5,906 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
Pseudoparticle Multipole Method: A Simple Method to Implement High-Accuracy Treecode
In this letter we describe the pseudoparticle multipole method (P2M2), a new
method to express multipole expansion by a distribution of pseudoparticles. We
can use this distribution of particles to calculate high order terms in both
the Barnes-Hut treecode and FMM. The primary advantage of P2M2 is that it works
on GRAPE. GRAPE is a special-purpose hardware for the calculation of
gravitational force between particles. Although the treecode has been
implemented on GRAPE, we could handle terms only up to dipole, since GRAPE can
calculate forces from point-mass particles only. Thus the calculation cost
grows quickly when high accuracy is required. With P2M2, the multipole
expansion is expressed by particles, and thus GRAPE can calculate high order
terms. Using P2M2, we implemented an arbitrary-order treecode on GRAPE-4.
Timing result shows GRAPE-4 accelerates the calculation by a factor between 10
(for low accuracy) to 150 (for high accuracy). Even on general-purpose
programmable computers, our method offers the advantage that the mathematical
formulae and therefore the actual program is much simpler than that of the
direct implementation of multipole expansion.Comment: 6 pages, 4 figures, latex, submitted to ApJ Letter
Evolution of Clusters of Galaxies: Mass Stripping from Galaxies and Growth of Common Halos
We investigated the evolution of clusters of galaxies using self-consistent
-body simulations in which each galaxy was modeled by many particles. We
carried out simulations for about 20 cases using different initial conditions.
In all simulations, clusters were initially in virial equilibrium. We found
that more than half of the total mass escaped from individual galaxies within a
few crossing times of the cluster, and that a diffuse halo was formed. The
growth rate of the common halo depended on the size of individual galaxies only
weakly. The stripping of the mass from galaxies was mainly due to the
interaction of galaxies, not due to the effect of the tidal field of the
cluster potential. The amount of stripped mass was larger for galaxies in the
central region than for those in the outer region, since the interactions were
more frequent in the central region. As a result, a positive correlation
between the distance from the center and the mass of the galaxy developed. The
volume-density profile of the common halo is expressed as
in the central region. This mass distribution is consistent with the mass
distribution in clusters estimated using X-ray observations.Comment: 12 pages with 12 figures; accepted for publication in PAS
Analysis on reflection spectra in strained ZnO thin films
Thin films of laser molecular-beam epitaxy grown ZnO films were studied with
respect to their optical properties. 4-K reflectivity was used to analyze
various samples grown at different biaxial in-plane strain. The spectra show
two structures at 3.37 eV corresponding to the A-free exciton transition and at
3.38 eV corresponding to the B-free exciton transition. Theoretical
reflectivity spectra were calculated using the spatial dispersion model. Thus,
the transverse energies, the longitudinal transversal splitting (ELT,), the
oscillator strengths, and the damping parameters were determined for both the
A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the
energy E_LT for the A-excitons is characterized by a negatively-peaking
behavior with a minimum around the zero strain, while ELT for the B-excitons is
an increasing function of the strain field values.Comment: 4 pages, 2 figures, 1 table, conference: ICMAT2005 (Singapore), to
appear in an issue of J. Cryst. Growt
Spherically symmetric equilibria for self-gravitating kinetic or fluid models in the non-relativistic and relativistic case - A simple proof for finite extension
We consider a self-gravitating collisionless gas as described by the
Vlasov-Poisson or Einstein-Vlasov system or a self-gravitating fluid ball as
described by the Euler-Poisson or Einstein-Euler system. We give a simple proof
for the finite extension of spherically symmetric equilibria, which covers all
these models simultaneously. In the Vlasov case the equilibria are
characterized by a local growth condition on the microscopic equation of state,
i.e., on the dependence of the particle distribution on the particle energy, at
the cut-off energy E_0, and in the Euler case by the corresponding growth
condition on the equation of state p=P(\rho) at \rho=0. These purely local
conditions are slight generalizations to known such conditions.Comment: 20 page
Binary Black Hole Mergers from Planet-like Migrations
If supermassive black holes (BHs) are generically present in galaxy centers,
and if galaxies are built up through hierarchical merging, BH binaries are at
least temporary features of most galactic bulges. Observations suggest,
however, that binary BHs are rare, pointing towards a binary lifetime far
shorter than the Hubble time. We show that, regardless of the detailed
mechanism, all stellar-dynamical processes are insufficient to reduce
significantly the orbital separation once orbital velocities in the binary
exceed the virial velocity of the system. We propose that a massive gas disk
surrounding a BH binary can effect its merger rapidly, in a scenario analogous
to the orbital decay of super-jovian planets due to a proto-planetary disk. As
in the case of planets, gas accretion onto the secondary (here a supermassive
BH) is integrally connected with its inward migration. Such accretion would
give rise to quasar activity. BH binary mergers could therefore be responsible
for many or most quasars.Comment: 8 pages, submitted to ApJ Letter
The dynamics of spiral arms in pure stellar disks
It has been believed that spirals in pure stellar disks, especially the ones
spontaneously formed, decay in several galactic rotations due to the increase
of stellar velocity dispersions. Therefore, some cooling mechanism, for example
dissipational effects of the interstellar medium, was assumed to be necessary
to keep the spiral arms. Here we show that stellar disks can maintain spiral
features for several tens of rotations without the help of cooling, using a
series of high-resolution three-dimensional -body simulations of pure
stellar disks. We found that if the number of particles is sufficiently large,
e.g., , multi-arm spirals developed in an isolated disk can
survive for more than 10 Gyrs. We confirmed that there is a self-regulating
mechanism that maintains the amplitude of the spiral arms. Spiral arms increase
Toomre's of the disk, and the heating rate correlates with the squared
amplitude of the spirals. Since the amplitude itself is limited by the value of
, this makes the dynamical heating less effective in the later phase of
evolution. A simple analytical argument suggests that the heating is caused by
gravitational scattering of stars by spiral arms, and that the self-regulating
mechanism in pure-stellar disks can effectively maintain spiral arms on a
cosmological timescale. In the case of a smaller number of particles, e.g.,
, spiral arms grow faster in the beginning of the simulation
(while is small) and they cause a rapid increase of . As a result, the
spiral arms become faint in several Gyrs.Comment: 18 pages, 19 figures, accepted for Ap
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