484 research outputs found
Mercury in the environs of the north slope of Alaska
The analysis of Greenland ice suggests that the flux of mercury from the continents
to the atmosphere has increased in recent times, perhaps partly as a result of the many of
man’s activities that effect an alteration of terrestrial surfaces. Upon the exposure of fresh
crustal matter, the natural outgassing of mercury vapor from the earth’s surface could be
enhanced.
Accordingly, mercury was measured in a variety of environmental materials gathered
from the North Slope of Alaska to provide background data prior to the anticipated increase
of activity in this environment. The materials were collected during the U. S. Coast Guard
WEBSEC 72-73 cruises as well as through the facilities provided by Naval Arctic Research
Laboratory in the spring of 1973.
The method of measurement depended upon radioactivation of mercury with neutrons
and the subsequent quantification of characteristic gamma radiations after radiochemical
purification.
Mercury concentrations in seawater at several locations in the vicinity of 151°W,
71°N averaged 20 parts per trillion. The waters from all stations east of this location showed
a significantly smaller concentration. This difference may relate to penetration o f Bering-
Chukchi Sea water into the southern Beaufort Sea to 151°W. Marine sediments on the shelf
and slope between 143°W and 153°W contained about 100 parts per billion mercury, except
for those on the continental shelf between Barter Island and the Canning River, where the
concentration was less than half this value. These results are consistent with sediment input
from the respective rivers when their mercury content and mineralogy are considered. The
mercury content of river waters was 18 ppt and in reasonable agreement with the average of
snow samples (13 ppt). The burden of mercury in plankton was 37 ppb.This work was supported by the office of Naval Research under grant N R 083-290
The Reaction of Potassium Amide in Liquid Ammonia with Diarylbromethenes
The behavior of certain diarylchloroethenes with potassium amide in liquid ammonia has been reported previously. A rapid reaction occurs and the ethenes are changed with the loss of hydrochloric acid to tolanes. Thus 1, 1-di-p-tolyl-2-chloroethene forms p. p\u27-dimethyltolane. A series of such para substituted compounds was studied and as anticipated only symmetrical tolanes were obtained
Uniformity in the Wiener-Wintner theorem for nilsequences
We prove a uniform extension of the Wiener-Wintner theorem for nilsequences
due to Host and Kra and a nilsequence extension of the topological
Wiener-Wintner theorem due to Assani. Our argument is based on (vertical)
Fourier analysis and a Sobolev embedding theorem.Comment: v3: 18 p., proof that the cube construction produces compact
homogeneous spaces added, measurability issues in the proof of Theorem 1.5
addressed. We thank the anonymous referees for pointing out these gaps in v
Measurement of the dark matter velocity anisotropy in galaxy clusters
The internal dynamics of a dark matter structure may have the remarkable
property that the local temperature in the structure depends on direction. This
is parametrized by the velocity anisotropy beta which must be zero for relaxed
collisional structures, but has been shown to be non-zero in numerical
simulations of dark matter structures. Here we present a method to infer the
radial profile of the velocity anisotropy of the dark matter halo in a galaxy
cluster from X-ray observables of the intracluster gas. This non-parametric
method is based on a universal relation between the dark matter temperature and
the gas temperature which is confirmed through numerical simulations. We apply
this method to observational data and we find that beta is significantly
different from zero at intermediate radii. Thus we find a strong indication
that dark matter is effectively collisionless on the dynamical time-scale of
clusters, which implies an upper limit on the self-interaction cross-section
per unit mass sigma/m < 1 cm2/g. Our results may provide an independent way to
determine the stellar mass density in the central regions of a relaxed cluster,
as well as a test of whether a cluster is in fact relaxed.Comment: 10 pages, 8 figures, submitted to Ap
Might we eventually understand the origin of the dark matter velocity anisotropy?
The density profile of simulated dark matter structures is fairly
well-established, and several explanations for its characteristics have been
put forward. In contrast, the radial variation of the velocity anisotropy has
still not been explained. We suggest a very simple origin, based on the shapes
of the velocity distributions functions, which are shown to differ between the
radial and tangential directions. This allows us to derive a radial variation
of the anisotropy profile which is in good agreement with both simulations and
observations. One of the consequences of this suggestion is that the velocity
anisotropy is entirely determined once the density profile is known. We
demonstrate how this explains the origin of the \gamma-\beta relation, which is
the connection between the slope of the density profile and the velocity
anisotropy. These findings provide us with a powerful tool, which allows us to
close the Jeans equations.Comment: 10 pages, 7 figures, subm to ap
A detailed statistical analysis of the mass profiles of galaxy clusters
The distribution of mass in the halos of galaxies and galaxy clusters has
been probed observationally, theoretically, and in numerical simulations. Yet
there is still confusion about which of several suggested parameterized models
is the better representation, and whether these models are universal. We use
the temperature and density profiles of the intracluster medium as measured by
X-ray observations of 11 relaxed galaxy clusters to investigate mass models for
the halo using a thorough Bayesian statistical analysis. We make careful
comparisons between two- and three-parameter models, including the issue of a
universal third parameter. We find that, of the two-parameter models, the NFW
is the best representation, but we also find moderate statistical evidence that
a generalized three-parameter NFW model with a freely varying inner slope is
preferred, despite penalizing against the extra degree of freedom. There is a
strong indication that this inner slope needs to be determined for each cluster
individually, i.e. some clusters have central cores and others have steep
cusps. The mass-concentration relation of our sample is in reasonable agreement
with predictions based on numerical simulations.Comment: 10 pages, 5 figures, accepted by ApJ. Matches accepted versio
The impact of going beyond the Maxwell distribution in direct dark matter detection rates
We consider direct dark matter detection rates and investigate the difference
between a standard Maxwell-Boltzmann velocity distribution and a "realistic"
distribution like the ones extracted from numerical N-body simulations. Sizable
differences are observed when such results are compared to the standard
Maxwell-Boltzmann distribution. For a light target both the total rate and the
annual modulation are reduced by ~25%. For a heavy target the total rate is
virtually unchanged, whereas the annual modulation is modified by up to 50%,
depending on the WIMP mass and detector energy threshold. We also consider the
effect of a possible velocity anisotropy, and the effect is found to be largest
for a light target For the realistic velocity distribution the anisotropy may
reduce the annual modulation, in contrast to the Maxwell-Boltzmann case.Comment: 13 pages, 10 figures, accepted by Phys. Rev.
What it takes to measure a fundamental difference between dark matter and baryons: the halo velocity anisotropy
Numerous ongoing experiments aim at detecting WIMP dark matter particles from
the galactic halo directly through WIMP-nucleon interactions. Once such a
detection is established a confirmation of the galactic origin of the signal is
needed. This requires a direction-sensitive detector. We show that such a
detector can measure the velocity anisotropy beta of the galactic halo.
Cosmological N-body simulations predict the dark matter anisotropy to be
nonzero, beta~0.2. Baryonic matter has beta=0 and therefore a detection of a
nonzero beta would be strong proof of the fundamental difference between dark
and baryonic matter. We estimate the sensitivity for various detector
configurations using Monte Carlo methods and we show that the strongest signal
is found in the relatively few high recoil energy events. Measuring beta to the
precision of ~0.03 will require detecting more than 10^4 WIMP events with
nuclear recoil energies greater than 100 keV for a WIMP mass of 100 GeV and a
32S target. This number corresponds to ~10^6 events at all energies. We discuss
variations with respect to input parameters and we show that our method is
robust to the presence of backgrounds and discuss the possible improved
sensitivity for an energy-sensitive detector.Comment: 15 pages, 8 figures, accepted by JCAP. Matches accepted versio
Stirring N-body systems: Universality of end states
We study the evolution of the phase-space of collisionless N-body systems
under repeated stirrings or perturbations. We find convergence towards a
limited solution group, in accordance with Hansen 2010, that is independent of
the initial system and environmental conditions, paying particular attention to
the assumed gravitational paradigm (Newtonian and MOND). We examine the effects
of changes to the perturbation scheme and in doing so identify a large group of
perturbations featuring radial orbit instability (ROI) which always lead to
convergence. The attractor is thus found to be a robust and reproducible effect
under a variety of circumstances
SARS-CoV-2 susceptibility and COVID-19 disease severity are associated with genetic variants affecting gene expression in a variety of tissues.
Variability in SARS-CoV-2 susceptibility and COVID-19 disease severity between individuals is partly due to genetic factors. Here, we identify 4 genomic loci with suggestive associations for SARS-CoV-2 susceptibility and 19 for COVID-19 disease severity. Four of these 23 loci likely have an ethnicity-specific component. Genome-wide association study (GWAS) signals in 11 loci colocalize with expression quantitative trait loci (eQTLs) associated with the expression of 20 genes in 62 tissues/cell types (range: 1:43 tissues/gene), including lung, brain, heart, muscle, and skin as well as the digestive system and immune system. We perform genetic fine mapping to compute 99% credible SNP sets, which identify 10 GWAS loci that have eight or fewer SNPs in the credible set, including three loci with one single likely causal SNP. Our study suggests that the diverse symptoms and disease severity of COVID-19 observed between individuals is associated with variants across the genome, affecting gene expression levels in a wide variety of tissue types
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