639 research outputs found
Evidence of Recent Treeline Dynamics in Southwest Yukon from Aerial Photographs
Small-scale vertical aerial photographs taken in 1947 and 1948 covering 200 km2 of the Kluane Ranges, southwest Yukon, were compared with corresponding photographs taken in 1989 for the purpose of characterizing changes in the distribution and abundance of white spruce (Picea glauca (Moench) Voss) at the alpine treeline. Digital photogrammetry, including orthorectification and on-screen interpretation, was supplemented by stereoscopic inspection of the original prints. Qualitative assessment of change across nine image pairs was accompanied by quantitative analysis of changes in spruce density and elevation using 1 hectare plots and 100 m wide elevational belt transects, respectively, superimposed on the orthorectified images. Significant changes were observed over the 41 years, but the degree of change varied throughout the study area. The most common changes were an increase in canopy size of individual trees and an increase in stand density resulting from the establishment of new individuals. Several instances of treeline advance were also observed. An absence of major natural disturbances or widespread land use change indicates that treeline change is attributable to climate. Results from concurrent dendroecological studies indicate that these dynamics represent only part of the total extent of change to occur during the 20th century.Des photographies aériennes verticales à petite échelle prises en 1947 et 1948 sur une aire de 200 km2 de la chaîne de montagnes Kluane, dans le sud-ouest du Yukon, ont été comparées à des photos correspondantes prises en 1989 dans le but de caractériser les changements en matière de répartition et d’abondance de l’épinette blanche (Picea glauca (Moench) Voss) à la hauteur de la limite alpine des arbres. La photogrammétrie numérique, dont les techniques de redressement différentiel et d’interprétation à l’écran, a été supplémentée par l’inspection stéréoscopique des épreuves photographiques originales. L’évaluation qualitative du changement par rapport à neuf paires d’images s’est accompagnée de l’analyse quantitative des changements en matière de densité des épinettes et d’élévation à l’aide de parcelles de 1 hectare et de transects d’élévation en bandes de 100 m de large, respectivement, superposés sur les images ayant fait l’objet du redressement différentiel. D’importants changements ont été observés au cours de la période de 41 ans, mais le degré de changement variait d’un endroit à l’autre visé par l’étude. Les changements les plus courants consistaient en l’augmentation de la taille du couvert des arbres individuels ainsi qu’en l’augmentation de la densité de peuplement découlant de l’établissement de nouveaux individus. Plusieurs occurrences d’avancement de la limite des arbres ont également été observées. L’absence de perturbations naturelles majeures ou d’importants changements sur le plan de l’utilisation des terres laisse croire que le changement de la limite des arbres est attribuable au climat. Les résultats découlant d’études dendroécologiques concurrentes indiquent que ces dynamiques ne représentent qu’une partie de l’ampleur totale du changement s’étant produit au cours du XXe siècle
Ultracold collisions between two light indistinguishable diatomic molecules: elastic and rotational energy transfer in HD+HD
A close coupling quantum-mechanical calculation is performed for rotational
energy transfer in a HD+HD collision at very low energy, down to the ultracold
temperatures: K. A global six-dimensional H-H
potential energy surface is adopted from a previous work [Boothroyd {\it et
al.}, J. Chem. Phys., {\bf 116}, 666 (2002).] State-resolved integral cross
sections of different
quantum-mechanical rotational transitions in the HD
molecules and corresponding state-resolved thermal rate coefficients
have been computed. Additionally, for comparison,
H+H calculations for a few selected rotational transitions have also
been performed. The hydrogen and deuterated hydrogen molecules are treated as
rigid rotors in this work. A pronounced isotope effect is identified in the
cross sections of these collisions at low and ultracold temperatures.Comment: 9 pages, 9 figures. Accepted for publication in Physical Review
Optimized coupling of cold atoms into a fiber using a blue-detuned hollow-beam funnel
We theoretically investigate the process of coupling cold atoms into the core
of a hollow-core photonic-crystal optical fiber using a blue-detuned
Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam
to couple the atoms, the blue-detuned hollow-beam can confine cold atoms to the
darkest regions of the beam thereby minimizing shifts in the internal states
and making the guide highly robust to heating effects. This single optical beam
is used as both a funnel and guide to maximize the number of atoms into the
fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical
trap (MOT) above a vertically-oriented optical fiber. We observe a
gravito-optical trapping effect for atoms with high orbital momentum around the
trap axis, which prevents atoms from coupling to the fiber: these atoms lack
the kinetic energy to escape the potential and are thus trapped in the laser
funnel indefinitely. We find that by reducing the dipolar force to the point at
which the trapping effect just vanishes, it is possible to optimize the
coupling of atoms into the fiber. Our simulations predict that by using a
low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a
20-{\mu}m radius core hollow-fiber it is possible to couple 11% of the atoms
from a MOT 9 mm away from the fiber. When MOT is positioned further away,
coupling efficiencies over 50% can be achieved with larger core fibers.Comment: 11 pages, 12 figures, 1 tabl
A Pre-Protostellar Core in L1551
Large field surveys of NH3, C2S, 13CO and C18O in the L1551 dark cloud have
revealed a prolate, pre-protostellar molecular core (L1551-MC) in a relatively
quiescent region to the northwest of the well-known IRS 5 source. The kinetic
temperature is measured to be 9K, the total mass is ~2Msun, and the average
particle density is 10^4-10^5 cm^(-3). L1551-MC is 2.25' x 1.11' in projection
oriented at a position angle of 133deg. The turbulent motions are on the order
of the sound speed in the medium and contain 4% of the gravitational energy,
E_{grav}, of the core. The angular momentum vector is projected along the major
axis of L1551-MC corresponding to a rotational energy of 2.5E-3(sin
i)^(-2)|E_{grav}|. The thermal energy constitutes about a third of |E_{grav}|
and the virial mass is approximately equal to the total mass. L1551-MC is
gravitationally bound and in the absence of strong, ~160 microgauss, magnetic
fields will likely contract on a ~0.3 Myr time scale. The line profiles of many
molecular species suggest that the cold quiescent interior is surrounded by a
dynamic, perhaps infalling envelope which is embedded within the ambient
molecular gas of L1551.Comment: 27 pages, 7 figures, ApJ accepte
Phase-space distribution of unbound dark matter near the Sun
We resolve discrepancies in previous analyses of the flow of collisionless
dark matter particles in the Sun's gravitational field. We determine the
phase-space distribution of the flow both numerically, tracing particle
trajectories back in time, and analytically, providing a simple correct
relation between the velocity of particles at infinity and at the Earth. We use
our results to produce sky maps of the distribution of arrival directions of
dark matter particles on Earth at various times of the year. We assume various
Maxwellian velocity distributions at infinity describing the standard dark halo
and streams of dark matter. We illustrate the formation of a ring, analogous to
the Einstein ring, when the Earth is directly downstream of the Sun.Comment: 17 pages, 10 figures (better rendered in ps than pdf
The influence of quintessence on the motion of a binary system in cosmology
We employ the metric of Schwarzschild space surrounded by quintessential
matter to study the trajectories of test masses on the motion of a binary
system. The results, which are obtained through the gradually approximate
approach, can be used to search for dark energy via the difference of the
azimuth angle of the pericenter. The classification of the motion is discussed.Comment: 7 pages, 1 figur
How Well Do We Know the Orbits of the Outer Planets?
This paper deals with the problem of astrometric determination of the orbital
elements of the outer planets, in particular by assessing the ability of
astrometric observations to detect perturbations of the sort expected from the
Pioneer effect or other small perturbations to gravity. We also show that while
using simplified models of the dynamics can lead to some insights, one must be
careful to not over-simplify the issues involved lest one be misled by the
analysis onto false paths. Specifically, we show that the current ephemeris of
Pluto does not preclude the existence of the Pioneer effect. We show that the
orbit of Pluto is simply not well enough characterized at present to make such
an assertion. A number of misunderstandings related to these topics have now
propagated through the literature and have been used as a basis for drawing
conclusions about the dynamics of the solar system. Thus, the objective of this
paper is to address these issues. Finally, we offer some comments dealing with
the complex topic of model selection and comparison.Comment: Accepted for publication in the Ap
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