36,807 research outputs found
Development of a platinum-thorium oxide alloy for resistojet thruster use
Platinum-thorium oxide alloy for resistojet thruster showing increase in stress rupture lif
Guidance algorithms for a free-flying space robot
Robotics is a promising technology for assembly, servicing, and maintenance of platforms in space. Several aspects of planning and guidance for telesupervised and fully autonomous robotic servicers are investigated. Guidance algorithms for proximity operation of a free flyer are described. Numeric trajectory optimization is combined with artificial intelligence based obstacle avoidance. An initial algorithm and the results of its simulating platform servicing scenario are discussed. A second algorithm experiment is then proposed
Initial state preparation with dynamically generated system-environment correlations
The dependence of the dynamics of open quantum systems upon initial
correlations between the system and environment is an utterly important yet
poorly understood subject. For technical convenience most prior studies assume
factorizable initial states where the system and its environments are
uncorrelated, but these conditions are not very realistic and give rise to
peculiar behaviors. One distinct feature is the rapid build up or a sudden jolt
of physical quantities immediately after the system is brought in contact with
its environments. The ultimate cause of this is an initial imbalance between
system-environment correlations and coupling. In this note we demonstrate
explicitly how to avoid these unphysical behaviors by proper adjustments of
correlations and/or the coupling, for setups of both theoretical and
experimental interest. We provide simple analytical results in terms of
quantities that appear in linear (as opposed to affine) master equations
derived for factorized initial states.Comment: 6 pages, 2 figure
Validity of the Adiabatic Approximation
We analyze the validity of the adiabatic approximation, and in particular the
reliability of what has been called the "standard criterion" for validity of
this approximation. Recently, this criterion has been found to be insufficient.
We will argue that the criterion is sufficient only when it agrees with the
intuitive notion of slowness of evolution of the Hamiltonian. However, it can
be insufficient in cases where the Hamiltonian varies rapidly but only by a
small amount. We also emphasize the distinction between the adiabatic {\em
theorem} and the adiabatic {\em approximation}, two quite different although
closely related ideas.Comment: 4 pages, 1 figur
Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions
Accurate models of alkali and halide ions in aqueous solution are necessary
for computer simulations of a broad variety of systems. Previous efforts to
develop ion force fields have generally focused on reproducing experimental
measurements of aqueous solution properties such as hydration free energies and
ion-water distribution functions. This dependency limits transferability of the
resulting parameters because of the variety and known limitations of water
models. We present a solvent-independent approach to calibrating ion parameters
based exclusively on crystal lattice properties. Our procedure relies on
minimization of lattice sums to calculate lattice energies and interionic
distances instead of equilibrium ensemble simulations of dense fluids. The gain
in computational efficiency enables simultaneous optimization of all parameters
for Li+, Na+, K+, Rb+, Cs+, F-, Cl-, Br-, and I- subject to constraints that
enforce consistency with periodic table trends. We demonstrate the method by
presenting lattice-derived parameters for the primitive model and the
Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting
parameters successfully reproduce the lattice properties used to derive them
and are free from the influence of any water model. To assess the
transferability of the Lennard-Jones parameters to aqueous systems, we used
them to estimate hydration free energies and found that the results were in
quantitative agreement with experimentally measured values. These
lattice-derived parameters are applicable in simulations where coupling of ion
parameters to a particular solvent model is undesirable. The simplicity and low
computational demands of the calibration procedure make it suitable for
parametrization of crystallizable ions in a variety of force fields.Comment: 9 pages, 5 table
Investigating the nature of the Fried Egg nebula: CO mm-line and optical spectroscopy of IRAS 17163-3907
Through CO mm-line and optical spectroscopy, we investigate the properties of
the Fried Egg nebula IRAS 17163-3907, which has recently been proposed to be
one of the rare members of the yellow hypergiant class. The CO J=2-1 and J=3-2
emission arises from a region within 20" of the star and is clearly associated
with the circumstellar material. The CO lines show a multi-component
asymmetrical profile, and an unexpected velocity gradient is resolved in the
east-west direction, suggesting a bipolar outflow. This is in contrast with the
apparent symmetry of the dust envelope as observed in the infrared. The optical
spectrum of IRAS 17163-3907 between 5100 and 9000 {\AA} was compared with that
of the archetypal yellow hypergiant IRC+10420 and was found to be very similar.
These results build on previous evidence that IRAS 17163-3907 is a yellow
hypergiant.Comment: 14 pages including appendix, accepted for publication in A&
Growth rates for subclasses of Av(321)
Pattern classes which avoid 321 and other patterns are shown to have the same growth rates as similar (but strictly larger) classes obtained by adding articulation points to any or all of the other patterns. The method of proof is to show that the elements of the latter classes can be represented as bounded merges of elements of the original class, and that the bounded merge construction does not change growth rates
The low-mass population of the Rho Ophiuchi molecular cloud
Star formation theories are currently divergent regarding the fundamental
physical processes that dominate the substellar regime. Observations of nearby
young open clusters allow the brown dwarf (BD) population to be characterised
down to the planetary mass regime, which ultimately must be accommodated by a
successful theory. We hope to uncover the low-mass population of the Rho
Ophiuchi molecular cloud and investigate the properties of the newly found
brown dwarfs. We use near-IR deep images (reaching completeness limits of
approximately 20.5 mag in J, and 18.9 mag in H and Ks) taken with the Wide
Field IR Camera (WIRCam) at the Canada France Hawaii Telescope (CFHT) to
identify candidate members of Rho Oph in the substellar regime. A spectroscopic
follow-up of a small sample of the candidates allows us to assess their
spectral type, and subsequently their temperature and membership. We select 110
candidate members of the Rho Ophiuchi molecular cloud, from which 80 have not
previously been associated with the cloud. We observed a small sample of these
and spectroscopically confirm six new brown dwarfs with spectral types ranging
from M6.5 to M8.25
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