21,417 research outputs found
Proof of the Umbral Moonshine Conjecture
The Umbral Moonshine Conjectures assert that there are infinite-dimensional
graded modules, for prescribed finite groups, whose McKay-Thompson series are
certain distinguished mock modular forms. Gannon has proved this for the
special case involving the largest sporadic simple Mathieu group. Here we
establish the existence of the umbral moonshine modules in the remaining 22
cases.Comment: 56 pages, to appear in Research in the Mathematical Science
Brayton heat exchanger unit development program (alternate design)
A Brayton Heat Exchanger Unit Alternate Design (BHXU-Alternate) consisting of a recuperator, a heat sink heat exchanger, and a gas ducting system, was designed and fabricated. The design was formulated to provide a high performance unit suitable for use in a long-life Brayton-cycle powerplant. Emphasis was on double containment against external leakage and leakage of the organic coolant into the gas stream. A parametric analysis and design study was performed to establish the optimum component configurations to achieve low weight and size and high reliability, while meeting the requirements of high effectiveness and low pressure drop. Layout studies and detailed mechanical and structural design were performed to obtain a flight-type packaging arrangement, including the close-coupled integration of the BHXU-Alternate with the Brayton Rotating Unit (BRU)
Independent Evaluation of the Marie Curie Cancer Care Delivering Choice Programme in Somerset and North Somerset
Quantum Dynamics of the Slow Rollover Transition in the Linear Delta Expansion
We apply the linear delta expansion to the quantum mechanical version of the
slow rollover transition which is an important feature of inflationary models
of the early universe. The method, which goes beyond the Gaussian
approximation, gives results which stay close to the exact solution for longer
than previous methods. It provides a promising basis for extension to a full
field theoretic treatment.Comment: 12 pages, including 4 figure
Correlated radial velocity and X-ray variations in HD 154791/4U 1700+24
We present evidence for approximately 400-d variations in the radial velocity
of HD 154791 (V934 Her), the suggested optical counterpart of 4U 1700+24. The
variations are correlated with the previously reported approximately 400 d
variations in the X-ray flux of 4U 1700+24, which supports the association of
these two objects, as well as the identification of this system as the second
known X-ray binary in which a neutron star accretes from the wind of a red
giant. The HD 154791 radial velocity variations can be fit with an eccentric
orbit with period 404 +/- 3 d, amplitude K=0.75 +/- 0.12 km/s and eccentricity
e=0.26 +/- 0.15. There are also indications of variations on longer time scales
>~ 2000 d. We have re-examined all available ASM data following an unusually
large X-ray outburst in 1997-98, and confirm that the 1-d averaged 2-10 keV
X-ray flux from 4U 1700+24 is modulated with a period of 400 +/- 20 d. The mean
profile of the persistent X-ray variations was approximately sinusoidal, with
an amplitude of 0.108 +/- 0.012 ASM count/s (corresponding to 31% rms). The
epoch of X-ray maximum was approximately 40 d after the time of periastron
according to the eccentric orbital fit. If the 400 d oscillations from HD
154791/4U 1700+24 are due to orbital motion, then the system parameters are
probably close to those of the only other neutron-star symbiotic-like binary,
GX 1+4. We discuss the similarities and differences between these two systems.Comment: 6 pages, 2 figures; accepted by Ap
A Lagrangian Integrator for Planetary Accretion and Dynamics (LIPAD)
We presented the first particle based, Lagrangian code that can follow the
collisional/accretional/dynamical evolution of a large number of km-sized
planetesimals through the entire growth process to become planets. We refer to
it as the 'Lagrangian Integrator for Planetary Accretion and Dynamics' or
LIPAD. LIPAD is built on top of SyMBA, which is a symplectic -body
integrator. In order to handle the very large number of planetesimals required
by planet formation simulations, we introduce the concept of a `tracer'
particle. Each tracer is intended to represent a large number of disk particles
on roughly the same orbit and size as one another, and is characterized by
three numbers: the physical radius, the bulk density, and the total mass of the
disk particles represented by the tracer. We developed statistical algorithms
that follow the dynamical and collisional evolution of the tracers due to the
presence of one another. The tracers mainly dynamically interact with the
larger objects (`planetary embryos') in the normal N-body way. LIPAD's greatest
strength is that it can accurately model the wholesale redistribution of
planetesimals due to gravitational interaction with the embryos, which has
recently been shown to significantly affect the growth rate of planetary
embryos . We verify the code via a comprehensive set of tests which compare our
results with those of Eulerian and/or direct N-body codes.Comment: Accepted to the Astronomical Journal. See
http://www.boulder.swri.edu/~hal/LIPAD.html for more detail including
animation
Modeling the Formation of Giant Planet Cores I: Evaluating Key Processes
One of the most challenging problems we face in our understanding of planet
formation is how Jupiter and Saturn could have formed before the the solar
nebula dispersed. The most popular model of giant planet formation is the
so-called 'core accretion' model. In this model a large planetary embryo formed
first, mainly by two-body accretion. This is then followed by a period of
inflow of nebular gas directly onto the growing planet. The core accretion
model has an Achilles heel, namely the very first step. We have undertaken the
most comprehensive study of this process to date. In this study we numerically
integrate the orbits of a number of planetary embryos embedded in a swarm of
planetesimals. In these experiments we have included: 1) aerodynamic gas drag,
2) collisional damping between planetesimals, 3) enhanced embryo cross-sections
due to their atmospheres, 4) planetesimal fragmentation, and 5) planetesimal
driven migration. We find that the gravitational interaction between the
embryos and the planetesimals lead to the wholesale redistribution of material
- regions are cleared of material and gaps open near the embryos. Indeed, in
90% of our simulations without fragmentation, the region near that embryos is
cleared of planetesimals before much growth can occur. The remaining 10%,
however, the embryos undergo a burst of outward migration that significantly
increases growth. On timescales of ~100,000 years, the outer embryo can migrate
~6 AU and grow to roughly 30 Earth-masses. We also find that the inclusion of
planetesimal fragmentation tends to inhibit growth.Comment: Accepted to AJ, 62 pages 11 figure
Activities of \gamma-ray emitting isotopes in rainwater from Greater Sudbury, Canada following the Fukushima incident
We report the activity measured in rainwater samples collected in the Greater
Sudbury area of eastern Canada on 3, 16, 20, and 26 April 2011. The samples
were gamma-ray counted in a germanium detector and the isotopes 131I and 137Cs,
produced by the fission of 235U, and 134Cs, produced by neutron capture on
133Cs, were observed at elevated levels compared to a reference sample of
ice-water. These elevated activities are ascribed to the accident at the
Fukushima Dai-ichi nuclear reactor complex in Japan that followed the 11 March
earthquake and tsunami. The activity levels observed at no time presented
health concerns.Comment: 4 pages, 8 figure
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