3,331 research outputs found
Roche volume filling and the dissolution of open star clusters
From direct N-body simulations we find that the dynamical evolution of star
clusters is strongly influenced by the Roche volume filling factor. We present
a parameter study of the dissolution of open star clusters with different Roche
volume filling factors and different particle numbers. We study both Roche
volume underfilling and overfilling models and compare with the Roche volume
filling case. We find that in the Roche volume overfilling limit of our
simulations two-body relaxation is no longer the dominant dissolution mechanism
but the changing cluster potential. We call this mechnism "mass-loss driven
dissolution" in contrast to "two-body relaxation driven dissolution" which
occurs in the Roche volume underfilling regime. We have measured scaling
exponents of the dissolution time with the two-body relaxation time. In this
experimental study we find a decreasing scaling exponent with increasing Roche
volume filling factor. The evolution of the escaper number in the Roche volume
overfilling limit can be described by a log-logistic differential equation. We
report the finding of a resonance condition which may play a role for the
evolution of star clusters and may be calibrated by the main periodic orbit in
the large island of retrograde quasiperiodic orbits in the Poincar\'e surfaces
of section. We also report on the existence of a stability curve which may be
of relevance with respect to the structure of star clusters.Comment: 14 pages, 10+1 figures, accepted by Astronomische Nachrichte
The star cluster survivability after gas expulsion is independent of the impact of the Galactic tidal field
We study the impact of the tidal field on the survivability of star clusters
following instantaneous gas expulsion. Our model clusters are formed with a
centrally-peaked star-formation efficiency profile as a result of
star-formation taking place with a constant efficiency per free-fall time. We
define the impact of the tidal field as the ratio of the cluster half-mass
radius to its Jacobi radius immediately after gas expulsion, . We vary by varying either the Galactocentric distance,
or the size (hence volume density) of star clusters.
We propose a new method to measure the violent relaxation duration, in which
we compare the total mass-loss rate of star clusters with their stellar
evolutionary mass-loss rate. That way, we can robustly estimate the bound mass
fraction of our model clusters at the end of violent relaxation. The duration
of violent relaxation correlates linearly with the Jacobi radius, when
considering identical clusters at different Galactocentric distances. In
contrast, it is nearly constant for the solar neighbourhood clusters, slightly
decreasing with . The violent relaxation does not last longer than 50
Myr in our simulations.
Identical model clusters placed at different Galactocentric distances have
the same final bound fraction, despite experiencing different impacts of the
tidal field. The solar neighbourhood clusters with different densities
experience only limited variations of their final bound fraction.
In general, we conclude that the cluster survivability after instantaneous
gas expulsion, as measured by their bound mass fraction at the end of violent
relaxation, , is independent of the impact of the tidal field,
.Comment: accepted for publication in MNRAS, 8 pages, 5 figures,3 table
Two Program Packages for Deriving Multistate-Multiregion Life Tables and Two-Sex (Female Dominant) Population Projections
This paper describes two program packages for carrying out multistate/multiregion life table analyses and a female dominant two-sex projection by states and regions. The computer programs are adaptations of those published earlier for multistate life tables (Willekens 1979b) and for multiregional population projections (Willekens and Rogers 1978). Users not familiar with all the details of the theory of multistate demography nevertheless should be able to analyze populations disaggregated by age, sex, states, and regions, using these computer programs
Effects of galaxy--satellite interactions on bar formation
Aims. We aim to show how encounters with low-mass satellite galaxies may
alter the bar formation in a Milky Way-like disc galaxy. Methods. We use
high-resolution N-body simulations of a disc galaxy prone to mild bar
instability. For realistic initial conditions of satellites, we take advantage
of cosmological simulations of Milky Way-like dark matter haloes. Results. The
satellites may have a significant impact on the time of bar formation. Some
runs with satellites demonstrate a delay, while others show an advancement in
bar formation compared to the isolated run, with such time differences reaching
1 Gyr. Meanwhile, the final bar configuration, including its very
appearance and the bar characteristics such as the pattern speed and the
exponential growth rate of its amplitude are independent of the number of
encounters and their orbits. The contribution of satellites with masses below
is insignificant, unless their pericentre distances are small.
We suggest that the encounters act indirectly via inducing perturbations across
the disc that evolve to delayed waves in the central part and interfere with an
emerging seed bar. The predicted effect for the present-day host galaxy is
expected to be even more significant at redshifts .Comment: 16 pages, 14 figures and 4 table
Metropolitan Growth and Population Development at a National Level
This paper demonstrates the importance of treating urban populations as heterogeneous when making national population projections. Large metropolitan areas are shown to exhibit specific patterns of migration, age composition, and fertility when compared with urban areas as a whole. It is argued that the path of demographic change at a national scale is not independent of the urbanization scenario selected, i.e., one emphasizing the expansion of large cities or, alternatively, a balanced settlement hierarchy
On the degenerated soft-mode instability
We consider instabilities of a single mode with finite wavenumber in
inversion symmetric spatially one dimensional systems, where the character of
the bifurcation changes from sub- to supercritical behaviour. Starting from a
general equation of motion the full amplitude equation is derived
systematically and formulas for the dependence of the coefficients on the
system parameters are obtained. We emphasise the importance of nonlinear
derivative terms in the amplitude equation for the behaviour in the vicinity of
the bifurcation point. Especially the numerical values of the corresponding
coefficients determine the region of coexistence between the stable trivial
solution and stable spatially periodic patterns. Our approach clearly shows
that similar considerations fail for the case of oscillatory instabilities.Comment: 16 pages, uses iop style files, manuscript also available at
ftp://athene.fkp.physik.th-darmstadt.de/pub/publications/wolfram/jpa_97/ or
at http://athene.fkp.physik.th-darmstadt.de/public/wolfram_publ.html. J.
Phys. A in pres
Global Prospects for Population Growth and Distribution
Accurate estimates of future populations are out of reach of present techniques; the methods actually used, however they are described, are extrapolations of past trends and cannot anticipate turning points in births or other components. The present paper is no exception. It summarizes the errors of past work in the expectation that these will indicate the accuracy of the figures here contained. The method used in this paper is to suppose some year in which each country will attain bare replacement (an average of about 2.3 children per fertile couple) along with a smooth transition to that condition, and a continuance of replacement until the age distribution stabilizes and the population becomes stationary. For most countries we took the turn of the century as about the time when replacement would be reached. Estimates of population and labor force are shown by age and sex at 25 year intervals for 150 countries; groupings are given by continent and degree of development. Printouts are available showing much more detail than we have been able to publish in this working paper
Calibration of radii and masses of open clusters with a simulation
Context: A recent new approach to apply a simple dynamical mass estimate of
tidally limited star clusters is based on the identification of the tidal
radius in a King profile with the dynamical Jacobi radius. The application to
an unbiased open cluster catalogue yields significantly higher cluster masses
compared to the classical methods. Aims: We quantify the bias in the mass
determination as function of projection direction and cluster age by analysing
a simulated star cluster. Methods: We use direct -body simulations of a star
cluster including stellar evolution in an analytic Milky Way potential and
apply a best fit to the projected number density of cluster stars. Results: We
obtain significantly overestimated star cluster masses which depend strongly on
the viewing direction. The overestimation is typically in the range of 10-50
percent and reaches a factor of 3.5 for young clusters. Mass segregation
reduces the derived limiting radii systematically.Comment: 9 pages, 10+1 figures, accepted by Astronomy and Astrophysic
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