648 research outputs found
Scattering outcomes of close-in planets: constraints on planet migration
Many exoplanets in close-in orbits are observed to have relatively high
eccentricities and large stellar obliquities. We explore the possibility that
these result from planet-planet scattering by studying the dynamical outcomes
from a large number of orbit integrations in systems with two and three
gas-giant planets in close-in orbits (0.05 AU < a < 0.15 AU). We find that at
these orbital separations, unstable systems starting with low eccentricities
and mutual inclinations (, ) generally lead to
planet-planet collisions in which the collision product is a planet on a
low-eccentricity, low-inclination orbit. This result is inconsistent with the
observations. We conclude that eccentricity and inclination excitation from
planet-planet scattering must precede migration of planets into short-period
orbits. This result constrains theories of planet migration: the semi-major
axis must shrink by 1-2 orders of magnitude without damping the eccentricity
and inclination.Comment: 11 pages, 3 figures, accepted for publication in Ap
Mid-infrared transmission properties of step index and large mode area ZnSe microstructured optical fibers
ZnSe microstructured fibers have been designed and fabricated using silica capillaries and an air-silica photonic band-gap optical fiber as high-pressure microfluidic templates for semiconductor growth via chemical fluid deposition. We examine their transmission properties over a wide spectral range
Modelling applications of photonic bandgap fibres
Photonic crystal fibres (PCFs)[1] are one of the most exciting developments in the field of photonics that has emerged in recent years. Not only have they already led to cheap all-fibre high brightness white light sources and have sparked a renaissance in the field of nonlinear optics but they also have the potential to dramatically change the next generation of telecommunication systems. PCFs can be split into two categories, the first have a solid core and guide light by modified total internal reflection, while the second photonic bandgap fibres (PBF) guide light by photonic bandgap effects and typically have a low index core compared to the cladding. Also of interest are "arrow" fibres which have a solid core and guide light due to the arrangement of high index defects in the cladding. In this paper we will be concentrating on designing and manipulating the properties of PBFs. etc..
Bipartite representations and many-body entanglement of pure states of indistinguishable particles
We analyze a general bipartite-like representation of arbitrary pure states
of -indistinguishable particles, valid for both bosons and fermions, based
on - and -particle states. It leads to exact Schmidt-like
expansions of the state for any and is directly related to the
isospectral reduced - and -body density matrices and
. The formalism also allows for reduced yet still exact
Schmidt-like decompositions associated with blocks of these densities, in
systems having a fixed fraction of the particles in some single particle
subspace. Monotonicity of the ensuing -body entanglement under a certain set
of quantum operations is also discussed. Illustrative examples in fermionic and
bosonic systems with pairing correlations are provided, which show that in the
presence of dominant eigenvalues in , approximations based on a few
terms of the pertinent Schmidt expansion can provide a reliable description of
the state. The associated one- and two-body entanglement spectrum and entropies
are also analyzed.Comment: 17 pages, 5 figure
The origin of the negative torque density in disk-satellite interaction
Tidal interaction between a gaseous disk and a massive orbiting perturber is
known to result in angular momentum exchange between them. Understanding
astrophysical manifestations of this coupling such as gap opening by planets in
protoplanetary disks or clearing of gas by binary supermassive black holes
(SMBHs) embedded in accretion disks requires knowledge of the spatial
distribution of the torque exerted on the disk by a perturber. Recent
hydrodynamical simulations by Dong et al (2011) have shown evidence for the
tidal torque density produced in a uniform disk to change sign at the radial
separation of scale heights from the perturber's orbit, in clear
conflict with the previous studies. To clarify this issue we carry out a linear
calculation of the disk-satellite interaction putting special emphasis on
understanding the behavior of the perturbed fluid variables in physical space.
Using analytical as well as numerical methods we confirm the reality of the
negative torque density phenomenon and trace its origin to the overlap of
Lindblad resonances in the vicinity of the perturber's orbit - an effect not
accounted for in previous studies. These results suggest that calculations of
the gap and cavity opening in disks by planets and binary SMBHs should rely on
more realistic torque density prescriptions than the ones used at present.Comment: 18 pages, 6 figures, accepted to Ap
Advances and limitations in the modelling of fabricated photonic bandgap fibers
Copyright © 2006 IEEEWe model fabricated silica photonic bandgap fibers and achieve good agreement between simulated and measured properties. We identify the size of the SEM bitmap image as the ultimate limit to the accurate calculation of surfaces modes within the bandgapF. Poletti, M. N. Petrovich, R. Amezcua-Correa, N. G. Broderick, T. M. Monro and D. J. Richardsonhttp://eprints.soton.ac.uk/47883
Density Waves Excited by Low-Mass Planets in Protoplanetary Disks I: Linear Regime
Density waves excited by planets embedded in protoplanetary disks play a
central role in planetary migration and gap opening processes. We carry out 2D
shearing sheet simulations to study the linear regime of wave evolution with
the grid-based code Athena, and provide detailed comparisons with the
theoretical predictions. Low mass planets (down to ~0.03 Earth mass at 1 AU)
and high spatial resolution (256 grid points per scale height) are chosen to
mitigate the effects of wave nonlinearity. To complement the existing numerical
studies, we focus on the primary physical variables such as the spatial profile
of the wave, torque density, and the angular momentum flux carried by the wave,
instead of secondary quantities such as the planetary migration rate. Our
results show percent level agreement with theory in both physical and Fourier
space. New phenomena such as the change of the toque density sign far from the
planet are discovered and discussed. Also, we explore the effect of the
numerical algorithms, and find that a high order of accuracy, high resolution,
and an accurate planetary potential are crucial to achieve good agreement with
the theory. We find that the use of a too large time-step without properly
resolving the dynamical time scale around the planet produces incorrect
results, and may lead to spurious gap opening. Global simulations of planet
migration and gap opening violating this requirement may be affected by
spurious effects resulting in e.g. the incorrect planetary migration rate and
gap opening mass.Comment: single column, 44 pages, 12 figures, ApJ in press, minor corrections
mad
Disk-satellite interaction in disks with density gaps
Gravitational coupling between a gaseous disk and an orbiting perturber leads
to angular momentum exchange between them which can result in gap opening by
planets in protoplanetary disks and clearing of gas by binary supermassive
black holes (SMBHs) embedded in accretion disks. Understanding the co-evolution
of the disk and the orbit of the perturber in these circumstances requires
knowledge of the spatial distribution of the torque exerted by the latter on a
highly nonuniform disk. Here we explore disk-satellite interaction in disks
with gaps in linear approximation both in Fourier and in physical space,
explicitly incorporating the disk non-uniformity in the fluid equations.
Density gradients strongly displace the positions of Lindblad resonances in the
disk (which often occur at multiple locations), and the waveforms of modes
excited close to the gap edge get modified compared to the uniform disk case.
The spatial distribution of the excitation torque density is found to be quite
different from the existing prescriptions: most of the torque is exerted in a
rather narrow region near the gap edge where Lindblad resonances accumulate,
followed by an exponential fall-off with the distance from the perturber.
Despite these differences, for a given gap profile the full integrated torque
exerted on the disk agrees with the conventional uniform disk theory prediction
at the level of ~10%. The nonlinearity of the density wave excited by the
perturber is shown to decrease as the wave travels out of the gap, slowing down
its nonlinear evolution and damping. Our results suggest that gap opening in
protoplanetary disks and gas clearing around SMBH binaries can be more
efficient than the existing theories predict. They pave the way for
self-consistent calculations of the gap structure and the orbital evolution of
the perturber using accurate prescription for the torque density behavior.Comment: corrected typos in reference
Anisotropic low field behavior and the observation of flux jumps in CeCoIn5
The magnetic behavior of the heavy fermion superconductor CeCoIn5 has been
investigated. The low field magnetization data show flux jumps in the mixed
state of the superconducting phase in a restricted range of temperature. These
flux jumps begin to disappear below 1.7 K, and are completely absent at 1.5 K.
The magnetization loops are asymmetric, suggesting that surface and geometrical
factors dominate the pinning in this system. The lower critical field (Hc1),
obtained from the magnetization data, shows a linear temperature dependence and
is anisotropic. The calculated penetration depth is also anisotropic, which is
consistent with the observation of an anisotropic superconducting gap in
CeCoIn5. The critical currents, determined from the high field isothermal
magnetization loops, are comparatively low (around 4000 A/cm2 at 1.6 K and 5
kOe).Comment: 4 pages 3 figure
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