885 research outputs found
Synthesis, vacuum ultraviolet and near ultraviolet-excited luminescent properties of GdCaAl3O7: RE3+ (RE=Eu, Tb)
Vacuum ultraviolet (VUV) excitation and photoluminescent (PL) properties of Eu3+ and Tb3+ ion-doped aluminate phosphors, GdCaAl 3O7:Eu3+ and GdCaAl3O 7:Tb3+ have been investigated. X-ray diffraction (XRD) patterns indicate that the phosphor GdCaAl3O7 forms without impurity phase at 900 °C. Field emission scanning electron microscopy (FE-SEM) images show that the particle size of the phosphor is less than 3 μm. Upon excitation with VUV irradiation, the phosphors show a strong emission at around 619 nm corresponding to the forced electric dipole 5D0→7F2 transition of Eu 3+, and at around 545 nm corresponding to the 5D 4→7F5 transition of Tb3+. The results reveal that both GdCaAl3O7:RE3+ (RE=Eu, Tb) are potential candidates as red and green phosphors, respectively, for use in plasma display panel (PDP). © 2005 Elsevier Inc. All rights reserved.postprin
General Relativistic Simulations of Slowly and Differentially Rotating Magnetized Neutron Stars
We present long-term (~10^4 M) axisymmetric simulations of differentially
rotating, magnetized neutron stars in the slow-rotation, weak magnetic field
limit using a perturbative metric evolution technique. Although this approach
yields results comparable to those obtained via nonperturbative (BSSN)
evolution techniques, simulations performed with the perturbative metric solver
require about 1/4 the computational resources at a given resolution. This
computational efficiency enables us to observe and analyze the effects of
magnetic braking and the magnetorotational instability (MRI) at very high
resolution. Our simulations demonstrate that (1) MRI is not observed unless the
fastest-growing mode wavelength is resolved by more than about 10 gridpoints;
(2) as resolution is improved, the MRI growth rate converges, but due to the
small-scale turbulent nature of MRI, the maximum growth amplitude increases,
but does not exhibit convergence, even at the highest resolution; and (3)
independent of resolution, magnetic braking drives the star toward uniform
rotation as energy is sapped from differential rotation by winding magnetic
fields.Comment: 21 pages, 11 figures, published in Phys.Rev.
{BOAO Photometric Survey of Galactic Open Clusters. II. Physical Parameters of 12 Open Clusters
We have initiated a long-term project, the BOAO photometric survey of open
clusters, to enlarge our understanding of galactic structure using UBVI CCD
photometry of open clusters which have been little studied before. This is the
second paper of the project in which we present the photometry of 12 open
clusters. We have determined the cluster parameters by fitting the Padova
isochrones to the color-magnitude diagrams of the clusters. All the clusters
except for Be 0 and NGC 1348 are found to be intermediate-age to old (0.2 - 4.0
Gyrs) open clusters with a mean metallicity of [Fe/H] = 0.0.Comment: 11 page
Evolution of magnetized, differentially rotating neutron stars: Simulations in full general relativity
We study the effects of magnetic fields on the evolution of differentially
rotating neutron stars, which can form in stellar core collapse or binary
neutron star coalescence. Magnetic braking and the magnetorotational
instability (MRI) both redistribute angular momentum; the outcome of the
evolution depends on the star's mass and spin. Simulations are carried out in
axisymmetry using our recently developed codes which integrate the coupled
Einstein-Maxwell-MHD equations. For initial data, we consider three categories
of differentially rotating, equilibrium configurations, which we label normal,
hypermassive and ultraspinning. Hypermassive stars have rest masses exceeding
the mass limit for uniform rotation. Ultraspinning stars are not hypermassive,
but have angular momentum exceeding the maximum for uniform rotation at the
same rest mass. We show that a normal star will evolve to a uniformly rotating
equilibrium configuration. An ultraspinning star evolves to an equilibrium
state consisting of a nearly uniformly rotating central core, surrounded by a
differentially rotating torus with constant angular velocity along magnetic
field lines, so that differential rotation ceases to wind the magnetic field.
In addition, the final state is stable against the MRI, although it has
differential rotation. For a hypermassive neutron star, the MHD-driven angular
momentum transport leads to catastrophic collapse of the core. The resulting
rotating black hole is surrounded by a hot, massive, magnetized torus
undergoing quasistationary accretion, and a magnetic field collimated along the
spin axis--a promising candidate for the central engine of a short gamma-ray
burst. (Abridged)Comment: 27 pages, 30 figure
Temporal and Spatial Variability of Precipitation from Observations and Models
Principal component analysis (PCA) is utilized to explore the temporal and spatial variability of precipitation from GPCP and a CAM5 simulation from 1979 to 2010. In the tropical region, the interannual variability of tropical precipitation is characterized by two dominant modes (El Niño and El Niño Modoki). The first and second modes of tropical GPCP precipitation capture 31.9% and 15.6% of the total variance, respectively. The first mode has positive precipitation anomalies over the western Pacific and negative precipitation anomalies over the central and eastern Pacific. The second mode has positive precipitation anomalies over the central Pacific and negative precipitation anomalies over the western and eastern Pacific. Similar variations are seen in the first two modes of tropical precipitation from a CAM5 simulation, although the magnitudes are slightly weaker than in the observations. Over the Northern Hemisphere (NH) high latitudes, the first mode, capturing 8.3% of the total variance of NH GPCP precipitation, is related to the northern annular mode (NAM). During the positive phase of NAM, there are negative precipitation anomalies over the Arctic and positive precipitation anomalies over the midlatitudes. Over the Southern Hemisphere (SH) high latitudes, the first mode, capturing 13.2% of the total variance of SH GPCP precipitation, is related to the southern annular mode (SAM). During the positive phase of the SAM, there are negative precipitation anomalies over the Antarctic and positive precipitation anomalies over the midlatitudes. The CAM5 precipitation simulation demonstrates similar results to those of the observations. However, they do not capture both the high precipitation anomalies over the northern Pacific Ocean or the position of the positive precipitation anomalies in the SH
Magnetorotational collapse of massive stellar cores to neutron stars: Simulations in full general relativity
We study magnetohydrodynamic (MHD) effects arising in the collapse of
magnetized, rotating, massive stellar cores to proto-neutron stars (PNSs). We
perform axisymmetric numerical simulations in full general relativity with a
hybrid equation of state. The formation and early evolution of a PNS are
followed with a grid of 2500 x 2500 zones, which provides better resolution
than in previous (Newtonian) studies. We confirm that significant differential
rotation results even when the rotation of the progenitor is initially uniform.
Consequently, the magnetic field is amplified both by magnetic winding and the
magnetorotational instability (MRI). Even if the magnetic energy E_EM is much
smaller than the rotational kinetic energy T_rot at the time of PNS formation,
the ratio E_EM/T_rot increases to 0.1-0.2 by the magnetic winding. Following
PNS formation, MHD outflows lead to losses of rest mass, energy, and angular
momentum from the system. The earliest outflow is produced primarily by the
increasing magnetic stress caused by magnetic winding. The MRI amplifies the
poloidal field and increases the magnetic stress, causing further angular
momentum transport and helping to drive the outflow. After the magnetic field
saturates, a nearly stationary, collimated magnetic field forms near the
rotation axis and a Blandford-Payne type outflow develops along the field
lines. These outflows remove angular momentum from the PNS at a rate given by
\dot{J} \sim \eta E_EM C_B, where \eta is a constant of order 0.1 and C_B is a
typical ratio of poloidal to toroidal field strength. As a result, the rotation
period quickly increases for a strongly magnetized PNS until the degree of
differential rotation decreases. Our simulations suggest that rapidly rotating,
magnetized PNSs may not give rise to rapidly rotating neutron stars.Comment: 28 pages, 20 figures, accepted for publication in Phys. Rev.
Magnetorotational collapse of very massive stars to black holes in full general relativity
We perform axisymmetric simulations of the magnetorotational collapse of very
massive stars in full general relativity. Our simulations are applicable to the
collapse of supermassive stars (M > 10^3M_sun) and to very massive Pop III
stars. We model our initial configurations by n=3 polytropes. The ratio of
magnetic to rotational kinetic energy in these configurations is chosen to be
small (1% and 10%). We find that such magnetic fields do not affect the initial
collapse significantly. The core collapses to a black hole, after which black
hole excision is employed to continue the evolution long enough for the hole to
reach a quasi-stationary state. We find that the black hole mass is M_h = 0.95M
and its spin parameter is J_h/M_h^2 = 0.7, with the remaining matter forming a
torus around the black hole. We freeze the spacetime metric ("Cowling
approximation") and continue to follow the evolution of the torus after the
black hole has relaxed to quasi-stationary equilibrium. In the absence of
magnetic fields, the torus settles down following ejection of a small amount of
matter due to shock heating. When magnetic fields are present, the field lines
gradually collimate along the hole's rotation axis. MHD shocks and the MRI
generate MHD turbulence in the torus and stochastic accretion onto the central
black hole. When the magnetic field is strong, a wind is generated in the
torus, and the torus undergoes radial oscillations that drive episodic
accretion onto the hole. These oscillations produce long-wavelength
gravitational waves potentially detectable by LISA. The final state of the
magnetorotational collapse always consists of a central black hole surrounded
by a collimated magnetic field and a hot, thick accretion torus. This system is
a viable candidate for the central engine of a long-soft gamma-ray burst.Comment: 17 pages, 13 figures, replaced with the published versio
Mars Aeronomy Observer: Report of the Science Working Team
The Mars Aeronomy Observer (MAO) is a candidate follow-on mission to Mars Observer (MO) in the Planetary Observer Program. The four Mariner and two Viking spacecraft sent to Mars between 1965 and 1976 have provided a wealth of information concerning Martian planetology. The Mars Observer, to be launched in 1990, will build on their results by further examining the elemental and mineralogical composition of the surface, the strength and multipolar composition of the planetary magnetic field, the gravitational field and topography, and the circulation of the lower atmosphere. The Mars Aeronomy Observer is intended to address the last major aspects of Martian environment which have yet to be investigated: the upper atmosphere, the ionsphere, and the solar wind interaction region
The Spectroscopy of Plasma Evolution from Astrophysical Radiation Mission
The Spectroscopy of Plasma Evolution from Astrophysical Radiation (or the
Far-ultraviolet Imaging Spectrograph) instruments, flown aboard the STSAT-1
satellite mission, have provided the first large-area spectral mapping of the
cosmic far ultraviolet (FUV, lambda 900-1750 Ang) background. We observe
diffuse radiation from hot (10^4 to 10^6 K) and ionized plasmas, molecular
hydrogen, and dust scattered starlight. These data provide for the
unprecedented detection and discovery of spectral emission from a variety of
interstellar environments, including the general medium, molecular clouds,
supernova remnants, and super-bubbles. We describe the mission and its data,
present an overview of the diffuse FUV sky's appearance and spectrum, and
introduce the scientific findings detailed later in this volume
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