372 research outputs found
Irregular Satellites of the Planets: Products of Capture in the Early Solar System
All four giant planets in the Solar system possess irregular satellites,
characterized by large, highly eccentric and/or inclined orbits that are
distinct from the nearly circular, uninclined orbits of the regular satellites.
This difference can be traced directly to different modes of formation. Whereas
the regular satellites grew by accretion within circumplanetary disks the
irregular satellites were captured from initially heliocentric orbits at an
early epoch. Recently, powerful survey observations have greatly increased the
number of known irregular satellites, permitting a fresh look at the group
properties of these objects and motivating a re-examination of the mechanisms
of capture. None of the suggested mechanisms, including gas-drag, pull-down,
and three-body capture, convincingly fit the group characteristics of the
irregular satellites. The sources of the satellites also remain unidentified.Comment: 51 pages, 17 figures, 5 tables, to appear in ARAA 200
Revisiting the ABC flow dynamo
The ABC flow is a prototype for fast dynamo action, essential to the origin
of magnetic field in large astrophysical objects. Probably the most studied
configuration is the classical 1:1:1 flow. We investigate its dynamo properties
varying the magnetic Reynolds number Rm. We identify two kinks in the growth
rate, which correspond respectively to an eigenvalue crossing and to an
eigenvalue coalescence. The dominant eigenvalue becomes purely real for a
finite value of the control parameter. Finally we show that even for Rm =
25000, the dominant eigenvalue has not yet reached an asymptotic behaviour. Its
still varies very significantly with the controlling parameter. Even at these
very large values of Rm the fast dynamo property of this flow cannot yet be
established
Capture of field stars by globular clusters in dense bulge regions
The recent detection of a double Red Giant Branch in the optical
color-magnitude diagram (CMD) of the bulge globular cluster HP1 (Ortolani et
al. 1997), a more populated metal-poor steep one corresponding to the cluster
itself, and another metal-rich curved, led us to explore in the present Letter
the possibility of capture of field stars by a globular cluster orbiting in
dense bulge regions over several gigayears. Analytical arguments, as well as
N-body calculations for a cluster model of 10^5 solar masses in a bulge-like
environment, suggest that a significant fraction of cluster stars may consist
of captures. Metal-poor globular clusters in the inner bulge, like HP1,
contrasting at least in Delta [Fe/H] = 1.0 dex with respect to the surrounding
metal-rich stars, are ideal probes to further test the capture scenario. In
turn, if this scenario is confirmed, the double RGB of HP1 could provide direct
estimates of blanketing amounts, which is fundamental for the photometric
calibration of metal-rich stellar populations.Comment: 6 pages, 2 included figures, aas2pp4,sty Latex style. To be published
in Astrophysical Journal Letter
Two-Component Fokker-Planck Models for the Evolution of Isolated Globular Clusters
Two-component (normal and degenerate stars) models are the simplest
realization of clusters with a mass spectrum because high mass stars evolve
quickly into degenerates, while low mass stars remain on the main-sequence for
the age of the universe. Here we examine the evolution of isolated globular
clusters using two-component Fokker-Planck (FP) models that include heating by
binaries formed in tidal capture and in three-body encounters. Three-body
binary heating dominates and the postcollapse expansion is self-similar, at
least in models with total mass M <= 3 x 10^5 M_\odot, initial half-mass radius
r_{h,i} >= 5 pc, component mass ratio m_2/m_1 <= 2, and number ratio N_1/N_2 <=
300 when m_2=1.4 M_\odot. We derive scaling laws for \rho_c, v_c, r_c, and r_h
as functions of m_1/m_2, N, M, and time t from simple energy-balance arguments,
and these agree well with the FP simulations. We have studied the conditions
under which gravothermal oscillations (GTOs) occur. If E_{tot} and E_c are the
energies of the cluster and of the core, respectively, and t_{rh} and t_c are
their relaxation times, then \epsilon \equiv (E_{tot}/t_{rh})/(E_c/t_{rc}) is a
good predictor of GTOs: all models with \epsilon>0.01 are stable, and all but
one with \epsilon < 0.01 oscillate. We derive a scaling law for \epsilon
against N and m_1/m_2 and compared with our numerical results. Clusters with
larger m_2/m_1 or smaller N are stabler.Comment: 15 pages (LaTeX) with 8 figures. To appear in ApJ March 10, 1998
issu
Long-Term Evolution of Massive Black Hole Binaries. III. Binary Evolution in Collisional Nuclei
[Abridged] In galactic nuclei with sufficiently short relaxation times,
binary supermassive black holes can evolve beyond their stalling radii via
continued interaction with stars. We study this "collisional" evolutionary
regime using both fully self-consistent N-body integrations and approximate
Fokker-Planck models. The N-body integrations employ particle numbers up to
0.26M and a direct-summation potential solver; close interactions involving the
binary are treated using a new implementation of the Mikkola-Aarseth chain
regularization algorithm. Even at these large values of N, two-body scattering
occurs at high enough rates in the simulations that they can not be simply
scaled to the large-N regime of real galaxies. The Fokker-Planck model is used
to bridge this gap; it includes, for the first time, binary-induced changes in
the stellar density and potential. The Fokker-Planck model is shown to
accurately reproduce the results of the N-body integrations, and is then
extended to the much larger N regime of real galaxies. Analytic expressions are
derived that accurately reproduce the time dependence of the binary semi-major
axis as predicted by the Fokker-Planck model. Gravitational wave coalescence is
shown to occur in <10 Gyr in nuclei with velocity dispersions below about 80
km/s. Formation of a core results from a competition between ejection of stars
by the binary and re-supply of depleted orbits via two-body scattering. Mass
deficits as large as ~4 times the binary mass are produced before coalescence.
After the two black holes coalesce, a Bahcall-Wolf cusp appears around the
single hole in one relaxation time, resulting in a nuclear density profile
consisting of a flat core with an inner, compact cluster, similar to what is
observed at the centers of low-luminosity spheroids.Comment: 21 page
A Method for Determining the Transient Process Duration in Dynamic Systems in the Regime of Chaotic Oscillations
We describe a method for determining the transient process duration in a
standard two-dimensionaldynamic system with discrete time (Henon map),
occurring in the regime of chaotic oscillationsComment: 4 pages, 2 figure
Analytic Central Orbits and their Transformation Group
A useful crude approximation for Abelian functions is developed and applied
to orbits. The bound orbits in the power-law potentials A*r^{-alpha} take the
simple form (l/r)^k = 1 + e cos(m*phi), where k = 2 - alpha > 0 and 'l' and 'e'
are generalisations of the semi-latus-rectum and the eccentricity. 'm' is given
as a function of 'eccentricity'. For nearly circular orbits 'm' is sqrt{k},
while the above orbit becomes exact at the energy of escape where 'e' is one
and 'm' is 'k'. Orbits in the logarithmic potential that gives rise to a
constant circular velocity are derived via the limit of small alpha. For such
orbits, r^2 vibrates almost harmonically whatever the 'eccentricity'. Unbound
orbits in power-law potentials are given in an appendix. The transformation of
orbits in one potential to give orbits in a different potential is used to
determine orbits in potentials that are positive powers of r. These
transformations are extended to form a group which associates orbits in sets of
six potentials, e.g. there are corresponding orbits in the potentials
proportional to r, r^{-2/3}, r^{-3}, r^{-6}, r^{4/3} and r^{-4}. A degeneracy
reduces this to three, which are r^{-1}, r^2 and r^{-4} for the Keplerian case.
A generalisation of this group includes the isochrone with the Kepler set.Comment: 12 pages, 8 figures; updated version with minor typographical
corrections; published in MNRA
Forming Galaxies with MOND
Beginning with a simple model for the growth of structure, I consider the
dissipationless evolution of a MOND-dominated region in an expanding Universe
by means of a spherically symmetric N-body code. I demonstrate that the final
virialized objects resemble elliptical galaxies with well-defined relationships
between the mass, radius, and velocity dispersion. These calculations suggest
that, in the context of MOND, massive elliptical galaxies may be formed early
(z > 10) as a result of monolithic dissipationless collapse. Then I reconsider
the classic argument that a galaxy of stars results from cooling and
fragmentation of a gas cloud on a time scale shorter than that of dynamical
collapse. Qualitatively, the results are similar to that of the traditional
picture; moreover, the existence, in MOND, of a density-temperature relation
for virialized, near isothermal objects as well as a mass-temperature relation
implies that there is a definite limit to the mass of a gas cloud where this
condition can be met-- an upper limit corresponding to that of presently
observed massive galaxies.Comment: 9 pages, 9 figures, revised in response to comments of referee. Table
added, extended discussion, accepted MNRA
Analytical solutions of the lattice Boltzmann BGK model
Analytical solutions of the two dimensional triangular and square lattice
Boltzmann BGK models have been obtained for the plain Poiseuille flow and the
plain Couette flow. The analytical solutions are written in terms of the
characteristic velocity of the flow, the single relaxation time and the
lattice spacing. The analytic solutions are the exact representation of these
two flows without any approximation.Comment: 10 pages, no postscript figure provide
A Two-Temperature Model of the Intracluster Medium
We investigate evolution of the intracluster medium (ICM), considering the
relaxation process between the ions and electrons. According to the standard
scenario of structure formation, ICM is heated by the shock in the accretion
flow to the gravitational potential well of the dark halo. The shock primarily
heats the ions because the kinetic energy of an ion entering the shock is
larger than that of an electron by the ratio of masses. Then the electrons and
ions exchange the energy through coulomb collisions and reach the equilibrium.
From simple order estimation we find that the region where the electron
temperature is considerably lower than the ion temperature spreads out on a Mpc
scale. We then calculate the ion and electron temperature profiles by combining
the adiabatic model of two-temperature plasma by Fox & Loeb (1997) with
spherically symmetric N-body and hydrodynamic simulations based on three
different cosmological models. It is found that the electron temperature is
about a half of the mean temperature at radii 1 Mpc. This could lead to
an about 50 % underestimation in the total mass contained within 1 Mpc
when the electron temperature profiles are used. The polytropic indices of the
electron temperature profiles are whereas those of mean
temperature for Mpc. This result is consistent both
with the X-ray observations on electron temperature profiles and with some
theoretical and numerical predictions about mean temperature profiles.Comment: 20 pages with 6 figures. Accepted for publication in Ap
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