5,347 research outputs found
Fluorine-doping concentration and fictive temperature dependence of self-trapped holes in SiO[sub 2] glasses
Fictive temperature (Tf) and fluorine (F)-doping concentration dependences of self-trapped holes (STHs) in silicaglasses created by UVirradiation at low temperatures have been studied by the electron-paramagnetic-resonance method. It was found that the yield of STH decreases with decreasing Tf and increasing F-doping concentration. In combination with infrared spectra measurements, the correlation among Tf, F-doping concentration, Si–O bond length, and Si–O–Si bond angle was elucidated. We conclude that the change in both Tf and F doping can modify the network of SiO₂glass, leading to the suppression of the formation of STHs
Impact of 2000–2050 climate change on fine particulate matter (PM<sub>2.5</sub>) air quality inferred from a multi-model analysis of meteorological modes
Studies of the effect of climate change on fine particulate matter (PM<sub>2.5</sub> air quality using general circulation models (GCMs) show inconsistent results including in the sign of the effect. This reflects uncertainty in the GCM simulations of the regional meteorological variables affecting PM<sub>2.5</sub>. Here we use the CMIP3 archive of data from fifteen different IPCC AR4 GCMs to obtain improved statistics of 21st-century trends in the meteorological modes driving PM<sub>2.5</sub> variability over the contiguous US. We analyze 1999–2010 observations to identify the dominant meteorological modes driving interannual PM<sub>2.5</sub> variability and their synoptic periods T. We find robust correlations (<i>r</i> > 0.5) of annual mean PM<sub>2.5</sub> with T, especially in the eastern US where the dominant modes represent frontal passages. The GCMs all have significant skill in reproducing present-day statistics for T and we show that this reflects their ability to simulate atmospheric baroclinicity. We then use the local PM<sub>2.5</sub>-to-period sensitivity (dPM<sub>2.5</sub>/dT) from the 1999–2010 observations to project PM<sub>2.5</sub> changes from the 2000–2050 changes in T simulated by the 15 GCMs following the SRES A1B greenhouse warming scenario. By weighted-average statistics of GCM results we project a likely 2000–2050 increase of ~ 0.1 μg m<sup>−3</sup> in annual mean PM<sub>2.5</sub> in the eastern US arising from less frequent frontal ventilation, and a likely decrease albeit with greater inter-GCM variability in the Pacific Northwest due to more frequent maritime inflows. Potentially larger regional effects of 2000–2050 climate change on PM<sub>2.5</sub> may arise from changes in temperature, biogenic emissions, wildfires, and vegetation, but are still unlikely to affect annual PM<sub>2.5</sub> by more than 0.5 μg m<sup>−3</sup>
A viable mouse model of factor X deficiency provides evidence for maternal transfer of factor X
Background:Activated factor X (FXa) is a vitamin K-dependent serine protease that plays a pivotal role in blood coagulation by converting prothrombin to thrombin. There are no reports of humans with complete deficiency of FX, and knockout of murine F10 is embryonic or perinatal lethal. Objective:We sought to generate a viable mouse model of FX deficiency. Methods:We used a socket-targeting construct to generate F10-knockout mice by eliminating F10 exon 8 (knockout allele termed F10tm1Ccmt, abbreviated as ‘−’; wild-type ‘+’), and a plug-targeting construct to generate mice expressing a FX variant with normal antigen levels but low levels of FX activity [4–9% normal in humans carrying the defect, Pro343→Ser, termed FX Friuli (mutant allele termed F10tm2Ccmt, abbreviated as F)]. Results:F10 knockout mice exhibited embryonic or perinatal lethality. In contrast, homozygous Friuli mice [F10 (F/F)] had FX activity levels of ∼5.5% (sufficient to rescue both embryonic and perinatal lethality), but developed age-dependent iron deposition and cardiac fibrosis. Interestingly, F10 (−/F) mice with FX activity levels of 1–3% also showed complete rescue of lethality. Further study of this model provides evidence supporting a role of maternal FX transfer in the embryonic survival. Conclusions:We demonstrate that, while complete absence of FX is incompatible with murine survival, minimal FX activity as low as 1–3% is sufficient to rescue the lethal phenotype. This viable low-FX mouse model will facilitate the development of FX-directed therapies as well as investigation of the FX role in embryonic development
Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in the formation
of low- and intermediate-mass stars in OB associations (the Lambda Ori region,
Ori OB1, and Lac OB1 associations). We find that the classical T Tauri stars
and Herbig Ae/Be stars tend to line up between luminous O stars and
bright-rimmed or comet-shaped clouds; the closer to a cloud the progressively
younger they are. Our positional and chronological study lends support to the
validity of the radiation-driven implosion mechanism, where the Lyman continuum
photons from a luminous O star create expanding ionization fronts to evaporate
and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive
pressure then causes dense clumps to collapse, prompting the formation of
low-mass stars on the cloud surface (i.e., the bright rim) and
intermediate-mass stars somewhat deeper in the cloud. These stars are a
signpost of current star formation; no young stars are seen leading the
ionization fronts further into the cloud. Young stars in bright-rimmed or
comet-shaped clouds are likely to have been formed by triggering, which would
result in an age spread of several megayears between the member stars or star
groups formed in the sequence.Comment: 2007, ApJ, 657, 88
Threshold production in nucleon-nucleon collisions
We analyze production in nucleon-nucleon collisions near threshold
in the framework of a general model independent formalism, which can be applied
to any reaction , where , , or . Such
reactions show large isotopic effects: a large difference for - and
-collisions, which is due to the different spin structure of the
corresponding matrix elements. The analysis of the spin structure and of the
polarization observables is based on symmetry properties of the strong
interaction. Using existing experimental data on the different decays of
meson, we suggest a model for , based on
channel -exchanges. We predict polarization phenomena for the
-reaction and the ratio of cross sections for and
-collisions. For the processes we apply two
different approaches: vector meson exchange and local four-particle
interaction. In both cases we find larger -production in
-collisions, with respect to -collisions.Comment: 17 pages, 6 figure
Coupling between surface ozone and leaf area index in a chemical transport model: strength of feedback and implications for ozone air quality and vegetation health
Tropospheric ozone is an air pollutant that substantially harms vegetation
and is also strongly dependent on various vegetation-mediated processes. The
interdependence between ozone and vegetation may constitute feedback
mechanisms that can alter ozone concentration itself but have not been
considered in most studies to date. In this study we examine the importance
of dynamic coupling between surface ozone and leaf area index (LAI) in
shaping ozone air quality and vegetation. We first implement an empirical
scheme for ozone damage on vegetation in the Community Land Model (CLM) and
simulate the steady-state responses of LAI to long-term exposure to a range
of prescribed ozone levels (from 0 to 100 ppb). We find that most plant
functional types suffer a substantial decline in LAI as ozone level
increases. Based on the CLM-simulated results, we develop and implement in
the GEOS-Chem chemical transport model a parameterization that computes
fractional changes in monthly LAI as a function of local mean ozone levels.
By forcing LAI to respond to ozone concentrations on a monthly timescale, the
model simulates ozone–LAI coupling dynamically via biogeochemical processes
including biogenic volatile organic compound (VOC) emissions and dry
deposition, without the complication from meteorological changes. We find
that ozone-induced damage on LAI can lead to changes in ozone concentrations
by −1.8 to +3 ppb in boreal summer, with a corresponding ozone feedback
factor of −0.1 to +0.6 that represents an overall self-amplifying effect
from ozone–LAI coupling. Substantially higher simulated ozone due to strong
positive feedbacks is found in most tropical forests, mainly due to the
ozone-induced reductions in LAI and dry deposition velocity, whereas reduced
isoprene emission plays a lesser role in these low-NOx
environments. In high-NOx regions such as the eastern US,
Europe,
and China, however, the feedback effect is much weaker and even negative in
some regions, reflecting the compensating effects of reduced dry deposition
and reduced isoprene emission (which reduces ozone in
high-NOx environments). In remote, low-LAI regions, including
most of the Southern Hemisphere, the ozone feedback is generally slightly
negative due to the reduced transport of NOx–VOC reaction
products that serve as NOx reservoirs. This study represents
the first step to accounting for dynamic ozone–vegetation coupling in a chemical
transport model with ramifications for a more realistic joint assessment of
ozone air quality and ecosystem health.</p
Topological spin excitations of Heisenberg antiferromagnets in two dimensions
In this paper we discuss the construction and the dynamics of vortex-like
topological spin excitations in the Schwinger-boson description of Heisenberg
antiferromagnets in two dimensions. The topological spin excitations are Dirac
fermions (with gap) when spin value is a half-integer. Experimental and
theoretical implications of these excitations are being investigated.Comment: Latex file, no figur
On the possibility of a metallic phase in granular superconducting films
We investigate the possibility of finding a zero-temperature metallic phase
in granular superconducting films. We are able to identify the breakdown of the
conventional treatment of these systems as dissipative Bose systems. We do not
find a metallic state at zero temperature. At finite temperatures, we find that
the system exhibit crossover behaviour which may have implications for the
analysis of experimental results. We also investigate the effect of vortex
dissipation in these systems.Comment: 7 pages, ReVTeX3.0, 3 EPS figure
Heuristic derivation of continuum kinetic equations from microscopic dynamics
We present an approximate and heuristic scheme for the derivation of
continuum kinetic equations from microscopic dynamics for stochastic,
interacting systems. The method consists of a mean-field type, decoupled
approximation of the master equation followed by the `naive' continuum limit.
The Ising model and driven diffusive systems are used as illustrations. The
equations derived are in agreement with other approaches, and consequences of
the microscopic dependences of coarse-grained parameters compare favorably with
exact or high-temperature expansions. The method is valuable when more
systematic and rigorous approaches fail, and when microscopic inputs in the
continuum theory are desirable.Comment: 7 pages, RevTeX, two-column, 4 PS figures include
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