441 research outputs found
Polarimetric Observations of 15 AGNs at High Frequencies
Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the PacificWe have obtained total and polarized intensity images of 15 AGNs with the VLBA at 7 mm at 17 epochs from 25/26 March 1998 to 14 April 2001. The VLBA observations are accompanied at many epochs by simultaneous mea- surements of polarization at 1.35/0.85 mm as well as less frequent simultaneous optical polarization measurements. We discuss the similarities and complexities of polarization behavior at different frequencies along with the VLBI properties
Phase mixing of standing Alfven waves with shear flows in solar spicules
Alfvenic waves are thought to play an important role in coronal heating and
solar wind acceleration. Here we investigate the dissipation of such waves due
to phase mixing at the presence of shear flow and field in the stratified
atmosphere of solar spicules. The initial flow is assumed to be directed along
spicule axis and to vary linearly in the x direction and the equilibrium
magnetic field is taken 2-dimensional and divergence-free. It is determined
that the shear flow and field can fasten the damping of standing Alfven waves.
In spite of propagating Alfven waves, standing Alfven waves in Solar spicules
dissipate in a few periods. As height increases, the perturbed velocity
amplitude does increase in contrast to the behavior of perturbed magnetic
field. Moreover, it should be emphasized that the stratification due to
gravity, shear flow and field are the facts that should be considered in MHD
models in spicules.Comment: Accepted for publication in Astrophysics & Space Scienc
High-temperature oxidation of nickel-based alloys and estimation of the adhesion strength of resulting oxide layers
The kinetics of isothermal oxidation (1100°C) of commercial nickel-based alloys with different content of sulfur (0.22–3.2 wt ppm) is studied. The adhesion strength in a metal/oxide system is estimated as a function of sulfur content and duration of high-temperature exposure. The scratch-test technique is proposed to quantitatively estimate the work of adhesion of resulting oxide films. It is found that the film microstructure is composed of an inner α-Al2O3 layer and an outer NiAl2O4 spinel layer, which are separated by discrete inclusions of TiO2. Residual stresses in the oxide film are experimentally determined by X-ray diffraction. spinel layer, which are separated by discrete inclusions of TiO2. Residual stresses in the oxide film are experimentally determined by X-ray diffractio
Parameterizing scalar-tensor theories for cosmological probes
We study the evolution of density perturbations for a class of models
which closely mimic CDM background cosmology. Using the quasi-static
approximation, and the fact that these models are equivalent to scalar-tensor
gravity, we write the modified Friedmann and cosmological perturbation
equations in terms of the mass of the scalar field. Using the perturbation
equations, we then derive an analytic expression for the growth parameter
in terms of , and use our result to reconstruct the linear matter
power spectrum. We find that the power spectrum at is characterized
by a tilt relative to its General Relativistic form, with increased power on
small scales. We discuss how one has to modify the standard, constant
prescription in order to study structure formation for this class of models.
Since is now scale and time dependent, both the amplitude and transfer
function associated with the linear matter power spectrum will be modified. We
suggest a simple parameterization for the mass of the scalar field, which
allows us to calculate the matter power spectrum for a broad class of
models
Pkd1 Regulates Lymphatic Vascular Morphogenesis during Development.
Lymphatic vessels arise during development through sprouting of precursor cells from veins, which is regulated by known signaling and transcriptional mechanisms. The ongoing elaboration of vessels to form a network is less well understood. This involves cell polarization, coordinated migration, adhesion, mixing, regression, and shape rearrangements. We identified a zebrafish mutant, lymphatic and cardiac defects 1 (lyc1), with reduced lymphatic vessel development. A mutation in polycystic kidney disease 1a was responsible for the phenotype. PKD1 is the most frequently mutated gene in autosomal dominant polycystic kidney disease (ADPKD). Initial lymphatic precursor sprouting is normal in lyc1 mutants, but ongoing migration fails. Loss of Pkd1 in mice has no effect on precursor sprouting but leads to failed morphogenesis of the subcutaneous lymphatic network. Individual lymphatic endothelial cells display defective polarity, elongation, and adherens junctions. This work identifies a highly selective and unexpected role for Pkd1 in lymphatic vessel morphogenesis during development
Torsional Alfven waves in stratified and expanding magnetic flux tubes
The effects of both density stratification and magnetic field expansion on
torsional Alfven waves in magnetic flux tubes are studied. The frequencies, the
period ratio P1/P2 of the fundamental and its first-overtone, and
eigenfunctions of torsional Alfven modes are obtained. Our numerical results
show that the density stratification and magnetic field expansion have opposite
effects on the oscillating properties of torsional Alfven waves.Comment: 13 pages, 7 figures, Accepted for publication in Astrophysics and
Space Scienc
Interstellar Plasma Turbulence Spectrum Toward the Pulsars PSR B0809+74 and B0950+08
Interstellar scintillations of pulsars PSR B0809+74 and B0950+08 have been
studied using observations at low frequencies (41, 62, 89, and 112 MHz).
Characteristic temporal and frequency scales of diffractive scintillations at
these frequencies have been determined. The comprehensive analysis of the
frequency and temporal structure functions reduced to the same frequency has
shown that the spectrum of interstellar plasma inhomogeneities toward both
pulsars is described by a power law. The exponent of the spectrum of
fluctuations of interstellar plasma inhomogeneities toward PSR B0950+08 (n =
3.00 +- 0.05) appreciably differs from the Kolmogorov exponent. Toward PSR
B0809+74 the spectrum is a power law with an exponent n = 3.7 +- 0.1. A strong
angular refraction has been detected toward PSR B0950+08. The distribution of
inhomogeneities along the line of sight has been analyzed; it has been shown
that the scintillations of PSR B0950+08 take place on a turbulent layer with
enhanced electron density, which is localized at approximately 10 pc from the
observer. For PSR B0809+74 the distribution of inhomogeneities is
quasi-uniform. Mean-square fluctuations of electron density on inhomogeneities
with a characteristic scale rho_0 = 10^7 m toward four pulsars have been
estimated. On this scale the local turbulence level in the 10-pc layer is 20
times higher than in an extended region responsible for the scintillations of
PSR B0809+74.Comment: 13 pages, 11 figure
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
System Size and Energy Dependence of Jet-Induced Hadron Pair Correlation Shapes in Cu+Cu and Au+Au Collisions at sqrt(s_NN) = 200 and 62.4 GeV
We present azimuthal angle correlations of intermediate transverse momentum
(1-4 GeV/c) hadrons from {dijets} in Cu+Cu and Au+Au collisions at sqrt(s_NN) =
62.4 and 200 GeV. The away-side dijet induced azimuthal correlation is
broadened, non-Gaussian, and peaked away from \Delta\phi=\pi in central and
semi-central collisions in all the systems. The broadening and peak location
are found to depend upon the number of participants in the collision, but not
on the collision energy or beam nuclei. These results are consistent with sound
or shock wave models, but pose challenges to Cherenkov gluon radiation models.Comment: 464 authors from 60 institutions, 6 pages, 3 figures, 2 tables.
Submitted to Physical Review Letters. Plain text data tables for the points
plotted in figures for this and previous PHENIX publications are (or will be)
publicly available at http://www.phenix.bnl.gov/papers.htm
Improved Measurement of Double Helicity Asymmetry in Inclusive Midrapidity pi^0 Production for Polarized p+p Collisions at sqrt(s)=200 GeV
We present an improved measurement of the double helicity asymmetry for pi^0
production in polarized proton-proton scattering at sqrt(s) = 200 GeV employing
the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The
improvements to our previous measurement come from two main factors: Inclusion
of a new data set from the 2004 RHIC run with higher beam polarizations than
the earlier run and a recalibration of the beam polarization measurements,
which resulted in reduced uncertainties and increased beam polarizations. The
results are compared to a Next to Leading Order (NLO) perturbative Quantum
Chromodynamics (pQCD) calculation with a range of polarized gluon
distributions.Comment: 389 authors, 4 pages, 2 tables, 1 figure. Submitted to Phys. Rev. D,
Rapid Communications. Plain text data tables for the points plotted in
figures for this and previous PHENIX publications are (or will be) publicly
available at http://www.phenix.bnl.gov/papers.htm
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