89,326 research outputs found
Tidal Barrier and the Asymptotic Mass of Proto Gas-Giant Planets
Extrasolar planets found with radial velocity surveys have masses ranging
from several Earth to several Jupiter masses. While mass accretion onto
protoplanetary cores in weak-line T-Tauri disks may eventually be quenched by a
global depletion of gas, such a mechanism is unlikely to have stalled the
growth of some known planetary systems which contain relatively low-mass and
close-in planets along with more massive and longer period companions. Here, we
suggest a potential solution for this conundrum. In general, supersonic infall
of surrounding gas onto a protoplanet is only possible interior to both of its
Bondi and Roche radii. At a critical mass, a protoplanet's Bondi and Roche
radii are equal to the disk thickness. Above this mass, the protoplanets' tidal
perturbation induces the formation of a gap. Although the disk gas may continue
to diffuse into the gap, the azimuthal flux across the protoplanets' Roche lobe
is quenched. Using two different schemes, we present the results of numerical
simulations and analysis to show that the accretion rate increases rapidly with
the ratio of the protoplanet's Roche to Bondi radii or equivalently to the disk
thickness. In regions with low geometric aspect ratios, gas accretion is
quenched with relatively low protoplanetary masses. This effect is important
for determining the gas-giant planets' mass function, the distribution of their
masses within multiple planet systems around solar type stars, and for
suppressing the emergence of gas-giants around low mass stars
Blue flag with yellow tiger? Flags, authenticity and identity
The Flag of the Formosa Republic in the collection of the National Taiwan Museum is a national icon. It is a copy of one made in 1895 to mark the formation of a new Taiwanese republic; this replica, described in a contemporary newspaper account as an exact copy, was made in Japan in 1909. The painted flag was an intriguing puzzle. Instrumental analysis and a close study of the flag itself and of surviving historic photographs and records were used to try to establish whether what looked like later additions and repairs were actually part of the original construction. An international team of conservators and scientists from Taiwan, the UK, the USA and Germany carried out the investigation and the conservation treatment. Although dye analysis was inconclusive and it has not yet been possible to ascertain the original colour, it was felt that an addition in the upper right corner and some of the repairs could well be part of the original construction and these were left in situ though other repairs were removed. The paper lining was removed, revealing that the flag was painted on both sides. The fabric was cleaned using a vacuum suction table, while the paint surface was cleaned with swabs. The flag was supported using an adhesive treatment with Lascaux acrylic resin
A user's guide for the signal processing software for image and speech compression developed in the Communications and Signal Processing Laboratory (CSPL), version 1
A complete documentation of the software developed in the Communication and Signal Processing Laboratory (CSPL) during the period of July 1985 to March 1986 is provided. Utility programs and subroutines that were developed for a user-friendly image and speech processing environment are described. Additional programs for data compression of image and speech type signals are included. Also, programs for the zero-memory and block transform quantization in the presence of channel noise are described. Finally, several routines for simulating the perfromance of image compression algorithms are included
Eccentricity Evolution of Extrasolar Multiple Planetary Systems due to the Depletion of Nascent Protostellar Disks
Most extrasolar planets are observed to have eccentricities much larger than
those in the solar system. Some of these planets have sibling planets, with
comparable masses, orbiting around the same host stars. In these multiple
planetary systems, eccentricity is modulated by the planets' mutual secular
interaction as a consequence of angular momentum exchange between them. For
mature planets, the eigenfrequencies of this modulation are determined by their
mass and semi-major axis ratios. But, prior to the disk depletion, self gravity
of the planets' nascent disks dominates the precession eigenfrequencies. We
examine here the initial evolution of young planets' eccentricity due to the
apsidal libration or circulation induced by both the secular interaction
between them and the self gravity of their nascent disks. We show that as the
latter effect declines adiabatically with disk depletion, the modulation
amplitude of the planets' relative phase of periapse is approximately invariant
despite the time-asymmetrical exchange of angular momentum between planets.
However, as the young planets' orbits pass through a state of secular
resonance, their mean eccentricities undergo systematic quantitative changes.
For applications, we analyze the eccentricity evolution of planets around
Upsilon Andromedae and HD168443 during the epoch of protostellar disk
depletion. We find that the disk depletion can change the planets' eccentricity
ratio. However, the relatively large amplitude of the planets' eccentricity
cannot be excited if all the planets had small initial eccentricities.Comment: 50 pages including 11 figures, submitted to Ap
Gamma-Ray Bursts are Produced Predominately in the Early Universe
It is known that some observed gamma-ray bursts (GRBs) are produced at
cosmological distances and that the GRB production rate may follow the star
formation rate. We model the BATSE-detected intensity distribution of long GRBs
in order to determine their space density distribution and opening angle
distribution. Our main results are: the lower and upper distance limits to the
GRB production are z 0.24 and >10, respectively; the GRB opening angle follows
an exponential distribution and the mean opening angle is about 0.03 radians;
and the peak luminosity appears to be a better standard candle than the total
energy of a GRB.Comment: 12 pages, 2 figur
Hypervelocity binary stars: smoking gun of massive binary black holes
The hypervelocity stars recently found in the Galactic halo are expelled from
the Galactic center through interactions between binary stars and the central
massive black hole or between single stars and a hypothetical massive binary
black hole. In this paper, we demonstrate that binary stars can be ejected out
of the Galactic center with velocities up to 10^3 km/s, while preserving their
integrity, through interactions with a massive binary black hole. Binary stars
are unlikely to attain such high velocities via scattering by a single massive
black hole or through any other mechanisms. Based on the above theoretical
prediction, we propose a search for binary systems among the hypervelocity
stars. Discovery of hypervelocity binary stars, even one, is a definitive
evidence of the existence of a massive binary black hole in the Galactic
center.Comment: 5 pages, 3 figures, shortened version, ApJL in pres
Experimental studies on the impact properties of water ice
Experimental studies on the impact of ice particles at very low velocity were continued. These measurements have applications in the dynamics of Saturn's rings. Initially data were obtained on the coefficient of restitution for ice spheres of one radius of curvature. The type of measurements were expanded to include restitution data for balls with a variety of surfaces as well as sticking forces between ice particles. Significant improvements were made to this experiment, the most important being the construction of a new apparatus. The new apparatus consists of a smaller version of the disk pendulum and a stainless steel, double-walled cryostat. The apparatus has proved to be a significant improvement over the old one. Measurements can now be made at temperatures near 90 K, comparable to the temperature of the environment of Saturn's rings, and with much greater temperature stability. It was found that a roughened contact surface or the presence of frost can cause a much larger change in the restitution measure than the geometrical effect of the radius of curvature
Twisted and Nontwisted Bifurcations Induced by Diffusion
We discuss a diffusively perturbed predator-prey system. Freedman and
Wolkowicz showed that the corresponding ODE can have a periodic solution that
bifurcates from a homoclinic loop. When the diffusion coefficients are large,
this solution represents a stable, spatially homogeneous time-periodic solution
of the PDE. We show that when the diffusion coefficients become small, the
spatially homogeneous periodic solution becomes unstable and bifurcates into
spatially nonhomogeneous periodic solutions.
The nature of the bifurcation is determined by the twistedness of an
equilibrium/homoclinic bifurcation that occurs as the diffusion coefficients
decrease. In the nontwisted case two spatially nonhomogeneous simple periodic
solutions of equal period are generated, while in the twisted case a unique
spatially nonhomogeneous double periodic solution is generated through
period-doubling.
Key Words: Reaction-diffusion equations; predator-prey systems; homoclinic
bifurcations; periodic solutions.Comment: 42 pages in a tar.gz file. Use ``latex2e twisted.tex'' on the tex
files. Hard copy of figures available on request from
[email protected]
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