2,977 research outputs found
Selection of the SIM Astrometric Grid
We investigate the choice of stellar population for use as the Astrometric
Grid for the Space Interferometry Mission (SIM). SIM depends on the astrometric
stability of about 2000 stars, the so called Grid, against which the science
measures are referenced. Low metallicity, and thus relatively high luminosity K
giants are shown to be the population of choice, when available. The
alternative, nearby G dwarfs, are shown to be suseptable to unmodeled motions
induced by gas-giant planetary companions, should there be a significant
population of such companions.
Radial velocity filtering is quite efficient in selecting Grid members from
the K giants with yields exceeding 50% if filtering at 30m/s (1-sigma) is
available. However if the binary fraction of the G dwarfs approaches 100% as
some studies suggest, the yield of stable systems would be in the range of 15%
at best (with 10m/s filtering). Use of the initial SIM measurement as a final
filter is shown not to be critical in either case, although it could improve
the yield of stable grid members.
For a Grid composed of weak-lined K giants, the residual contamination by
large unmodeled motions will amount to about 3% (and rises to about 6% if a
60m/s radial velocity criterion is used). The selective introduction of
quadratic terms in the proper motion solutions during the post-mission phase of
data reduction can reduce contamination to a remarkable 1% or better in either
case.
Analytic estimates based on circular orbits are developed which show how
these results come about.Comment: 42 pages including 13 eps figures. To be published Sept 2002 in PAS
The secular evolution of the Kuiper belt after a close stellar encounter
We show the effects of the perturbation caused by a passing by star on the
Kuiper belt objects (KBOs) of our Solar System. The dynamics of the Kuiper belt
(KB) is followed by direct -body simulations. The sampling of the KB has
been done with up to , setting the KBOs on initially nearly
circular orbits distributed in a ring of surface density .
This modelization allowed us to investigate the secular evolution of the KB
upon the encounter with the perturbing star. Actually, the encounter itself
usually leads toward eccentricity and inclination distributions similar to
observed ones, but tends also to excite the low-eccentricity population ( around \, from the Sun), depleting this region of
low eccentricities. The following long-term evolution shows a "cooling" of the
eccentricities repopulating the low-eccentricity area. In dependence on the
assumed KBO mass spectrum and sampled number of bodies, this repopulation takes
place in a time that goes from 0.5 Myr to 100 Myr. Due to the unavoidable
limitation in the number of objects in our long-term simulations (), we could not consider a detailed KBO mass spectrum, accounting for low
mass objects, thus our present simulations are not reliable in constraining
correlations among inclination distribution of the KBOs and other properties,
such as their size distribution. However, our high precision long term
simulations are a starting point for future larger studies on massively
parallel computational platforms which will provide a deeper investigation of
the secular evolution (Myr) of the KB over its whole mass spectrum.Comment: 13 pages, 12 figures, 5 table
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
De Toekomst van Waterkwaliteitsmonitoring, Deel 3: Biomonitoring
In general terms the problems with the existing water quality monitoring approach concern effective and efficient monitoring strategies. In 1993 the project "Monitoring water quality in the future" started in order to address these problems which will only increase in the future. In the framework of this project five reports have been produced, focussing on: Chemical Monitoring (Volume 1) ; Mixture toxicity parameters (Volume 2) ; Biomonitoring (Volume 3) ; Monitoring strategies for complex mixtures (Volume 4); and Organizational aspects (Volume 5). The specific objectives were to produce concise reviews of methods to signal changes in and control water quality (Volumes 1-3), to give a review of testing strategies for complex mixtures of chemical substances which can give more complete information at less costs (Volume 4) and to review existing practices and make recommendations concerning standardization, optimization and organization of monitoring activities in the European Union, with a focus on effectiveness and efficiency (Volume 5). In an executive summary overall recommendations are also made by drawing these together from the individual studies. The present report (Volume 3) includes a short description of existing biomonitoring methodologies and measurement strategies, as well as a discussion on possibilities, developments, limitations and financial consequences
Monte-Carlo Simulations of Globular Cluster Evolution - I. Method and Test Calculations
We present a new parallel supercomputer implementation of the Monte-Carlo
method for simulating the dynamical evolution of globular star clusters. Our
method is based on a modified version of Henon's Monte-Carlo algorithm for
solving the Fokker-Planck equation. Our code allows us to follow the evolution
of a cluster containing up to 5x10^5 stars to core collapse in < 40 hours of
computing time. In this paper we present the results of test calculations for
clusters with equal-mass stars, starting from both Plummer and King model
initial conditions. We consider isolated as well as tidally truncated clusters.
Our results are compared to those obtained from approximate, self-similar
analytic solutions, from direct numerical integrations of the Fokker-Planck
equation, and from direct N-body integrations performed on a GRAPE-4
special-purpose computer with N=16384. In all cases we find excellent agreement
with other methods, establishing our new code as a robust tool for the
numerical study of globular cluster dynamics using a realistic number of stars.Comment: 35 pages, including 8 figures, submitted to ApJ. Revised versio
MYRIAD: A new N-body code for simulations of Star Clusters
We present a new C++ code for collisional N-body simulations of star
clusters. The code uses the Hermite fourth-order scheme with block time steps,
for advancing the particles in time, while the forces and neighboring particles
are computed using the GRAPE-6 board. Special treatment is used for close
encounters, binary and multiple sub-systems that either form dynamically or
exist in the initial configuration. The structure of the code is modular and
allows the appropriate treatment of more physical phenomena, such as stellar
and binary evolution, stellar collisions and evolution of close black-hole
binaries. Moreover, it can be easily modified so that the part of the code that
uses GRAPE-6, could be replaced by another module that uses other
accelerating-hardware like the Graphics Processing Units (GPUs). Appropriate
choice of the free parameters give a good accuracy and speed for simulations of
star clusters up to and beyond core collapse. Simulations of Plummer models
consisting of equal-mass stars reached core collapse at t~17 half-mass
relaxation times, which compares very well with existing results, while the
cumulative relative error in the energy remained below 0.001. Also, comparisons
with published results of other codes for the time of core collapse for
different initial conditions, show excellent agreement. Simulations of King
models with an initial mass-function, similar to those found in the literature,
reached core collapse at t~0.17, which is slightly smaller than the expected
result from previous works. Finally, the code accuracy becomes comparable and
even better than the accuracy of existing codes, when a number of close binary
systems is dynamically created in a simulation. This is due to the high
accuracy of the method that is used for close binary and multiple sub-systems.Comment: 24 pages, 29 figures, accepted for publication to Astronomy &
Astrophysic
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
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