5,419 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
FAST: A Fully Asynchronous Split Time-Integrator for Self-Gravitating Fluid
We describe a new algorithm for the integration of self-gravitating fluid
systems using SPH method. We split the Hamiltonian of a self-gravitating fluid
system to the gravitational potential and others (kinetic and internal
energies) and use different time-steps for their integrations. The time
integration is done in the way similar to that used in the mixed variable or
multiple stepsize symplectic schemes. We performed three test calculations. One
was the spherical collapse and the other was an explosion. We also performed a
realistic test, in which the initial model was taken from a simulation of
merging galaxies. In all test calculations, we found that the number of
time-steps for gravitational interaction were reduced by nearly an order of
magnitude when we adopted our integration method. In the case of the realistic
test, in which the dark matter potential dominates the total system, the total
calculation time was significantly reduced. Simulation results were almost the
same with those of simulations with the ordinary individual time-step method.
Our new method achieves good performance without sacrificing the accuracy of
the time integration.Comment: 14 pages, 8 figures, accepted for publication in PAS
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
The Giant Impact Simulations with Density Independent Smoothed Particle Hydrodynamics
At present, the giant impact (GI) is the most widely accepted model for the
origin of the Moon. Most of the numerical simulations of GI have been carried
out with the smoothed particle hydrodynamics (SPH) method. Recently, however,
it has been pointed out that standard formulation of SPH (SSPH) has
difficulties in the treatment of a contact discontinuity such as a core-mantle
boundary and a free surface such as a planetary surface. This difficulty comes
from the assumption of differentiability of density in SSPH. We have developed
an alternative formulation of SPH, density independent SPH (DISPH), which is
based on differentiability of pressure instead of density to solve the problem
of a contact discontinuity. In this paper, we report the results of the GI
simulations with DISPH and compare them with those obtained with SSPH. We found
that the disk properties, such as mass and angular momentum produced by DISPH
is different from that of SSPH. In general, the disks formed by DISPH are more
compact: while formation of a smaller mass moon for low-oblique impacts is
expected with DISPH, inhibition of ejection would promote formation of a larger
mass moon for high-oblique impacts. Since only the improvement of core-mantle
boundary significantly affects the properties of circumplanetary disks
generated by GI and DISPH has not been significantly improved from SSPH for a
free surface, we should be very careful when some conclusions are drawn from
the numerical simulations for GI. And it is necessary to develop the numerical
hydrodynamical scheme for GI that can properly treat the free surface as well
as the contact discontinuity.Comment: Accepted for publication in Icaru
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
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