1,716 research outputs found
The Nature of [Ar III] Bright Knots in the Crab Nebula
The kinematic and morphological properties of a string of [Ar III] bright
knots in the Crab Nebula are examined using 1994 - 1999 HST WFPC-2 images of
the remnant. We find that five southern [Ar III] bright knots exhibit ordinary
radial motions away from the nebula's center of expansion with magnitudes
consistent with their projected radial displacements. These results do not
support the suggestion by MacAlpine et al.(1994) that these knots might be
moving rapidly away from the Crab pulsar due to a collimated wind. The HST
images also do not show that the [Ar III] knots have unusual morphologies
relative to other features in the remnant. Our proper motion results, when
combined with radial velocity estimates, suggest these knots have relatively
low space velocities implying relatively interior remnant locations thus
placing them closer to the ionizing radiation from the Crab's synchrotron
nebula. This might lead to higher knot gas temperatures thereby explaining the
knots' unusual line emission strengths as MacAlpine et al.(1994) suspected.Comment: 11 pages including three figures. Submitted to the Astronomical
Journa
Spectroscopy and 3D imaging of the Crab nebula
Spectroscopy of the Crab nebula along different slit directions reveals the 3
dimensional structure of the optical nebula. On the basis of the linear radial
expansion result first discovered by Trimble (1968), we make a 3D model of the
optical emission. Results from a limited number of slit directions suggest that
optical lines originate from a complicated array of wisps that are located in a
rather thin shell, pierced by a jet. The jet is certainly not prominent in
optical emission lines, but the direction of the piercing is consistent with
the direction of the X-ray and radio jet. The shell's effective radius is ~ 79
seconds of arc, its thickness about a third of the radius and it is moving out
with an average velocity 1160 km/s.Comment: 21 pages, 14 figures, submitted to ApJ, 3D movie of the Crab nebula
available at http://www.fiz.uni-lj.si/~vidrih
Diluted Random Fields in Mixed Cyanide Crystals
A percolation argument and a dilute compressible random field Ising model are
used to present a simple model for mixed cyanide crystals. The model reproduces
quantitatively several features of the phase diagrams altough some crude
approximations are made. In particular critical thresholds x_c at which
ferroelastic first order transitions disappear, are calculated. Moreover,
transitions are found to remain first order down to x_c for all mixtures except
for bromine, for which the transition becomes continuous. All the results are
in full agreement with experimental data.Comment: 8 pages, late
Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals
From first principles calculations we determine the Coulomb interaction
between two holes on oligo-acene and -thiophene molecules in a crystal, as a
function of the oligomer length. The relaxation of the molecular geometry in
the presence of holes is found to be small. In contrast, the electronic
polarization of the molecules that surround the charged oligomer, reduces the
bare Coulomb repulsion between the holes by approximately a factor of two. In
all cases the effective hole-hole repulsion is much larger than the calculated
valence bandwidth, which implies that at high doping levels the properties of
these organic semiconductors are determined by electron-electron correlations.Comment: 5 pages, 3 figure
Electronic properties of silica nanowires
Thin nanowires of silicon oxide were studied by pseudopotential density
functional electronic structure calculations using the generalized gradient
approximation. Infinite linear and zigzag Si-O chains were investigated. A wire
composed of three-dimensional periodically repeated Si4O8 units was also
optimized, but this structure was found to be of limited stability. The
geometry, electronic structure, and Hirshfeld charges of these silicon oxide
nanowires were computed. The results show that the Si-O chain is metallic,
whereas the zigzag chain and the Si4O8 nanowire are insulators
Temperature dependence of the electronic structure of semiconductors and insulators
The renormalization of electronic eigenenergies due to electron-phonon
coupling is sizable in many materials with light atoms. This effect, often
neglected in ab-initio calculations, can be computed using the
perturbation-based Allen-Heine-Cardona theory in the adiabatic or non-adiabatic
harmonic approximation. After a short description of the numerous recent
progresses in this field, and a brief overview of the theory, we focus on the
issue of phonon wavevector sampling convergence, until now poorly understood.
Indeed, the renormalization is obtained numerically through a q-point sampling
inside the BZ. For q-points close to G, we show that a divergence due to
non-zero Born effective charge appears in the electron-phonon matrix elements,
leading to a divergence of the integral over the BZ for band extrema. Although
it should vanish for non-polar materials, unphysical residual Born effective
charges are usually present in ab-initio calculations. Here, we propose a
solution that improves the coupled q-point convergence dramatically. For polar
materials, the problem is more severe: the divergence of the integral does not
disappear in the adiabatic harmonic approximation, but only in the
non-adiabatic harmonic approximation. In all cases, we study in detail the
convergence behavior of the renormalization as the q-point sampling goes to
infinity and the imaginary broadening parameter goes to zero. This allows
extrapolation, thus enabling a systematic way to converge the renormalization
for both polar and non-polar materials. Finally, the adiabatic and
non-adiabatic theory, with corrections for the divergence problem, are applied
to the study of five semiconductors and insulators: a-AlN, b-AlN, BN, diamond
and silicon. For these five materials, we present the zero-point
renormalization, temperature dependence, phonon-induced lifetime broadening and
the renormalized electronic bandstructure.Comment: 27 pages and 26 figure
Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins
We study implications for the apparent alignment of the spin axes,
proper-motions, and polarization vectors of the Crab and Vela pulsars. The spin
axes are deduced from recent Chandra X-ray Observatory images that reveal jets
and nebular structure having definite symmetry axes. The alignments indicate
these pulsars were born either in isolation or with negligible velocity
contributions from binary motions. We examine the effects of rotation and the
conditions under which spin-kick alignment is produced for various models of
neutron star kicks. If the kick is generated when the neutron star first forms
by asymmetric mass ejection or/and neutrino emission, then the alignment
requires that the protoneutron star possesses an original spin with period
much less than the kick timescale, thus spin-averaging the kick forces.
The kick timescale ranges from 100 ms to 10 s depending on whether the kick is
hydrodynamically driven or neutrino-magnetic field driven. For hydrodynamical
models, spin-kick alignment further requires the rotation period of an
asymmetry pattern at the radius near shock breakout (>100 km) to be much less
than ~100 ms; this is difficult to satisfy unless rotation plays a dynamically
important role in the core collapse and explosion (P_s\lo 1 ms). Aligned kick
and spin vectors are inherent to the slow process of asymmetric electromagnetic
radiation from an off-centered magnetic dipole. We reassess the viability of
this effect, correcting a factor of 4 error in Harrison and Tademaru's
calculation that increases the size of the effect. To produce a kick velocity
of order a few hundred km/s requires that the neutron star be born with an
initial spin close to 1 ms and that spindown due to r-mode driven gravitational
radiation be inefficient compared to standard magnetic braking.Comment: Small changes/additions; final version to be published in ApJ,
Vol.549 (March 10, 2001
Path integral Monte Carlo simulations of silicates
We investigate the thermal expansion of crystalline SiO in the --
cristobalite and the -quartz structure with path integral Monte Carlo
(PIMC) techniques. This simulation method allows to treat low-temperature
quantum effects properly. At temperatures below the Debye temperature, thermal
properties obtained with PIMC agree better with experimental results than those
obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press
Correlation effects in total energy of transition metals and related properties
We present an accurate implementation of total energy calculations into the
local density approximation plus dynamical mean-field theory (LDA+DMFT) method.
The electronic structure problem is solved through the full potential linear
Muffin-Tin Orbital (FP-LMTO) and Korringa-Kohn-Rostoker (FP-KKR) methods with a
perturbative solver for the effective impurity suitable for moderately
correlated systems. We have tested the method in detail for the case of Ni and
investigated the sensitivity of the results to the computational scheme and to
the complete self-consistency. It is demonstrated that the LDA+DMFT method can
resolve a long-standing controversy between the LDA/GGA density functional
approach and experiment for equilibrium lattice constant and bulk modulus of
Mn.Comment: 14 pages, 5 figure
The Distribution, Excitation and Formation of Cometary Molecules: Methanol, Methyl Cyanide and Ethylene Glycol
We present an interferometric and single dish study of small organic species
toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA
interferometer at 3 mm and the ARO 12m telescope at 2 mm. For Comet Hale-Bopp,
both the single-dish and interferometer observations of CH3OH indicate an
excitation temperature of 105+/-5 K and an average production rate ratio
Q(CH3OH)/Q(H2O)~1.3% at ~1 AU. Additionally, the aperture synthesis
observations of CH3OH suggest a distribution well described by a spherical
outflow and no evidence of significant extended emission. Single-dish
observations of CH3CN in Comet Hale-Bopp indicate an excitation temperature of
200+/-10 K and a production rate ratio of Q(CH3CN)/Q(H2O)~0.017% at ~1 AU. The
non-detection of a previously claimed transition of cometary (CH2OH)2 toward
Comet Hale-Bopp with the 12m telescope indicates a compact distribution of
emission, D<9'' (<8500 km). For the single-dish observations of Comet T7
LINEAR, we find an excitation temperature of CH3OH of 35+/-5 K and a CH3OH
production rate ratio of Q(CH3OH)/Q(H2O)~1.5% at ~0.3 AU. Our data support
current chemical models that CH3OH, CH3CN and (CH2OH)2 are parent nuclear
species distributed into the coma via direct sublimation off cometary ices from
the nucleus with no evidence of significant production in the outer coma.Comment: accepted for publication in Ap
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