2,956 research outputs found
Local Lagrangian Approximations for the Evolution of the Density Distribution Function in Large-Scale Structure
We examine local Lagrangian approximations for the gravitational evolution of
the density distribution function. In these approximations, the final density
at a Lagrangian point q at a time t is taken to be a function only of t and of
the initial density at the same Lagrangian point. A general expression is given
for the evolved density distribution function for such approximations, and we
show that the vertex generating function for a local Lagrangian mapping applied
to an initially Gaussian density field bears a simple relation to the mapping
itself. Using this result, we design a local Lagrangian mapping which
reproduces nearly exactly the hierarchical amplitudes given by perturbation
theory for gravitational evolution. When extended to smoothed density fields
and applied to Gaussian initial conditions, this mapping produces a final
density distribution function in excellent agreement with full numerical
simulations of gravitational clustering. We also examine the application of
these local Lagrangian approximations to non-Gaussian initial conditions.Comment: LaTeX, 22 pages, and 11 postscript figure
Mott metal-insulator transition on compressible lattices
The critical properties of the finite temperature Mott endpoint are
drastically altered by a coupling to crystal elasticity, i.e., whenever it is
amenable to pressure tuning. Similar as for critical piezoelectric
ferroelectrics, the Ising criticality of the electronic system is preempted by
an isostructural instability, and long-range shear forces suppress microscopic
fluctuations. As a result, the endpoint is governed by Landau criticality. Its
hallmark is thus a breakdown of Hooke's law of elasticity with a non-linear
strain-stress relation characterized by a mean-field exponent. Based on a
quantitative estimate, we predict critical elasticity to dominate the
temperature range DeltaT/Tc ~ 8% close to the Mott endpoint of
kappa-(BEDT-TTF)2X.Comment: 4 pages, 6 figure
Skewness of the Large-Scale Velocity Divergence from Non-Gaussian Initial Conditions
We compute the skewness and the corresponding hierarchical amplitude
of the divergence of the velocity field for arbitrary non-Gaussian
initial conditions. We find that qualitatively resembles the
corresponding hierarchical amplitude for the density field, , in that it
contains a term proportional to the initial skewness, which decays inversely as
the linear growth factor, plus a constant term which differs from the
corresponding Gaussian term by a complex function of the initial three- and
four- point functions. We extend the results for and with
non-Gaussian initial conditions to evolved fields smoothed with a spherical
tophat window function. We show that certain linear combinations, namely , , and , lead to expressions which are
much simpler, for non-Gaussian initial conditions, than and (or
and ) considered separately.Comment: 13 pages, latex, no figure
Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 19 radio stars in
the International Celestial Reference Frame (ICRF). The positions of these
stars were directly linked to the positions of distant quasars through phase
referencing observations. The positions of the ICRF quasars are known to 0.25
mas, thus providing an absolute reference at the angular resolution of our
radio observations. Average values for the errors in our derived positions for
all sources were 13 mas and 16 mas in R.A. and declination respectively, with
accuracies approaching 1-2 mas for some of the stars observed. Differences
between the ICRF positions of the 38 quasars, and those measured from our
observations showed no systematic offsets, with mean values of -0.3 mas in R.A.
and -1.0 mas in declination. Standard deviations of the quasar position
differences of 17 mas and 11 mas in R.A. and declination respectively, are
consistent with the mean position errors determined for the stars. Our measured
positions were combined with previous Very Large Array measurements taken from
1978-1995 to determine the proper motions of 15 of the stars in our list. With
mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions
approach those derived from Hipparcos, and for a few of the stars in our
program, are better than the Hipparcos values. Comparing the positions of our
radio stars with the Hipparcos catalog, we find that at the epoch of our
observations, the two frames are aligned to within formal errors of
approximately 3 mas. This result confirms that the Hipparcos frame is inertial
at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March
1
All-Sky spectrally matched UBVRI-ZY and u'g'r'i'z' magnitudes for stars in the Tycho2 catalog
We present fitted UBVRI-ZY and u'g'r'i'z' magnitudes, spectral types and
distances for 2.4M stars, derived from synthetic photometry of a library
spectrum that best matches the Tycho2 BtVt, NOMAD Rn and 2MASS JHK_{2/S}
catalog magnitudes. We present similarly synthesized multi-filter magnitudes,
types and distances for 4.8M stars with 2MASS and SDSS photometry to g<16
within the Sloan survey region, for Landolt and Sloan primary standards, and
for Sloan Northern (PT) and Southern secondary standards.
The synthetic magnitude zeropoints for BtVt, UBVRI, ZvYv, JHK_{2/S},
JHK_{MKO}, Stromgren uvby, Sloan u'g'r'i'z' and ugriz are calibrated on 20
calspec spectrophotometric standards. The UBVRI and ugriz zeropoints have
dispersions of 1--3%, for standards covering a range of color from -0.3 < V-I <
4.6; those for other filters are in the range 2--5%.
The spectrally matched fits to Tycho2 stars provide estimated 1-sigma errors
per star of ~0.2, 0.15, 0.12, 0.10 and 0.08 mags respectively in either UBVRI
or u'g'r'i'z'; those for at least 70% of the SDSS survey region to g<16 have
estimated 1-sigma errors per star of ~0.2, 0.06, 0.04, 0.04, 0.05 in u'g'r'i'z'
or UBVRI.
The density of Tycho2 stars, averaging about 60 stars per square degree,
provides sufficient stars to enable automatic flux calibrations for most
digital images with fields of view of 0.5 degree or more. Using several such
standards per field, automatic flux calibration can be achieved to a few
percent in any filter, at any airmass, in most workable observing conditions,
to facilitate inter-comparison of data from different sites, telescopes and
instruments.Comment: 36 pages, 30 figures, 3 printed tables, several electronic tables,
accepted PASP Dec 201
Heat conductivity of DNA double helix
Thermal conductivity of isolated single molecule DNA fragments is of
importance for nanotechnology, but has not yet been measured experimentally.
Theoretical estimates based on simplified (1D) models predict anomalously high
thermal conductivity. To investigate thermal properties of single molecule DNA
we have developed a 3D coarse-grained (CG) model that retains the realism of
the full all-atom description, but is significantly more efficient. Within the
proposed model each nucleotide is represented by 6 particles or grains; the
grains interact via effective potentials inferred from classical molecular
dynamics (MD) trajectories based on a well-established all-atom potential
function. Comparisons of 10 ns long MD trajectories between the CG and the
corresponding all-atom model show similar root-mean-square deviations from the
canonical B-form DNA, and similar structural fluctuations. At the same time,
the CG model is 10 to 100 times faster depending on the length of the DNA
fragment in the simulation. Analysis of dispersion curves derived from the CG
model yields longitudinal sound velocity and torsional stiffness in close
agreement with existing experiments. The computational efficiency of the CG
model makes it possible to calculate thermal conductivity of a single DNA
molecule not yet available experimentally. For a uniform (polyG-polyC) DNA, the
estimated conductivity coefficient is 0.3 W/mK which is half the value of
thermal conductivity for water. This result is in stark contrast with estimates
of thermal conductivity for simplified, effectively 1D chains ("beads on a
spring") that predict anomalous (infinite) thermal conductivity. Thus, full 3D
character of DNA double-helix retained in the proposed model appears to be
essential for describing its thermal properties at a single molecule level.Comment: 16 pages, 12 figure
Signal Structure of the Starlink Ku-Band Downlink
We develop a technique for blind signal identification of the Starlink
downlink signal in the 10.7 to 12.7 GHz band and present a detailed picture of
the signal's structure. Importantly, the signal characterization offered herein
includes the exact values of synchronization sequences embedded in the signal
that can be exploited to produce pseudorange measurements. Such an
understanding of the signal is essential to emerging efforts that seek to
dual-purpose Starlink signals for positioning, navigation, and timing, despite
their being designed solely for broadband Internet provision
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