3,523 research outputs found
Modelling diverse root density dynamics and deep nitrogen uptake — a simple approach
We present a 2-D model for simulation of root density and plant nitrogen (N) uptake for crops grown in agricultural systems, based on a modification of the root density equation originally proposed by Gerwitz and Page in J Appl Ecol 11:773–781, (1974). A root system form parameter was introduced to describe the distribution of root length vertically and horizontally in the soil profile. The form parameter can vary from 0 where root density is evenly distributed through the soil profile, to 8 where practically all roots are found near the surface. The root model has other components describing root features, such as specific root length and plant N uptake kinetics. The same approach is used to distribute root length horizontally, allowing simulation of root growth and plant N uptake in row crops. The rooting depth penetration rate and depth distribution of root density were found to be the most important parameters controlling crop N uptake from deeper soil layers. The validity of the root distribution model was tested with field data for white cabbage, red beet, and leek. The model was able to simulate very different root distributions, but it was not able to simulate increasing root density with depth as seen in the experimental results for white cabbage. The model was able to simulate N depletion in different soil layers in two field studies. One included vegetable crops with very different rooting depths and the other compared effects of spring wheat and winter wheat. In both experiments variation in spring soil N availability and depth distribution was varied by the use of cover crops. This shows the model sensitivity to the form parameter value and the ability of the model to reproduce N depletion in soil layers. This work shows that the relatively simple root model developed, driven by degree days and simulated crop growth, can be used to simulate crop soil N uptake and depletion appropriately in low N input crop production systems, with a requirement of few measured parameters
CPT Violation Implies Violation of Lorentz Invariance
An interacting theory that violates CPT invariance necessarily violates
Lorentz invariance. On the other hand, CPT invariance is not sufficient for
out-of-cone Lorentz invariance. Theories that violate CPT by having different
particle and antiparticle masses must be nonlocal.Comment: Minor changes in the published versio
Hamiltonian Formalism for Space-time Non-commutative Theories
Space-time non-commutative theories are non-local in time. We develop the
Hamiltonian formalism for non-local field theories in d space-time dimensions
by considering auxiliary d+1 dimensional field theories which are local with
respect to the evolution time. The Hamiltonian path integral quantization is
considered and the Feynman rules in the Lagrangian formalism are derived. The
case of non-commutative \phi^3 theory is considered as an example.Comment: 6 pages, A new section is added with other comments and references.
To appear in PR
Perturbative Hamiltonian constraints for higher order theories
We present a method for constructing a consistent low energy canonical
formalism for higher order time-derivative theories, extending the Dirac method
to include perturbative Hamiltonian constraints. We apply it to two
paradigmatic examples: the Pais-Uhlenbeck oscillator and the Bernard-Duncan
scalar field. We also compare the results, both at the classical and quantum
level, with the ones corresponding to a direct perturbative construction
applied to the exact higher order theory. This comparison highligths the
soundness of the present formalism.Comment: 26 pages, 4 figures; review section shortened and appendices change
Rigged Hilbert Space Approach to the Schrodinger Equation
It is shown that the natural framework for the solutions of any Schrodinger
equation whose spectrum has a continuous part is the Rigged Hilbert Space
rather than just the Hilbert space. The difficulties of using only the Hilbert
space to handle unbounded Schrodinger Hamiltonians whose spectrum has a
continuous part are disclosed. Those difficulties are overcome by using an
appropriate Rigged Hilbert Space (RHS). The RHS is able to associate an
eigenket to each energy in the spectrum of the Hamiltonian, regardless of
whether the energy belongs to the discrete or to the continuous part of the
spectrum. The collection of eigenkets corresponding to both discrete and
continuous spectra forms a basis system that can be used to expand any physical
wave function. Thus the RHS treats discrete energies (discrete spectrum) and
scattering energies (continuous spectrum) on the same footing.Comment: 27 RevTex page
Observation of Quantum Asymmetry in an Aharonov-Bohm Ring
We have investigated the Aharonov-Bohm effect in a one-dimensional
GaAs/GaAlAs ring at low magnetic fields. The oscillatory magnetoconductance of
these systems are for the first time systematically studied as a function of
density. We observe phase-shifts of in the magnetoconductance
oscillations, and halving of the fundamental period, as the density is
varied. Theoretically we find agreement with the experiment, by introducing an
asymmetry between the two arms of the ring.Comment: 4 pages RevTex including 3 figures, submitted to Phys. Rev.
Luminous, pc-scale CO 6-5 emission in the obscured nucleus of NGC1377
High resolution submm observations are important in probing the morphology,
column density and dynamics of obscured active galactic nuclei (AGNs). With
high resolution (0.06 x 0.05) ALMA 690 GHz observations we have found bright
(TB >80 K) and compact (FWHM 10x7 pc) CO 6-5 line emission in the nucleus of
the extremely radio-quiet galaxy NGC1377. The integrated CO 6-5 intensity is
aligned with the previously discovered jet/outflow of NGC1377 and is tracing
the dense (n>1e4 cm-3), hot gas at the base of the outflow. The velocity
structure is complex and shifts across the jet/outflow are discussed in terms
of jet-rotation or separate, overlapping kinematical components. High velocity
gas (deltaV +-145 km/s) is detected inside r<2-3 pc and we suggest that it is
emerging from an inclined rotating disk or torus of position angle PA=140+-20
deg with a dynamical mass of approx 3e6 Msun. This mass is consistent with that
of a supermassive black hole (SMBH), as inferred from the M-sigma relation. The
gas mass of the proposed disk/torus constitutes <3% of the nuclear dynamical
mass. In contrast to the intense CO 6-5 line emission, we do not detect dust
continuum with an upper limit of S(690GHz)<2mJy. The corresponding, 5 pc, H2
column density is estimated to N(H2)<3e23 cm-2, which is inconsistent with a
Compton Thick (CT) source. We discuss the possibility that CT obscuration may
be occuring on small (subparsec) or larger scales. From SED fitting we suggest
that half of the IR emission of NGC1377 is nuclear and the rest (mostly the
far-infrared (FIR)) is more extended. The extreme radio quietness, and the lack
of emission from other star formation tracers, raise questions on the origin of
the FIR emission. We discuss the possibility that it is arising from the
dissipation of shocks in the molecular jet/outflow or from irradiation by the
nuclear source along the poles.Comment: 7 pages, 5 figures, submitted to Astronomy and Astrophysic
Direct UV written Michelson interferometer for RZ signal generation using phase-to-intensity modulation conversion
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