2,692 research outputs found
Determinants of grassland primary production in seasonally-dry silvopastoral systems in Central America
Grassland primary productivity is the function that underpins the majority of the fodder production in cattle-rearing silvopastoral farms. Hence, understanding the factors that determine grassland productivity is critical for the design and management of silvpastoral systems. We studied the effect of two factors with documented impact on grassland productivity in seasonally dry silvopastures of Nicaragua, rainfall and trees. We assessed the effects of three species that differed in crown size and phenology, one evergreen, Cassia grandis, and two deciduous species, Guazuma ulmifolia and Tabebuia rosea. Overall, grassland ANPP had a quadratic response to rainfall, with a decline at high rainfall that coincided with peak standing biomass and grassland cover. Trees had a predominately negative effect on grassland productivity, and the effect was concentrated in the rainy season at peak productivity. The effect of the trees corresponded with the tree crown area, but not with crown density. Trees reduced the standing biomass of graminoids and increased forb biomass; thus, the effect of trees on grassland ANPP appears in part to respond to changes in grassland composition. We also found higher levels of soil moisture content below the tree canopy, particularly at the peak of the rainy season when soils tend to become waterlogged. The evergreen species, C. grandis, affected grassland ANPP more strongly than the deciduous specie
Explosive Disintegration of a Massive Young Stellar System in Orion
Young massive stars in the center of crowded star clusters are expected to
undergo close dynamical encounters that could lead to energetic, explosive
events. However, there has so far never been clear observational evidence of
such a remarkable phenomenon. We here report new interferometric observations
made with the Submillimeter Array (SMA) that indicate the well known enigmatic
wide-angle outflow located in the Orion BN/KL star-forming region to have been
produced by such a violent explosion during the disruption of a massive young
stellar system, and that this was caused by a close dynamical interaction about
500 years ago. This outflow thus belongs to a totally different family of
molecular flows which is not related to the classical bipolar flows that are
generated by stars during their formation process. Our molecular data allow us
to create a 3D view of the debris flow and to link this directly to the well
known Orion H "fingers" farther outComment: Accepted by ApJ Letters The 3D movie can be found in:
ftp://ftp.mpifr-bonn.mpg.de/outgoing/lzapata/movie.gi
Deterministic ratchet from stationary light fields
Ratchets are dynamic systems where particle transport is induced by
zero-average forces due to the interplay between nonlinearity and asymmetry.
Generally, they rely on the effect of a strong external driving. We show that
stationary optical lattices can be designed to generate particle flow in one
direction while requiring neither noise nor driving. Such optical fields must
be arranged to yield a combination of conservative (dipole) and nonconservative
(radiation pressure) forces. Under strong friction all paths converge to a
discrete set of limit periodic trajectories flowing in the same direction.Comment: 6 pages, 4 figure
Continuum spin foam model for 3d gravity
An example illustrating a continuum spin foam framework is presented. This
covariant framework induces the kinematics of canonical loop quantization, and
its dynamics is generated by a {\em renormalized} sum over colored polyhedra.
Physically the example corresponds to 3d gravity with cosmological constant.
Starting from a kinematical structure that accommodates local degrees of
freedom and does not involve the choice of any background structure (e. g.
triangulation), the dynamics reduces the field theory to have only global
degrees of freedom. The result is {\em projectively} equivalent to the
Turaev-Viro model.Comment: 12 pages, 3 figure
On the kinematics of massive star forming regions: the case of IRAS 17233-3606
Direct observations of accretion disks around high-mass young stellar objects
would help to discriminate between different models of formation of massive
stars. However, given the complexity of massive star forming regions, such
studies are still limited in number. Additionally, there is still no general
consensus on the molecular tracers to be used for such investigations. Because
of its close distance and high luminosity, IRAS 17233-3606 is a potential good
laboratory to search for traces of rotation in the inner gas around the
protostar(s). Therefore, we selected the source for a detailed analysis of its
molecular emission at 230 GHz with the SMA. We systematically investigated the
velocity fields of transitions in the SMA spectra which are not affected by
overlap with other transitions, and searched for coherent velocity gradients to
compare them to the distribution of outflows in the region. Beside CO emission
we also used high-angular H2 images to trace the outflow motions driven by the
IRAS 17233-3606 cluster. We find linear velocity gradients in many transitions
of the same molecular species and in several molecules. We report the first
detection of HNCO in molecular outflows from massive YSOs. We discuss the CH3CN
velocity gradient taking into account various scenarios: rotation, presence of
multiple unresolved sources with different velocities, and outflow(s). Although
other interpretations cannot be ruled out, we propose that the CH3CN emission
might be affected by the outflows of the region. Higher angular observations
are needed to discriminate between the different scenarios. The present
observations, with the possible association of CH3CN with outflows in a few
thousands AU around the YSOs' cluster, (i) question the choice of the tracer to
probe rotating structures, and (ii) show the importance of the use of H2 images
for detailed studies of kinematics.Comment: accepted for publication in A&
Voltage rectification by a SQUID ratchet
We argue that the phase across an asymmetric dc SQUID threaded by a magnetic
flux can experience an effective ratchet (periodic and asymmetric) potential.
Under an external ac current, a rocking ratchet mechanism operates whereby one
sign of the time derivative of the phase is favored. We show that there exists
a range of parameters in which a fixed sign (and, in a narrower range, even a
fixed value) of the average voltage across the ring occurs, regardless of the
sign of the external current dc component.Comment: 4 pages, 4 EPS figures, uses psfig.sty. Revised version, to appear in
Physical Review Letters (26 August 1996
Dissipation Enhanced Asymmetric Transport in Quantum Ratchets
Quantum mechanical motion of a particle in a periodic asymmetric potential is
studied theoretically at zero temperature. It is shown based on semi-classical
approximation that the tunneling probability from one local minimum to the next
becomes asymmetric in the presence of weak oscillating field, even though there
is no macroscopic field gradient in average. Dissipation enhances this
asymmetry, and leads to a steady unidirectional current, resulting in a quantum
ratchet system.Comment: 12 pages, 2 Figures, submitted to J. Phys. Soc. Jp
- …