4,336 research outputs found
Modeling radiation belt radial diffusion in ULF wave fields: 1. Quantifying ULF wave power at geosynchronous orbit in observations and in global MHD model
[1] To provide critical ULF wave field information for radial diffusion studies in the radiation belts, we quantify ULF wave power (f = 0.5â8.3 mHz) in GOES observations and magnetic field predictions from a global magnetospheric model. A statistical study of 9 years of GOES data reveals the wave local time distribution and power at geosynchronous orbit in field-aligned coordinates as functions of wave frequency, solar wind conditions (Vx, ÎPd and IMF Bz) and geomagnetic activity levels (Kp, Dst and AE). ULF wave power grows monotonically with increasing solar wind Vx, dynamic pressure variations ÎPd and geomagnetic indices in a highly correlated way. During intervals of northward and southward IMF Bz, wave activity concentrates on the dayside and nightside sectors, respectively, due to different wave generation mechanisms in primarily open and closed magnetospheric configurations. Since global magnetospheric models have recently been used to trace particles in radiation belt studies, it is important to quantify the wave predictions of these models at frequencies relevant to electron dynamics (mHz range). Using 27 days of real interplanetary conditions as model inputs, we examine the ULF wave predictions modeled by the Lyon-Fedder-Mobarry magnetohydrodynamic code. The LFM code does well at reproducing, in a statistical sense, the ULF waves observed by GOES. This suggests that the LFM code is capable of modeling variability in the magnetosphere on ULF time scales during typical conditions. The code provides a long-missing wave field model needed to quantify the interaction of radiation belt electrons with realistic, global ULF waves throughout the inner magnetosphere
Is the Convergence of Accounting Standards Good for Stock Markets?
This paper examines the impact of the convergence of Hong Kong Accounting Standard 40 (HKAS 40) with the International Financial Reporting Standard (IFRS) on the stock prices of firms in the property industry. Using a sample of 22111 firm-day observations, we show that the new standard has a negative impact on the stock performance of these firms.Hong Kong Accounting Standard 40, Event Window, Stock Return.
Dynamic concentration of motors in microtubule arrays
We present experimental and theoretical studies of the dynamics of molecular
motors in microtubule arrays and asters. By solving a convection-diffusion
equation we find that the density profile of motors in a two-dimensional aster
is characterized by continuously varying exponents. Simulations are used to
verify the assumptions of the continuum model. We observe the concentration
profiles of kinesin moving in quasi two-dimensional artificial asters by
fluorescent microscopy and compare with our theoretical results.Comment: 4pages, 4 figures revte
HAT-P-57b: A Short-Period Giant Planet Transiting A Bright Rapidly Rotating A8V Star Confirmed Via Doppler Tomography
We present the discovery of HAT-P-57b, a P = 2.4653 day transiting planet
around a V = 10.465 +- 0.029 mag, Teff = 7500 +- 250 K main sequence A8V star
with a projected rotation velocity of v sin i = 102.1 +- 1.3 km s^-1. We
measure the radius of the planet to be R = 1.413 +- 0.054 R_J and, based on RV
observations, place a 95% confidence upper limit on its mass of M < 1.85 M_J .
Based on theoretical stellar evolution models, the host star has a mass and
radius of 1.47 +- 0.12 M_sun, and 1.500 +- 0.050 R_sun, respectively.
Spectroscopic observations made with Keck-I/HIRES during a partial transit
event show the Doppler shadow of HAT-P-57b moving across the average spectral
line profile of HAT-P- 57, confirming the object as a planetary system. We use
these observations, together with analytic formulae that we derive for the line
profile distortions, to determine the projected angle between the spin axis of
HAT-P-57 and the orbital axis of HAT-P-57b. The data permit two possible
solutions, with -16.7 deg < lambda < 3.3 deg or 27.6 deg < lambda < 57.4 deg at
95% confidence, and with relative probabilities for the two modes of 26% and
74%, respectively. Adaptive optics imaging with MMT/Clio2 reveals an object
located 2.7" from HAT-P-57 consisting of two point sources separated in turn
from each other by 0.22". The H and L -band magnitudes of the companion stars
are consistent with their being physically associated with HAT-P-57, in which
case they are stars of mass 0.61 +- 0.10 M_sun and 0.53 +- 0.08 M_sun. HAT-P-57
is the most rapidly rotating star, and only the fourth main sequence A star,
known to host a transiting planet.Comment: 18 pages, 14 figures, 5 tables, accepted for publication in A
Bayesian optimization for materials design
We introduce Bayesian optimization, a technique developed for optimizing
time-consuming engineering simulations and for fitting machine learning models
on large datasets. Bayesian optimization guides the choice of experiments
during materials design and discovery to find good material designs in as few
experiments as possible. We focus on the case when materials designs are
parameterized by a low-dimensional vector. Bayesian optimization is built on a
statistical technique called Gaussian process regression, which allows
predicting the performance of a new design based on previously tested designs.
After providing a detailed introduction to Gaussian process regression, we
introduce two Bayesian optimization methods: expected improvement, for design
problems with noise-free evaluations; and the knowledge-gradient method, which
generalizes expected improvement and may be used in design problems with noisy
evaluations. Both methods are derived using a value-of-information analysis,
and enjoy one-step Bayes-optimality
Method to compute the stress-energy tensor for the massless spin 1/2 field in a general static spherically symmetric spacetime
A method for computing the stress-energy tensor for the quantized, massless,
spin 1/2 field in a general static spherically symmetric spacetime is
presented. The field can be in a zero temperature state or a non-zero
temperature thermal state. An expression for the full renormalized
stress-energy tensor is derived. It consists of a sum of two tensors both of
which are conserved. One tensor is written in terms of the modes of the
quantized field and has zero trace. In most cases it must be computed
numerically. The other tensor does not explicitly depend on the modes and has a
trace equal to the trace anomaly. It can be used as an analytic approximation
for the stress-energy tensor and is equivalent to other approximations that
have been made for the stress-energy tensor of the massless spin 1/2 field in
static spherically symmetric spacetimes.Comment: 34 pages, no figure
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A 2 R_â Planet Orbiting the Bright Nearby K Dwarf Wolf 503
Since its launch in 2009, the Kepler telescope has found thousands of planets with radii between that of Earth and Neptune. Recent studies of the distribution of these planets have revealed a gap in the population near 1.5â2.0 Râ, informally dividing these planets into "super-Earths" and "sub-Neptunes." The origin of this division is difficult to investigate directly because the majority of planets found by Kepler orbit distant, dim stars and are not amenable to radial velocity follow-up or transit spectroscopy, making bulk density and atmospheric measurements difficult. Here, we present the discovery and validation of a newly found 2.03^(+0.08)_(-0.07) Râ planet in direct proximity to the radius gap, orbiting the bright (J = 8.32 mag), nearby (D = 44.5 pc) high proper motion K3.5V star Wolf 503 (EPIC 212779563). We determine the possibility of a companion star and false positive detection to be extremely low using both archival images and high-contrast adaptive optics images from the Palomar observatory. The brightness of the host star makes Wolf 503b a prime target for prompt radial velocity follow-up, and with the small stellar radius (0.690 ± 0.025Râ), it is also an excellent target for HST transit spectroscopy and detailed atmospheric characterization with JWST. With its measured radius near the gap in the planet radius and occurrence rate distribution, Wolf 503b offers a key opportunity to better understand the origin of this radius gap as well as the nature of the intriguing populations of "super-Earths" and "sub-Neptunes" as a whole
Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization
Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.
A 2 Earth Radius Planet Orbiting the Bright Nearby K-Dwarf Wolf 503
Since its launch in 2009, the Kepler telescope has found thousands of planets
with radii between that of Earth and Neptune. Recent studies of the
distribution of these planets have revealed a rift in the population near
1.5-2.0, informally dividing these planets into "super-Earths"
and "sub-Neptunes". The origin of this division is not well understood, largely
because the majority of planets found by Kepler orbit distant, dim stars and
are not amenable to radial velocity follow-up or transit spectroscopy, making
bulk density and atmospheric measurements difficult. Here, we present the
discovery and validation of a newly found
planet in direct proximity to the radius gap, orbiting the bright
(~mag), nearby (~pc) high proper motion star Wolf 503 (EPIC
212779563). We classify Wolf 503 as a K3.5V star and member of the thick disc
population. We determine the possibility of a companion star and false positive
detection to be extremely low using both archival images and high-contrast
adaptive optics images from the Palomar observatory. The brightness of the host
star makes Wolf 503b a prime target for prompt radial velocity follow-up, HST
transit spectroscopy, as well as detailed atmospheric characterization with
JWST. With its measured radius near the gap in the planet radius and occurrence
rate distribution, Wolf 503b offers a key opportunity to better understand the
origin of this radius gap as well as the nature of the intriguing populations
of "super-Earths" and "sub-Neptunes" as a whole
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