4,349 research outputs found
Precise measurements of UV atomic lines: Hyperfine structure and isotope shifts in the 398.8 nm line of Yb
We demonstrate a technique for frequency measurements of UV transitions with
sub-MHz precision. The frequency is measured using a ring-cavity resonator
whose length is calibrated against a reference laser locked to the line
of Rb. We have used this to measure the 398.8 nm line of atomic Yb. We report isotope shifts of all the
seven stable isotopes, including the rarest isotope Yb. We have been
able to resolve the overlapping Yb() and Yb
transitions for the first time. We also obtain high-precision measurements of
excited-state hyperfine structure in the odd isotopes, Yb and
Yb. The measurements resolve several discrepancies among earlier
measurements.Comment: 7 pages, 6 figure
15 years of comet photometry: A comparative analysis of 80 comets
In 1976 we began a program of narrowband photometry of comets that has encompassed well over 400 nights of observations. To date, the program has provided detailed information on 80 comets, 11 of which have been observed on multiple apparitions. In this paper we present the observed range of compositions (molecular production rate ratios) and dustiness (gas production compared with AF-rho) for a well sampled group of comets. Based on these results we present preliminary analysis of taxonomic groupings as well as the abundance ratios we associate with a 'typical' comet
Reconstruction of Solar Subsurfaces by Local Helioseismology
Local helioseismology has opened new frontiers in our quest for understanding
of the internal dynamics and dynamo on the Sun. Local helioseismology
reconstructs subsurface structures and flows by extracting coherent signals of
acoustic waves traveling through the interior and carrying information about
subsurface perturbations and flows, from stochastic oscillations observed on
the surface. The initial analysis of the subsurface flow maps reconstructed
from the 5 years of SDO/HMI data by time-distance helioseismology reveals the
great potential for studying and understanding of the dynamics of the quiet Sun
and active regions, and the evolution with the solar cycle. In particular, our
results show that the emergence and evolution of active regions are accompanied
by multi-scale flow patterns, and that the meridional flows display the
North-South asymmetry closely correlating with the magnetic activity. The
latitudinal variations of the meridional circulation speed, which are probably
related to the large-scale converging flows, are mostly confined in shallow
subsurface layers. Therefore, these variations do not necessarily affect the
magnetic flux transport. The North-South asymmetry is also pronounced in the
variations of the differential rotation ("torsional oscillations"). The
calculations of a proxy of the subsurface kinetic helicity density show that
the helicity does not vary during the solar cycle, and that supergranulation is
a likely source of the near-surface helicity.Comment: 17 pages, 10 figures, in "Cartography of the Sun and the Stars",
Editors: Rozelot, Jean-Pierre, Neiner, Corali
High Pressure X-Ray Diffraction Study of UMn2Ge2
Uranium manganese germanide, UMn2Ge2, crystallizes in body-centered
tetragonal ThCr2Si2 structure with space group I4/mmm, a = 3.993A and c =
10.809A under ambient conditions. Energy dispersive X-ray diffraction was used
to study the compression behaviour of UMn2Ge2 in a diamond anvil cell. The
sample was studied up to static pressure of 26 GPa and a reversible structural
phase transition was observed at a pressure of ~ 16.1 GPa. Unit cell parameters
were determined up to 12.4 GPa and the calculated cell volumes were found to be
well reproduced by a Murnaghan equation of state with K0 = 73.5 GPa and K' =
11.4. The structure of the high pressure phase above 16.0 GPa is quite
complicated with very broad lines and could not be unambiguously determined
with the available instrument resolution
Tight-binding study of high-pressure phase transitions in titanium: alpha to omega and beyond
We use a tight-binding total energy method, with parameters determined from a
fit to first-principles calculations, to examine the newly discovered gamma
phase of titanium. Our parameters were adjusted to accurately describe the
alpha Ti-omega Ti phase transition, which is misplaced by density functional
calculations. We find a transition from omega Ti to gamma Ti at 102 GPa, in
good agreement with the experimental value of 116 GPa. Our results suggest that
current density functional calculations will not reproduce the omega Ti-gamma
Ti phase transition, but will instead predict a transition from omega Ti to the
bcc beta Ti phase.Comment: 3 encapsulated Postscript figures, submitted to Phyical Review
Letter
Mars climate engineering using orbiting solar reflectors
The manned mission is seen as a first step towards a Mars surface exploration base-station and, later, establishing permanent settlement. The location and use of Mars's natural resources is vital to enable cost-effective long-duration human exploration and exploitation missions as well as subsequent human colonization. Planet resources include various crust-lodged materials, a low-pressure natural atmosphere, assorted forms of utilizable energy, lower gravity than Earth's, and ground placement advantages relative to human operability and living standards. Power resources may include using solar and wind energy, importation of nuclear reactors and the harvesting of geothermal potential. In fact, a new branch of human civilization could be established permanently on Mars in the next century. But, meantime, an inventory and proper social assessment of Mars's prospective energy and material resources is required. This book investigates the possibilities and limitations of various systems supplying manned bases on Mars with energy and other vital resources. The book collects together recent proposals and innovative options and solutions. It is a useful source of condensed information for specialists involved in current and impending Mars-related activities and a good starting point for young researchers
Energetics of hydrogen impurities in aluminum and their effect on mechanical properties
The effects of hydrogen impurities in the bulk and on the surface of aluminum
are theoretically investigated. Within the framework of density functional
theory, we have obtained the dependence on H concentration of the stacking
fault energy, the cleavage energy, the Al/H surface energy and the Al/H/Al
interface formation energy. The results indicate a strong dependence of the
slip energy barrier in the direction the cleavage energy in the
[111] direction and the Al/H/Al interface formation energy, on H concentration
and on tension. The dependence of the Al/H surface energy on H coverage is less
pronounced, while the optimal H coverage is monolayer. The
calculated activation energy for diffusion between high symmetry sites in the
bulk and on the surface is practically the same, 0.167 eV. From these results,
we draw conclusions about the possible effect of H impurities on mechanical
properties, and in particular on their role in embrittlement of Al.Comment: 9 pages, 5 figure
Astrophysical implications of hypothetical stable TeV-scale black holes
We analyze macroscopic effects of TeV-scale black holes, such as could
possibly be produced at the LHC, in what is regarded as an extremely
hypothetical scenario in which they are stable and, if trapped inside Earth,
begin to accrete matter. We examine a wide variety of TeV-scale gravity
scenarios, basing the resulting accretion models on first-principles, basic,
and well-tested physical laws. These scenarios fall into two classes, depending
on whether accretion could have any macroscopic effect on the Earth at times
shorter than the Sun's natural lifetime. We argue that cases with such effect
at shorter times than the solar lifetime are ruled out, since in these
scenarios black holes produced by cosmic rays impinging on much denser white
dwarfs and neutron stars would then catalyze their decay on timescales
incompatible with their known lifetimes. We also comment on relevant lifetimes
for astronomical objects that capture primordial black holes. In short, this
study finds no basis for concerns that TeV-scale black holes from the LHC could
pose a risk to Earth on time scales shorter than the Earth's natural lifetime.
Indeed, conservative arguments based on detailed calculations and the
best-available scientific knowledge, including solid astronomical data,
conclude, from multiple perspectives, that there is no risk of any significance
whatsoever from such black holes.Comment: Version2: Minor corrections/fixed typos; updated reference
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
Weak-Lensing by Large-Scale Structure and the Polarization Properties of Distant Radio-Sources
We estimate the effects of weak lensing by large-scale density
inhomogeneities and long-wavelength gravitational waves upon the polarization
properties of electromagnetic radiation as it propagates from cosmologically
distant sources. Scalar (density) fluctuations do not rotate neither the plane
of polarization of the electromagnetic radiation nor the source image. They
produce, however, an appreciable shear, which distorts the image shape, leading
to an apparent rotation of the image orientation relative to its plane of
polarization. In sources with large ellipticity the apparent rotation is rather
small, of the order (in radians) of the dimensionless shear. The effect is
larger at smaller source eccentricity. A shear of 1% can induce apparent
rotations of around 5 degrees in radio sources with the smallest eccentricity
among those with a significant degree of integrated linear polarization. We
discuss the possibility that weak lensing by shear with rms value around or
below 5% may be the cause for the dispersion in the direction of integrated
linear polarization of cosmologically distant radio sources away from the
perpendicular to their major axis, as expected from models for their magnetic
fields. A rms shear larger than 5% would be incompatible with the observed
correlation between polarization properties and source orientation in distant
radio galaxies and quasars. Gravity waves do rotate both the plane of
polarization as well as the source image. Their weak lensing effects, however,
are negligible.Comment: 23 pages, 2 eps figures, Aastex 4.0 macros. Final version, as
accepted by ApJ. Additional references and some changes in the introduction
and conclusion
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