1,660,278 research outputs found
Indication of Anisotropy in Electromagnetic Propagation over Cosmological Distances
We report a systematic rotation of the plane of polarization of
electromagnetic radiation propagating over cosmological distances. The effect
is extracted independently from Faraday rotation, and found to be correlated
with the angular positions and distances to the sources. Monte Carlo analysis
yields probabilistic P-values of order 10^(-3) for this to occur as a
fluctuation. A fit yields a birefringence scale of order 10^(25) meters.
Dependence on redshift z rules out a local effect. Barring hidden systematic
bias in the data, the correlation indicates a new cosmological effect.Comment: 5 pages, 1 figure, ReVTeX. For more information, see
http://www.cc.rochester.edu/college/rtc/Borge/aniso.htm
Imaginary Chemical Potential Approach for the Pseudo-Critical Line in the QCD Phase Diagram with Clover-Improved Wilson Fermions
The QCD phase diagram is studied in the lattice QCD simulation with the
imaginary chemical potential approach. We employ a clover-improved Wilson
fermion action of two-flavors and a renormalization-group improved gauge
action, and perform the simulation at an intermediate quark mass on a
lattice. The QCD phase diagram in the imaginary chemical
potential region is investigated by performing the simulation for more
than 150 points on the plane. We find that the Roberge-Weiss
phase transition at is first order and its endpoint is second
order, which are identified by the phase of the Polyakov loop. We determine the
pseudo-critical line from the susceptibility of the Polyakov loop modulus. We
find a clear deviation from a linear dependence of the pseudo-critical line on
.Comment: 10 pages, 20 figures, 3 tables. Revtex4. References are added and,
discussions are sharpene
Chiral corrections to the Gell-Mann-Oakes-Renner relation
The next to leading order chiral corrections to the
Gell-Mann-Oakes-Renner (GMOR) relation are obtained using the pseudoscalar
correlator to five-loop order in perturbative QCD, together with new finite
energy sum rules (FESR) incorporating polynomial, Legendre type, integration
kernels. The purpose of these kernels is to suppress hadronic contributions in
the region where they are least known. This reduces considerably the systematic
uncertainties arising from the lack of direct experimental information on the
hadronic resonance spectral function. Three different methods are used to
compute the FESR contour integral in the complex energy (squared) s-plane, i.e.
Fixed Order Perturbation Theory, Contour Improved Perturbation Theory, and a
fixed renormalization scale scheme. We obtain for the corrections to the GMOR
relation, , the value . This result
is substantially more accurate than previous determinations based on QCD sum
rules; it is also more reliable as it is basically free of systematic
uncertainties. It implies a light quark condensate . As a byproduct, the chiral perturbation theory (unphysical) low energy
constant is predicted to be , or .Comment: A comment about the value of the strong coupling has been added at
the end of Section 4. No change in results or conslusion
Electrical characterization of amorphous LiAlO2 thin films deposited by atomic layer deposition
LiAlO2 thin films deposited by atomic layer deposition (ALD) have a potential application as an electrolyte in three-dimensional (3D) all-solid-state microbatteries. In this study, Li-ion conductivity of such films is investigated by both in-plane and cross-plane methods. LiAlO2 thin films with a Li composition of [Li]/([Li] + [Al]) = 0.46 and an amorphous structure were grown by ALD with thicknesses of 90, 160 and 235 nm on different substrates. The electrical characterization was conducted by impedance spectroscopy using inert electrodes over a temperature range of 25-200 degrees C in an inert atmosphere. In-plane conductivities were obtained from films on insulating sapphire substrates, whereas cross-plane conductivities were measured from films on conducting titanium substrates. For the first time, comparison of the in-plane and cross-plane conductivities in these ALD LiAlO2 films has been achieved. More comparable results are obtained using a cross-plane method, whereas in-plane conductivity measurements demonstrate a considerable thickness-dependence with thinner film thickness. The room-temperature conductivity of the LiAlO2 films has been determined to be in the order of 10(-10) S cm(-1) with an activation energy of ca. 0.8 eV.Peer reviewe
The clearing of discs around late type T Tauri stars: constraints from the infrared two colour plane
We have undertaken SED modeling of discs around low mass T Tauri stars in
order to delineate regions of the infrared two colour plane (K - [8] versus K -
[24]) that correspond to discs in different evolutionary stages. This provides
a ready tool for classifying the nature of star-disc systems based on infrared
photometry. In particular we demonstrate the distinct loci followed by discs
that undergo `uniform draining' (reduction in surface density by a spatially
uniform factor) from those that clear from the inside out. We draw attention to
the absence of objects on this `draining locus' in those star forming regions
where the 24um sensitivity would permit their detection, as compared with the
~20 objects in these regions with colours suggestive of inner holes. We thus
conclude that discs predominantly clear from the inside out. We also apply our
classification of the infrared two colour plane to stars of spectral type M3-M5
in the IC 348 cluster and show that some of its members (dubbed `weak excess'
sources by Muzerolle et al 2010) that had previously been assumed to be in a
state of clearing are instead likely to be optically thick discs in which the
dust is well settled towards the mid-plane. Nevertheless, there are many discs
in a state of partial clearing in IC 348, with their abundance (relative to the
total population of disc bearing stars) being around four times higher than for
comparable stars in Taurus. However, the number of partially cleared discs
relative to the total number of late type stars is similarly low in both
regions (10 and 20 % respectively). We therefore conclude that IC 348
represents a more evolved version of the Taurus population (with more of its
discs being highly settled or partially cleared) but that the timescale for
clearing is similarly short (a few times 10^5 years) in both cases.Comment: 9 pages, 6 figures, accepted, MNRA
Electron Pre-Acceleration at Nonrelativistic High-Mach-Number Perpendicular Shocks
We perform particle-in-cell simulations of perpendicular nonrelativistic
collisionless shocks to study electron heating and pre-acceleration for
parameters that permit extrapolation to the conditions at young supernova
remnants. Our high-resolution large-scale numerical experiments sample a
representative portion of the shock surface and demonstrate that the efficiency
of electron injection is strongly modulated with the phase of the shock
reformation. For plasmas with low and moderate temperature (plasma beta
and ), we explore the
nonlinear shock structure and electron pre-acceleration for various
orientations of the large-scale magnetic field with respect to the simulation
plane while keeping it at to the shock normal. Ion reflection off
the shock leads to the formation of magnetic filaments in the shock ramp,
resulting from Weibel-type instabilities, and electrostatic Buneman modes in
the shock foot. In all cases under study, the latter provides first-stage
electron energization through the shock-surfing acceleration (SSA) mechanism.
The subsequent energization strongly depends on the field orientation and
proceeds through adiabatic or second-order Fermi acceleration processes for
configurations with the out-of-plane and in-plane field components,
respectively. For strictly out-of-plane field the fraction of supra-thermal
electrons is much higher than for other configurations, because only in this
case the Buneman modes are fully captured by the 2D simulation grid. Shocks in
plasma with moderate provide more efficient pre-acceleration.
The relevance of our results to the physics of fully three-dimensional systems
is discussed
Statistical Properties of Galactic Starlight Polarization
We present a statistical analysis of Galactic interstellar polarization from
the largest compilation available of starlight data. The data comprises ~ 9300
stars of which we have selected ~ 5500 for our analysis. We find a nearly
linear growth of mean polarization degree with extinction. The amplitude of
this correlation shows that interstellar grains are not fully aligned with the
Galactic magnetic field, which can be interpreted as the effect of a large
random component of the field. In agreement with earlier studies of more
limited scope, we estimate the ratio of the uniform to the random
plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8.
Moreover, a clear correlation exists between polarization degree and
polarization angle what provides evidence that the magnetic field geometry
follows Galactic structures on large-scales. The angular power spectrum C_l of
the starlight polarization degree for Galactic plane data (|b| < 10 deg) is
consistent with a power-law, C_l ~ l^{-1.5} (where l ~ 180 deg/\theta is the
multipole order), for all angular scales \theta > 10 arcmin. An investigation
of sparse and inhomogeneous sampling of the data shows that the starlight data
analyzed traces an underlying polarized continuum that has the same power
spectrum slope, C_l ~ l^{-1.5}. Our findings suggest that starlight data can be
safely used for the modeling of Galactic polarized continuum emission at other
wavelengths.Comment: 31 pages, 11 figures. Minor corrections and some clarifications
included. Matches version accepted for publication by the Astrophysical
Journa
Rings and Radial Waves in the Disk of the Milky Way
We show that in the anticenter region, between Galactic longitudes of
, there is an oscillating asymmetry in the main sequence
star counts on either side of the Galactic plane using data from the Sloan
Digital Sky Survey. This asymmetry oscillates from more stars in the north at
distances of about 2 kpc from the Sun to more stars in the south at 4-6 kpc
from the Sun to more stars in the north at distances of 8-10 kpc from the Sun.
We also see evidence that there are more stars in the south at distances of
12-16 kpc from the Sun. The three more distant asymmetries form roughly
concentric rings around the Galactic center, opening in the direction of the
Milky Way's spiral arms. The northern ring, 9 kpc from the Sun, is easily
identified with the previously discovered Monoceros Ring. Parts of the southern
ring at 14 kpc from the Sun (which we call the TriAnd Ring) have previously
been identified as related to the Monoceros Ring and others have been called
the Triangulum Andromeda Overdensity. The two nearer oscillations are
approximated by a toy model in which the disk plane is offset by of the order
100 pc up and then down at different radii. We also show that the disk is not
azimuthally symmetric around the Galactic anticenter and that there could be a
correspondence between our observed oscillations and the spiral structure of
the Galaxy. Our observations suggest that the TriAnd and Monoceros Rings (which
extend to at least 25 kpc from the Galactic center) are primarily the result of
disk oscillations.Comment: 19figures, 2tables, ApJ accepte
A Census of Large-Scale ( 10 pc), Velocity-Coherent, Dense Filaments in the Northern Galactic Plane: Automated Identification Using Minimum Spanning Tree
Large-scale gaseous filaments with length up to the order of 100 pc are on
the upper end of the filamentary hierarchy of the Galactic interstellar medium.
Their association with respect to the Galactic structure and their role in
Galactic star formation are of great interest from both observational and
theoretical point of view. Previous "by-eye" searches, combined together, have
started to uncover the Galactic distribution of large filaments, yet inherent
bias and small sample size limit conclusive statistical results to be drawn.
Here, we present (1) a new, automated method to identify large-scale
velocity-coherent dense filaments, and (2) the first statistics and the
Galactic distribution of these filaments. We use a customized minimum spanning
tree algorithm to identify filaments by connecting voxels in the
position-position-velocity space, using the Bolocam Galactic Plane Survey
spectroscopic catalog. In the range of , we
have identified 54 large-scale filaments and derived mass (), length (10-276 pc), linear mass density (54-8625 ), aspect ratio, linearity, velocity gradient, temperature,
fragmentation, Galactic location and orientation angle. The filaments
concentrate along major spiral arms. They are widely distributed across the
Galactic disk, with 50% located within 20 pc from the Galactic mid-plane
and 27% run in the center of spiral arms (aka "bones"). An order of 1% of the
molecular ISM is confined in large filaments. Massive star formation is more
favorable in large filaments compared to elsewhere. This is the first
comprehensive catalog of large filaments useful for a quantitative comparison
with spiral structures and numerical simulations.Comment: Accepted to ApJS. 20 pages (in aastex6 compact format), 6 figures, 1
table. See http://www.eso.org/~kwang/MSTpaper for (1) a preprint with full
resolution Fig 6, (2) filaments catalog (Table 1) in ASCII format, and (3) a
DS9 region file for the coordinates of the filament
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