2,765 research outputs found
Constrained Differential Renormalization of Yang-Mills Theories
We renormalize QCD to one loop in coordinate space using constrained
differential renormalization, and show explicitly that the Slavnov-Taylor
identities are preserved by this method.Comment: LaTex, 13 pages with 2 ps figure
Periodic Pattern in the Residual-Velocity Field of OB Associations
An analysis of the residual-velocity field of OB associations within 3 kpc of
the Sun has revealed periodic variations in the radial residual velocities
along the Galactic radius vector with a typical scale length of
lambda=2.0(+/-0.2) kpc and a mean amplitude of fR=7(+/-1) km/s. The fact that
the radial residual velocities of almost all OB-associations in rich
stellar-gas complexes are directed toward the Galactic center suggests that the
solar neighborhood under consideration is within the corotation radius. The
azimuthal-velocity field exhibits a distinct periodic pattern in the region
0<l<180 degrees, where the mean azimuthal-velocity amplitude is ft=6(+/-2)
km/s. There is no periodic pattern of the azimuthal-velocity field in the
region 180<l<360 degrees. The locations of the Cygnus arm, as well as the
Perseus arm, inferred from an analysis of the radial- and azimuthal-velocity
fields coincide. The periodic patterns of the residual-velocity fields of
Cepheids and OB associations share many common features.Comment: 21 page
Three-dimensional simulations of the orientation and structure of reconnection X-lines
This work employs Hall magnetohydrodynamic (MHD) simulations to study the
X-lines formed during the reconnection of magnetic fields with differing
strengths and orientations embedded in plasmas of differing densities. Although
random initial perturbations trigger the growth of X-lines with many
orientations, at late time a few robust X-lines sharing an orientation
reasonably consistent with the direction that maximizes the outflow speed, as
predicted by Swisdak and Drake [Geophys. Res. Lett., 34, L11106, (2007)],
dominate the system. The existence of reconnection in the geometry examined
here contradicts the suggestion of Sonnerup [J. Geophys. Res., 79, 1546 (1974)]
that reconnection occurs in a plane normal to the equilibrium current. At late
time the growth of the X-lines stagnates, leaving them shorter than the
simulation domain.Comment: Accepted by Physics of Plasma
New Insights into Cosmic Ray induced Biosignature Chemistry in Earth-like Atmospheres
With the recent discoveries of terrestrial planets around active M-dwarfs,
destruction processes masking the possible presence of life are receiving
increased attention in the exoplanet community. We investigate potential
biosignatures of planets having Earth-like (N-O) atmospheres orbiting
in the habitable zone of the M-dwarf star AD Leo. These are bombarded by high
energetic particles which can create showers of secondary particles at the
surface. We apply our cloud-free 1D climate-chemistry model to study the
influence of key particle shower parameters and chemical efficiencies of NOx
and HOx production from cosmic rays. We determine the effect of stellar
radiation and cosmic rays upon atmospheric composition, temperature, and
spectral appearance. Despite strong stratospheric O destruction by cosmic
rays, smog O can significantly build up in the lower atmosphere of our
modeled planet around AD Leo related to low stellar UVB. NO abundances
decrease with increasing flaring energies but a sink reaction for NO with
excited oxygen becomes weaker, stabilizing its abundance. CH is removed
mainly by Cl in the upper atmosphere for strong flaring cases and not via
hydroxyl as is otherwise usually the case. Cosmic rays weaken the role of
CH in heating the middle atmosphere so that HO absorption becomes more
important. We additionally underline the importance of HNO as a possible
marker for strong stellar particle showers. In a nutshell, uncertainty in NOx
and HOx production from cosmic rays significantly influences biosignature
abundances and spectral appearance.Comment: Manuscript version after addressing all referee comments. Published
in Ap
Interaction-induced current-reversals in driven lattices
We demonstrate that long-range interactions can cause, as time evolves,
consecutive reversals of directed currents for dilute ensembles of particles in
driven lattices. These current-reversals are based on a general mechanism which
leads to an interaction-induced accumulation of particles in the regular
regions of the underlying single-particle phase space and to a synchronized
single-particle motion as well as an enhanced efficiency of Hamiltonian
ratchets.Comment: 5 pages, 5 figure
Customer empowerment in new product development
Companies have traditionally taken responsibility for
deciding what products and product innovations are offered
to customers. However, a growing number of companies are
turning this relationship on its head by asking customers not
only for help in creating new products, but also in deciding
which ones should be produced
Prevalence of X-ray variability in the Chandra Deep Field South
We studied the X-ray variability of sources detected in the Chandra Deep
Field South (Giacconi et al. 2002), nearly all of which are low to moderate z
AGN (Tozzi et al. 2001). We find that 45% of the sources with >100 counts
exhibit significant variability on timescales ranging from a day up to a year.
The fraction of sources found to be variable increases with observed flux,
suggesting that >90% of all AGNs possess intrinsic variability. We also find
that the fraction of variable sources appears to decrease with increasing
intrinsic absorption; a lack of variability in hard, absorbed AGNs could be due
to an increased contribution of reflected X-rays to the total flux. We do not
detect significant spectral variability in the majority (~70%) of our sources.
In half of the remaining 30%, the hardness ratio is anti-correlated with flux,
mimicking the high/soft-low/hard states of galactic sources. The X-ray
variability appears anti-correlated with the luminosity of the sources, in
agreement with previous studies. High redshift sources, however, have larger
variability amplitudes than expected from extrapolations of their low-z
counterparts, suggesting a possible evolution in the accretion rate and/or size
of the X-ray emitting region. Finally, we discuss some effects that may produce
the observed decrease in the fraction of variable sources from z=0.5 out to
z=2.Comment: 24 pages, including 15 figures and 1 table. In press on Ap
Efficient photosynthesis of carbon monoxide from CO2 using perovskite photovoltaics
Artificial photosynthesis, mimicking nature in its efforts to store solar energy, has received considerable attention from the research community. Most of these attempts target the production of H2 as a fuel and our group recently demonstrated solar-to-hydrogen conversion at 12.3% efficiency. Here, in an effort to take this approach closer to real photosynthesis, which is based on the conversion of CO2, we demonstrate the efficient reduction of CO2 to carbon monoxide driven solely by simulated sunlight using water as the electron source. Employing series-connected perovskite photovoltaics and high-performance catalyst electrodes, we reach a solar-to-CO efficiency exceeding 6.5%, which represents a new benchmark in sunlight-driven CO2 conversion. Considering hydrogen as a secondary product, an efficiency exceeding 7% is observed. Furthermore, this study represents one of the first demonstrations of extended, stable operation of perovskite photovoltaics, whose large open-circuit voltage is shown to be particularly suited for this process
Parameter estimation and accuracy matching strategies for 2-D reactor models
AbstractThe mathematical modelling of a special modular catalytic reactor kit leads to a system of partial differential equation in two space dimensions. As customary, this model contains uncertain physical parameters, which may be adapted to fit experimental data. To solve this nonlinear least-squares problem we apply a damped Gauss–Newton method. A method of lines approach is used to evaluate the associated model equations. By an a priori spatial discretization, a large DAE system is derived and integrated with an adaptive, linearly implicit extrapolation method. For sensitivity evaluation we apply an internal numerical differentiation technique, which reuses linear algebra information from the model integration. In order not to interfere with the control of the Gauss–Newton iteration these computations are done usually very accurately and, therefore, with substantial cost. To overcome this difficulty, we discuss several accuracy adaptation strategies, e.g., a master–slave mode. Finally, we present some numerical experiments
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