
accretion can proceed, once the planetesimals become massive. We performed {\it
N}-body simulations of planetary formation in binary systems starting from
massive planetesimals whose size is about 100-500 km. We found that the
eccentricity vectors of planetesimals quickly converge to the forced
eccentricity due to the coupling of the perturbation of the companion and the
mutual interaction of planetesimals if the initial disk model is sufficiently
wide in radial distribution. This convergence decreases the collision velocity
and as a result accretion can proceed much in the same way as in isolated
systems. The basic processes of the planetary formation, such as runaway growth
and oligarchic growth and final configuration of the protoplanets are
essentially the same in binary systems and single star systems, at least in the
late stage where the effect of gas drag is small.Comment: 26pages, 11 figures. ApJ accepte
Long-Term Evolution of Massive Black Hole Binaries. II. Binary Evolution in Low-Density Galaxies
We use direct-summation N-body integrations to follow the evolution of binary
black holes at the centers of galaxy models with large, constant-density cores.
Particle numbers as large as 400K are considered. The results are compared with
the predictions of loss-cone theory, under the assumption that the supply of
stars to the binary is limited by the rate at which they can be scattered into
the binary's influence sphere by gravitational encounters. The agreement
between theory and simulation is quite good; in particular, we are able to
quantitatively explain the observed dependence of binary hardening rate on N.
We do not verify the recent claim of Chatterjee, Hernquist & Loeb (2003) that
the hardening rate of the binary stabilizes when N exceeds a particular value,
or that Brownian wandering of the binary has a significant effect on its
evolution. When scaled to real galaxies, our results suggest that massive black
hole binaries in gas-poor nuclei would be unlikely to reach gravitational-wave
coalescence in a Hubble time.Comment: 13 pages, 8 figure
Cluster Mass Estimate and a Cusp of the Mass Density Distribution in Clusters of Galaxies
We study density cusps in the center of clusters of galaxies to reconcile
X-ray mass estimates with gravitational lensing masses. For various mass
density models with cusps we compute X-ray surface brightness distribution, and
fit them to observations to measure the range of parameters in the density
models. The Einstein radii estimated from these density models are compared
with Einstein radii derived from the observed arcs for Abell 2163, Abell 2218,
and RX J1347.5-1145. The X-ray masses and lensing masses corresponding to these
Einstein radii are also compared. While steeper cusps give smaller ratios of
lensing mass to X-ray mass, the X-ray surface brightnesses estimated from
flatter cusps are better fits to the observations. For Abell 2163 and Abell
2218, although the isothermal sphere with a finite core cannot produce giant
arc images, a density model with a central cusp can produce a finite Einstein
radius, which is smaller than the observed radii. We find that a total mass
density profile which declines as produces the largest radius
in models which are consistent with the X-ray surface brightness profile. As
the result, the extremely large ratio of the lensing mass to the X-ray mass is
improved from 2.2 to 1.4 for Abell 2163, and from 3 to 2.4 for Abell 2218. For
RX J1347.5-1145, which is a cooling flow cluster, we cannot reduce the mass
discrepancy.Comment: 23 pages, 10 figures, Latex, uses aasms4.sty, accepted for
publication in ApJ, Part
Star cluster ecology IVa: Dissection of an open star cluster---photometry
The evolution of star clusters is studied using N-body simulations in which
the evolution of single stars and binaries are taken self-consistently into
account. Initial conditions are chosen to represent relatively young Galactic
open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations
include a realistic mass function, primordial binaries and the external
potential of the parent Galaxy. Our model clusters are generally significantly
flattened in the Galactic tidal field, and dissolve before deep core collapse
occurs. The binary fraction decreases initially due to the destruction of soft
binaries, but increases later because lower mass single stars escape more
easily than the more massive binaries. At late times, the cluster core is quite
rich in giants and white dwarfs. There is no evidence for preferential
evaporation of old white dwarfs, on the contrary the formed white dwarfs are
likely to remain in the cluster. Stars tend to escape from the cluster through
the first and second Lagrange points, in the direction of and away from the
Galactic center. Mass segregation manifests itself in our models well within an
initial relaxation time. As expected, giants and white dwarfs are much more
strongly affected by mass segregation than main-sequence stars. Open clusters
are dynamically rather inactive. However, the combined effect of stellar mass
loss and evaporation of stars from the cluster potential drives its dissolution
on a much shorter timescale than if these effects are neglected. The often-used
argument that a star cluster is barely older than its relaxation time and
therefore cannot be dynamically evolved is clearly in error for the majority of
star clusters.Comment: reduced abstract, 33 pages (three separate color .jpg figures),
submitted to MNRA
High Performance Direct Gravitational N-body Simulations on Graphics Processing Units -- II: An implementation in CUDA
We present the results of gravitational direct -body simulations using the
Graphics Processing Unit (GPU) on a commercial NVIDIA GeForce 8800GTX designed
for gaming computers. The force evaluation of the -body problem is
implemented in ``Compute Unified Device Architecture'' (CUDA) using the GPU to
speed-up the calculations. We tested the implementation on three different
-body codes: two direct -body integration codes, using the 4th order
predictor-corrector Hermite integrator with block time-steps, and one
Barnes-Hut treecode, which uses a 2nd order leapfrog integration scheme. The
integration of the equations of motions for all codes is performed on the host
CPU.
We find that for particles the GPU outperforms the GRAPE-6Af, if
some softening in the force calculation is accepted. Without softening and for
very small integration time steps the GRAPE still outperforms the GPU. We
conclude that modern GPUs offer an attractive alternative to GRAPE-6Af special
purpose hardware. Using the same time-step criterion, the total energy of the
-body system was conserved better than to one in on the GPU, only
about an order of magnitude worse than obtained with GRAPE-6Af. For N \apgt
10^5 the 8800GTX outperforms the host CPU by a factor of about 100 and runs at
about the same speed as the GRAPE-6Af.Comment: Accepted for publication in New Astronom
Young massive star clusters
Young massive clusters are dense aggregates of young stars that form the
fundamental building blocks of galaxies. Several examples exist in the Milky
Way Galaxy and the Local Group, but they are particularly abundant in starburst
and interacting galaxies. The few young massive clusters that are close enough
to resolve are of prime interest for studying the stellar mass function and the
ecological interplay between stellar evolution and stellar dynamics. The
distant unresolved clusters may be effectively used to study the star-cluster
mass function, and they provide excellent constraints on the formation
mechanisms of young cluster populations. Young massive clusters are expected to
be the nurseries for many unusual objects, including a wide range of exotic
stars and binaries. So far only a few such objects have been found in young
massive clusters, although their older cousins, the globular clusters, are
unusually rich in stellar exotica. In this review we focus on star clusters
younger than Myr, more than a few current crossing times old, and
more massive than \Msun, irrespective of cluster size or
environment. We describe the global properties of the currently known young
massive star clusters in the Local Group and beyond, and discuss the state of
the art in observations and dynamical modeling of these systems. In order to
make this review readable by observers, theorists, and computational
astrophysicists, we also review the cross-disciplinary terminology.Comment: Only 88 pages. To be published in ARAA. Final version to be submitted
on Friday 12 Februar
On the Spin History of the X-ray Pulsar in Kes 73: Further Evidence For an Utramagnetized Neutron Star
In previous papers, we presented the discovery of a 12-s X-ray pulsar in the
supernova remnant Kes 73, providing the first direct evidence for an
ultramagnetized neutron star, a magnetar, with an equivalent dipole field of
nearly twenty times the quantum critical magnetic field. Our conclusions were
based on two epochs of measurement of the spin, along with an age estimate of
the host supernova remnant. Herein, we present a spin chronology of the pulsar
using additional GINGA, ASCA, XTE, & SAX datasets spanning over a decade.
Timing and spectral analysis confirms our initial results and severely limit an
accretion origin for the observed flux. Over the 10 year baseline, the pulsar
is found to undergo a rapid, constant spindown, while maintaining a steady flux
and an invariant pulse profile. Within the measurement uncertainties, no
systematic departures from a linear spin-down are found - departures as in the
case of glitches or simply stochastic fluctuations in the pulse
times-of-arrival (e.g. red timing noise). We suggest that this pulsar is akin
to the soft gamma-ray repeaters, however, it is remarkably stable and has yet
to display similar outbursts; future gamma-ray activity from this object is
likely.Comment: 6 pages with 3 embedded figures, LaTex, emulateapj.sty. Submitted to
the ApJ Letter
On Randomized Fictitious Play for Approximating Saddle Points Over Convex Sets
Given two bounded convex sets X\subseteq\RR^m and Y\subseteq\RR^n, specified by membership oracles, and a continuous convex-concave function F:X\times Y\to\RR, we consider the problem of computing an \eps-approximate saddle point, that is, a pair such that \sup_{y\in Y} F(x^*,y)\le \inf_{x\in X}F(x,y^*)+\eps. Grigoriadis and Khachiyan (1995) gave a simple randomized variant of fictitious play for computing an \eps-approximate saddle point for matrix games, that is, when is bilinear and the sets and are simplices. In this paper, we extend their method to the general case. In particular, we show that, for functions of constant "width", an \eps-approximate saddle point can be computed using O^*(\frac{(n+m)}{\eps^2}\ln R) random samples from log-concave distributions over the convex sets and . It is assumed that and have inscribed balls of radius and circumscribing balls of radius . As a consequence, we obtain a simple randomized polynomial-time algorithm that computes such an approximation faster than known methods for problems with bounded width and when \eps \in (0,1) is a fixed, but arbitrarily small constant. Our main tool for achieving this result is the combination of the randomized fictitious play with the recently developed results on sampling from convex sets
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