50,536 research outputs found
Negative Specific Heat in a Quasi-2D Generalized Vorticity Model
Negative specific heat is a dramatic phenomenon where processes decrease in
temperature when adding energy. It has been observed in gravo-thermal collapse
of globular clusters. We now report finding this phenomenon in bundles of
nearly parallel, periodic, single-sign generalized vortex filaments in the
electron magnetohydrodynamic (EMH) model for the unbounded plane under strong
magnetic confinement. We derive the specific heat using a steepest descent
method and a mean field property. Our derivations show that as temperature
increases, the overall size of the system increases exponentially and the
energy drops. The implication of negative specific heat is a runaway reaction,
resulting in a collapsing inner core surrounded by an expanding halo of
filaments.Comment: 12 pages, 3 figures; updated with revision
Uncertainty in phylogenetic tree estimates
Estimating phylogenetic trees is an important problem in evolutionary
biology, environmental policy and medicine. Although trees are estimated, their
uncertainties are discarded by mathematicians working in tree space. Here we
explicitly model the multivariate uncertainty of tree estimates. We consider
both the cases where uncertainty information arises extrinsically (through
covariate information) and intrinsically (through the tree estimates
themselves). The importance of accounting for tree uncertainty in tree space is
demonstrated in two case studies. In the first instance, differences between
gene trees are small relative to their uncertainties, while in the second, the
differences are relatively large. Our main goal is visualization of tree
uncertainty, and we demonstrate advantages of our method with respect to
reproducibility, speed and preservation of topological differences compared to
visualization based on multidimensional scaling. The proposal highlights that
phylogenetic trees are estimated in an extremely high-dimensional space,
resulting in uncertainty information that cannot be discarded. Most
importantly, it is a method that allows biologists to diagnose whether
differences between gene trees are biologically meaningful, or due to
uncertainty in estimation.Comment: Final version accepted to Journal of Computational and Graphical
Statistic
Crystalline silicates as a probe of disk formation history
We present a new perspective on the crystallinity of dust in protoplanetary
disks. The dominant crystallization by thermal annealing happens in the very
early phases of disk formation and evolution. Both the disk properties and the
level of crystallinity are thereby directly linked to the properties of the
molecular cloud core from which the star+disk system was formed. We show that,
under the assumption of single star formation, rapidly rotating clouds produce
disks which, after the main infall phase (i.e. in the optically revealed class
II phase), are rather massive and have a high accretion rate but low
crystallinity. Slowly rotating clouds, on the other hand, produce less massive
disks with lower accretion rate, but high levels of crystallinity. Cloud
fragmentation and the formation of multiple stars complicates the problem and
necessitates further study. The underlying physics of the model is
insufficiently understood to provide the precise relationship between
crystallinity, disk mass and accretion rate. But the fact that with `standard'
input physics the model produces disks which, in comparison to observations,
appear to have either too high levels of crystallinity or too high disk masses,
demonstrates that the comparison of these models to observations can place
strong contraints on the disk physics. The question to ask is not why some
sources are so crystalline, but why some other sources have such a low level of
crystallinity.Comment: Accepted for publication in ApJ
The soft X-ray afterglow of gamma ray bursts, a stringent test for the fireball model
I consider the recent discovery of a soft X-ray source inside the error box
of the gamma ray burst GB 960720 by the SAX, ASCA and ROSAT satellites, in
terms of the fireball model. I show that the ejecta shell, which, after causing
the burst is cold and dense, but still relativistic, keeps plowing through the
interstellar medium, heating up the just-shocked matter which then emits
X-rays. I compute the radiation emitted by this matter. I show that, up to
about two months after the burst, in the cosmological scenario a soft X-ray
(0.1-10 keV) flux of at least 10^(-13) erg / s cm^2, well within current
observational capabilities, is generated, explaining the observations of the
three satellites. Instead, in the Galactic Halo scenario a flux 3 orders of
magnitude lower is expected. Detection of this non-thermal, declining flux in a
statistically significant number of objects would simultaneously establish the
fireball model and the cosmological nature of gamma ray bursts.Comment: Replaces previous version: now it does include figure. Accepted for
publication in the Astrophysical Journal Letter
Transfer of Nonclassical Properties from A Microscopic Superposition to Macroscopic Thermal States in The High Temperature Limit
We present several examples where prominent quantum properties are
transferred from a microscopic superposition to thermal states at high
temperatures. Our work is motivated by an analogy of Schrodinger's cat paradox,
where the state corresponding to the virtual cat is a mixed thermal state with
a large average photon number. Remarkably, quantum entanglement can be produced
between thermal states with nearly the maximum Bell-inequality violation even
when the temperatures of both modes approach infinity.Comment: minor corrections, acknowledgments added, Phys.Rev.Lett., in pres
A 3-component laser-Doppler velocimeter data acquisition and reduction system
A laser doppler velocimeter capable of measuring all three components of velocity simultaneously in low-speed flows is described. All the mean velocities, Reynolds stresses, and higher-order products can be evaluated. The approach followed is to split one of the two colors used in a 2-D system, thus creating a third set of beams which is then focused in the flow from an off-axis direction. The third velocity component is computed from the known geometry of the system. The laser optical hardware and the data acquisition electronics are described in detail. In addition, full operating procedures and listings of the software (written in BASIC and ASSEMBLY languages) are also included. Some typical measurements obtained with this system in a vortex/mixing layer interaction are presented and compared directly to those obtained with a cross-wire system
Two-player quantum pseudo-telepathy based on recent all-versus-nothing violations of local realism
We introduce two two-player quantum pseudo-telepathy games based on two
recently proposed all-versus-nothing (AVN) proofs of Bell's theorem [A.
Cabello, Phys. Rev. Lett. 95, 210401 (2005); Phys. Rev. A 72, 050101(R)
(2005)]. These games prove that Broadbent and Methot's claim that these AVN
proofs do not rule out local-hidden-variable theories in which it is possible
to exchange unlimited information inside the same light-cone (quant-ph/0511047)
is incorrect.Comment: REVTeX4, 5 page
Do Rotations Beyond the Cosmological Horizon Affect the Local Inertial Frame?
If perturbations beyond the horizon have the velocities prescribed everywhere
then the dragging of inertial frames near the origin is suppressed by an
exponential factor. However if perturbations are prescribed in terms of their
angular momenta there is no such suppression.
We resolve this paradox and in doing so give new explicit results on the
dragging of inertial frames in closed, flat and open universe with and without
a cosmological constant.Comment: 12 page
Evaluating the articulation of programme theory in practice as observed in Quality Improvement initiatives
Background: The Action-Effect Method(AEM) was co-developed by NIHR CLAHRC Northwest London (CLAHRC NWL) researchers and QI practitioners, building on Driver Diagrams(DD). This study aimed to determine AEM effectiveness in terms of technical aspects (how diagrams produced in practice compared with theoretical ideals) and social aspects (how engagement with the method related to social benefits). Methods Diagrams were scored on criteria developed on theoretical ideals of programme theory. 65 programme theory diagrams were reviewed (21 published Driver Diagrams (External DDs), 22 CLAHRC NWL Driver Diagrams (Internal DDs), and 21 CLAHRC NWL Action-Effect Diagrams(AEDs)). Social functions were studied through ethnographic observation of frontline QI teams in AEM sessions facilitated by QI experts. Qualitative analysis used inductive and deductive coding. Results ANOVA indicated the AEM significantly improved the quality of programme theory diagrams over Internal and External DDs on an average of 5 criteria from an 8-point assessment. Articulated aims were more likely to be patient-focused and high-level in AEDs than DDs. The cause/effect relationships from intervention to overall aim also tended to be clearer and were more likely than DDs to contain appropriate measure concepts. Using the AEM also served several social functions such as facilitating dialogue among multidisciplinary teams, and encouraging teams to act scientifically and pragmatically about planning and measuring QI interventions. Implications: The Action-Effect Method developed by CLAHRC NWL resulted in improvements over Driver Diagrams in articulating programme theory, which has wide-ranging benefits to quality improvement, including encouraging broad multi-disciplinary buy-in to clear aims and pre-planning a rigorous evaluation strategy
Visual Search for Galaxies near the Northern Crossing of the Supergalactic plane by the Milky Way
We have visually examined twelve Palomar red Plates for galaxies at low
Galactic latitude b, where the Supergalactic Plane (SGP) is crossed by the
Galactic Plane (GP), at Galactic longitude l ~135 degrees. The catalogue
consists of 2575 galaxy candidates, of which 462 have major axis diameters d >=
0.8 arc min (uncorrected for extinction). Galaxy candidates can be identified
down to |b| ~ 0 degrees. One of our galaxy candidates (J24 = Dwingeloo 1) has
recently been discovered independently in 21cm by Kraan-Korteweg et al. (1994)
as a nearby galaxy. Comparisons with the structures seen in the IRAS and UGC
catalogues are made. We compare the success rate of identifying galaxies using
the IRAS Point Source Catalogue under different colour selection criteria. The
criteria that require both the 60 micron and 100 micron fluxes to be of high
quality, have the highest probability of selecting a galaxy (with d >= 0.6 arc
min), but at the expense of selecting a smaller number of galaxies in total.Comment: uuencoded compressed postscript, without figures. The figures are
available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm
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