11,124 research outputs found
Ionization heating in rare-gas clusters under intense XUV laser pulses
The interaction of intense extreme ultraviolet (XUV) laser pulses
(, \,W/cm) with small rare-gas clusters
(Ar) is studied by quasi-classical molecular dynamics simulations. Our
analysis supports a very general picture of the charging and heating dynamics
in finite samples under short-wavelength radiation that is of relevance for
several applications of free-electron lasers. First, up to a certain photon
flux, ionization proceeds as a series of direct photoemission events producing
a jellium-like cluster potential and a characteristic plateau in the
photoelectron spectrum as observed in [Bostedt {\it et al.}, Phys. Rev. Lett.
{\bf 100}, 013401 (2008)]. Second, beyond the onset of photoelectron trapping,
nanoplasma formation leads to evaporative electron emission with a
characteristic thermal tail in the electron spectrum. A detailed analysis of
this transition is presented. Third, in contrast to the behavior in the
infrared or low vacuum ultraviolet range, the nanoplasma energy capture
proceeds via {\it ionization heating}, i.e., inner photoionization of localized
electrons, whereas collisional heating of conduction electrons is negligible up
to high laser intensities. A direct consequence of the latter is a surprising
evolution of the mean energy of emitted electrons as function of laser
intensity.Comment: figure problems resolve
Coefficient of tangential restitution for the linear dashpot model
The linear dashpot model for the inelastic normal force between colliding
spheres leads to a constant coefficient of normal restitution,
const., which makes this model very popular for the investigation
of dilute and moderately dense granular systems. For two frequently used models
for the tangential interaction force we determine the coefficient of tangential
restitution , both analytically and by numerical integration of
Newton's equation. Although const. for the linear-dashpot model,
we obtain pronounced and characteristic dependencies of the tangential
coefficient on the impact velocity . The
results may be used for event-driven simulations of granular systems of
frictional particles.Comment: 12 pages, 12 figure
Expansion and Collapse in the Cosmic Web
We study the kinematics of the gaseous cosmic web at high redshift with Lyman
alpha forest absorption in multiple QSO sightlines. Using a simple analytic
model and a cosmological hydrodynamic simulation we constrain the underlying
three-dimensional distribution of velocities from the observed line-of-sight
distribution of velocity shear across the plane of the sky. The distribution is
found to be in good agreement with the intergalactic medium (IGM) undergoing
large scale motions dominated by the Hubble flow. Modeling the Lyman alpha
clouds analytically and with a hydrodynamics simulation, the average expansion
velocity of the gaseous structures causing the Lyman alpha forest in the lower
redshift (z = 2) sample appears about 20 percent lower than the local Hubble
expansion velocity. We interpret this as tentative evidence for some clouds
undergoing gravitational collapse. However, the distribution of velocities is
highly skewed, and the majority of clouds at redshifts from 2 to 3.8 expand
typically about 5 - 20 percent faster than the Hubble flow. This behavior is
explained if most absorbers in the column density range typically detectable
are expanding filaments that stretch and drain into more massive nodes. We find
no evidence for the observed distribution of velocity shear being significantly
influenced by processes other than Hubble expansion and gravitational
instability, like galactic winds. To avoid overly disturbing the IGM, winds may
be old and/or limp by the time we observe them in the Lyman alpha forest, or
they may occupy only an insignificant volume fraction of the IGM. (abridged)Comment: 63 pages, 26 figures, AAS Latex; ApJ, in pres
A Framework for Intranet-Based Information Systems (I-BIS) Applications
A recent survey of business executives showed that of 169 decision makers who responded to a survey by the Business Research Group, over fifty percent have either already implemented an internal web site or are in the process of developing such a site (Engler, 1996). These results were similar to those reported in InformationWeek, which showed that 49% of those firms responding to the survey already had direct Internet access for their employees. According to this same survey, this figure is expected to increase to 67% of these same firms by the spring of 1997 with another 10% of the firms testing the viability of such a connectivity policy (Yankelovich, 1996). Gantz (1986) reported that the microcomputer had achieved the same level of penetration into the corporate world (in terms of the percentage of employees with desktop access) in ten years that the telephone required 75 years to achieve. With these systems increasingly tied together in local or wide area networks, it is likely that a comparable level of penetration of Intranet-Based Information System (I-BIS) will occur in an even shorter time span. The rapid proliferation of network communication technology points to a need to measure how and where these systems are being implemented, and what results are being achieved. Recently published reports concerning Intranet-Based Information Systems (I-BIS) have cited return on investment values exceeding 1300% and direct payback time periods of six to twelve weeks (Campbell, 1996). While these systems have already been widely accepted and implemented, scant attention has been paid to rigorous research in relation to users\u27 acceptance and utilization of these systems or the perceived quality of these system
How to Find More Supernovae with Less Work: Object Classification Techniques for Difference Imaging
We present the results of applying new object classification techniques to
difference images in the context of the Nearby Supernova Factory supernova
search. Most current supernova searches subtract reference images from new
images, identify objects in these difference images, and apply simple threshold
cuts on parameters such as statistical significance, shape, and motion to
reject objects such as cosmic rays, asteroids, and subtraction artifacts.
Although most static objects subtract cleanly, even a very low false positive
detection rate can lead to hundreds of non-supernova candidates which must be
vetted by human inspection before triggering additional followup. In comparison
to simple threshold cuts, more sophisticated methods such as Boosted Decision
Trees, Random Forests, and Support Vector Machines provide dramatically better
object discrimination. At the Nearby Supernova Factory, we reduced the number
of non-supernova candidates by a factor of 10 while increasing our supernova
identification efficiency. Methods such as these will be crucial for
maintaining a reasonable false positive rate in the automated transient alert
pipelines of upcoming projects such as PanSTARRS and LSST.Comment: 25 pages; 6 figures; submitted to Ap
Chiral sedimentation of extended objects in viscous media
We study theoretically the chirality of a generic rigid object's
sedimentation in a fluid under gravity in the low Reynolds number regime. We
represent the object as a collection of small Stokes spheres or stokeslets, and
the gravitational force as a constant point force applied at an arbitrary point
of the object. For a generic configuration of stokeslets and forcing point, the
motion takes a simple form in the nearly free draining limit where the
stokeslet radius is arbitrarily small. In this case, the internal hydrodynamic
interactions between stokeslets are weak, and the object follows a helical path
while rotating at a constant angular velocity about a fixed axis. This
is independent of initial orientation, and thus constitutes a chiral
response for the object. Even though there can be no such chiral response in
the absence of hydrodynamic interactions between the stokeslets, the angular
velocity obtains a fixed, nonzero limit as the stokeslet radius approaches
zero. We characterize empirically how depends on the placement of the
stokeslets, concentrating on three-stokeslet objects with the external force
applied far from the stokeslets. Objects with the largest are aligned
along the forcing direction. In this case, the limiting varies as the
inverse square of the minimum distance between stokeslets. We illustrate the
prevalence of this robust chiral motion with experiments on small macroscopic
objects of arbitrary shape.Comment: 35 pages, 10 figures; Section VII.A redone and other edits made for
clarity. Accepted by Phys. Rev.
Spatiotemporal Distribution of Waterfowl Disease Outbreaks in Kansas, USA
Causes and impacts of disease outbreaks in wild bird populations are rarely studied beyond documentation of large epizootic events. In Kansas, USA, a central disease surveillance and reporting protocol currently does not exist within the Kansas Department of Wildlife, Parks and Tourism, which has led to a lack of available knowledge of disease occurrences that could be used to predict and manage future outbreaks. We compiled historic records of documented waterbird disease outbreaks in Kansas from 1967â2014 and related the frequency of outbreaks with light geese (Rossâs goose [Anser rossii]; Snow goose [A. caerulescens]) populations from 1970â2014. We found 32 reports across 16 counties that documented various outbreaks of avian cholera, avian botulism, aspergillosis, renal coccidiosis, West Nile, aflatoxicosis, and mycotoxicosis across multiple waterbird taxa. Avian cholera and avian botulism represented nearly 70% of documented disease outbreaks. Frequency of disease outbreaks increased across the time period with a positive relationship between annual Midwinter Waterfowl Survey counts of light geese and number of reported of avian cholera outbreaks in Kansas. Changes in the continental abundance and distribution of light geese has resulted in increasing residence times in Kansas during winter, potentially increasing risk and severity of avian cholera outbreaks. Disease mitigation efforts by the Kansas Department of Wildlife, Parks and Tourism (KDWPT) should strategically plan for future avian disease outbreaks
Spatiotemporal Distribution of Waterfowl Disease Outbreaks in Kansas, USA
Causes and impacts of disease outbreaks in wild bird populations are rarely studied beyond documentation of large epizootic events. In Kansas, USA, a central disease surveillance and reporting protocol currently does not exist within the Kansas Department of Wildlife, Parks and Tourism, which has led to a lack of available knowledge of disease occurrences that could be used to predict and manage future outbreaks. We compiled historic records of documented waterbird disease outbreaks in Kansas from 1967â2014 and related the frequency of outbreaks with light geese (Rossâs goose [Anser rossii]; Snow goose [A. caerulescens]) populations from 1970â2014. We found 32 reports across 16 counties that documented various outbreaks of avian cholera, avian botulism, aspergillosis, renal coccidiosis, West Nile, aflatoxicosis, and mycotoxicosis across multiple waterbird taxa. Avian cholera and avian botulism represented nearly 70% of documented disease outbreaks. Frequency of disease outbreaks increased across the time period with a positive relationship between annual Midwinter Waterfowl Survey counts of light geese and number of reported of avian cholera outbreaks in Kansas. Changes in the continental abundance and distribution of light geese has resulted in increasing residence times in Kansas during winter, potentially increasing risk and severity of avian cholera outbreaks. Disease mitigation efforts by the Kansas Department of Wildlife, Parks and Tourism (KDWPT) should strategically plan for future avian disease outbreaks
Quaternary Stratigraphy in Northwestern Maine: A Progress Report
A preliminary Quaternary stratigraphy for northwestern Maine can be assembled from interpretation of ice-flow indicators (dispersal and erosional), exposed sections, and drill-hole logs. Evidence from the ice-flow indicators delineates at least four regions each with different ice-flow histories. The distribution of these regions may result from an eastward invasion of Laurentide source ice during the early portion of the late Wisconsinan and subsequent development of a local ice dome during the closing portion of the Late Wisconsinan substage. Exposed sections contribute the following probable sequence of events to the stratigraphy: 1) deposition of alluvial fans, 2) deposition of a gray compact till beneath eastward flowing (Laurentide) ice, 3) deposition of a brown till beneath northward flowing local ice, 4) deglaciation by a southward retreating ice margin. These events appear continuous and have all been provisionally assigned a Late Wisconsinan age. Drill-hole logs confirm the sequence derived from the exposed sections and allow extension of the Quaternary stratigraphy. The drill-log data show three associated groups of sediments that may in turn result from at least three separate ice margin advances and recessions. The uppermost group of sediments is correlated with those found in exposed sections. The position of the drill-hole logs in an over-deepened basin suggest erosion by at least one even earlier glaciation.Ă partir de l'interprĂ©tation des indicateurs de l'Ă©coulement glaciaire (de dispersion et d'Ă©rosion), des coupes naturelles et des rĂ©sultats des forages, on a tentĂ© d'Ă©tablir la stratigraphie prĂ©liminaire du Quaternaire pour le nord du Maine. Les renseignements tirĂ©s des indicateurs de l'Ă©coulement glaciaire ont permis de circonscrire au moins quatre rĂ©gions, chacune d'elles prĂ©sentant un processus diffĂ©rent d'Ă©coulement glaciaire. La rĂ©partition des rĂ©gions est probablement le rĂ©sultat de la progression vers l'est des glaces laurentidiennes au dĂ©but du Wisconsinien supĂ©rieur et de la formation subsĂ©quente d'un dĂŽme local Ă la fin du Wisconsinien supĂ©rieur. Les coupes naturelles ont permis de reconstituer la sĂ©quence d'Ă©vĂ©nements suivante : 1 ) formation de cĂŽnes de dĂ©jection; 2) dĂ©pĂŽt d'un till gris compact sous une des glaces laurentidiennes s'Ă©coulant vers l'est; 3) dĂ©pĂŽt d'un till brun sous les glaces locales s'Ă©coulant vers le nord; 4) dĂ©glaciation rĂ©sultant du retrait de la marge glaciaire vers le sud. Ces Ă©vĂ©nements semblent s'ĂȘtre dĂ©roulĂ©s de façon continue; on les attribue du moins provisoirement au Wisconsinien supĂ©rieur. Les rĂ©sultats des forages confirment la sĂ©quence reconstituĂ©e grĂące aux coupes naturelles et permettent mĂȘme de prolonger la stratigraphie du Quaternaire. Les donnĂ©es de forage dĂ©montrent la prĂ©sence de trois groupes associĂ©s de sĂ©diments qui, Ă leur tour, sont le rĂ©sultat d'au moins trois rĂ©currences et trois rĂ©cessions de la marge glaciaire. Le groupe supĂ©rieur correspond aux sĂ©diments observĂ©s dans les coupes. Les rĂ©sultats de forage dans un bassin surcreusĂ© laissont croire qu'au moins une glaciation antĂ©rieure y a laissĂ© des traces d'Ă©rosion.Eine vorlĂ ufige Stratigraphie fur Nord-West Maine im Quatemar kann durch Interpretation der Eis-FluB Indikatoren (Dispersion und Erosion), der natĂčrlichen Schnitte und der Bohrergebnisse erstellt werden. Durch die Eis-RuB Indikatoren ergeben sich mindestens vier Gebiete mit jeweils unterschiedlicher Eis-FluB Vergangenheit. Die Verteilung dieser Gebiete hat wohl ihren Ursprung im Eindringen von Laurentischem Quell-Eis in ĂŽstlicher Richtung wĂ hrend des Beginns des spĂ ten Wisconsin und der darauf folgenden Entwicklung eines lokalen Eis-Doms wĂ hrend der AbschluBphase des spĂąten Wisconsin. Die natĂčrlichen Schnitte ergeben die folgende mĂŽgliche Abfolge der Ereignisse fur die Stratigraphie; 1) Ablagerung von Alluvialkegeln, 2) Ablagerung eines grauen kompakten Tills unter ostwĂ rts flieBendem (Laurentischem) Eis, 3) Ablagerung eines braunen Tills unter nordwĂąrts flieBendem lokalem Eis, 4) Enteisung durch die sĂčdwĂ rts zurĂčckweichende Eisgrenze. DiĂšse Ereignisse scheinen kontinuierlich abgelaufen zu sein und sind aile vorlĂ ufig dem spĂąten Wisconsin zugeschrieben worden. Die Bohrergebnisse bestĂ tigen die aus den natĂčrlichen Schnitten gewonnene Abfolge und erlauben eine Ausdehnung der Stratigraphie des Quatemar. Die Bohrdaten zeigen drei Gruppen von Sedimenten, die ihrerseits ihren Ursprung in mindestens drei unterschiedlichen VorstoB- und RĂčckzugs-Phasen der Eisgrenze haben. Die oberste Gruppe der Sedimente steht in Wechselbeziehung mit den in den natĂčrlichen Schnitten gefundenen Sedimenten. Bohrergebnissg in einom ĂčbGrtieften Becken lassen vermuten, daB zumindest eine frĂčhre Vereisung Erosionsspuren hinterlassen hat
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