On Simulating Liouvillian Flow From Quantum Mechanics Via Wigner Functions

The interconnection between quantum mechanics and probabilistic classical mechanics for a free relativistic particle is derived in terms of Wigner functions (WF) for both Dirac and Klein-Gordon (K-G) equations. Construction of WF is achieved by first defining a bilocal 4-current and then taking its Fourier transform w.r.t. the relative 4-coordinate. The K-G and Proca cases also lend themselves to a closely parallel treatment provided the Kemmer- Duffin beta-matrix formalism is employed for the former. Calculation of WF is carried out in a Lorentz-covariant fashion by standard `trace' techniques. The results are compared with a recent derivation due to Bosanac.Comment: 9 pages, Latex; email: [email protected]

Toward a homogeneous set of transiting planet parameters

With 40 or more transiting exoplanets now known, the time is ripe to seek patterns and correlations among their observed properties, which may give important insights into planet formation, structure, and evolution. This task is made difficult by the widely different methodologies that have been applied to measure their properties in individual cases. Furthermore, in many systems our knowledge of the planet properties is limited by the knowledge of the properties of the parent stars. To address these difficulties we have undertaken the first comprehensive analysis of the data for 23 transiting planets using a uniform methodology. We revisit several of the recently proposed correlations, and find new ones involving the metallicity of the parent stars.Comment: 4 pages including figures. To appear in Proceedings of IAU Symposium 253, "Transiting Planets", May 2008, Cambridge, M

Uncertainties in stellar evolution models: convective overshoot

In spite of the great effort made in the last decades to improve our understanding of stellar evolution, significant uncertainties remain due to our poor knowledge of some complex physical processes that require an empirical calibration, such as the efficiency of the interior mixing related to convective overshoot. Here we review the impact of convective overshoot on the evolution of stars during the main Hydrogen and Helium burning phases.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban

Evolution of stellar structure in the Small Magellanic Cloud

The projected distribution of stars in the Small Magellanic Cloud (SMC) from the Magellanic Clouds Photometric Survey is analysed. Stars of different ages are selected via criteria based on V magnitude and V-I colour, and the degree of `grouping' as a function of age is studied. We quantify the degree of structure using the two-point correlation function and a method based on the Minimum Spanning Tree and find that the overall structure of the SMC is evolving from a high degree of sub-structure at young ages (~10 Myr) to a smooth radial density profile. This transition is gradual and at ~75 Myr the distribution is statistically indistinguishable from the background SMC distribution. This time-scale corresponds to approximately the dynamical crossing time of stars in the SMC. The spatial positions of the star clusters in the SMC show a similar evolution of spatial distribution with age. Our analysis suggests that stars form with a high degree of (fractal) sub-structure, probably imprinted by the turbulent nature of the gas from which they form, which is erased by random motions in the galactic potential on a time-scale of a galactic crossing time.Comment: Updated to match final journal styl

Numerical simulation study of the dynamical behavior of the Niedermayer algorithm

We calculate the dynamic critical exponent for the Niedermayer algorithm applied to the two-dimensional Ising and XY models, for various values of the free parameter $E_0$. For $E_0=-1$ we regain the Metropolis algorithm and for $E_0=1$ we regain the Wolff algorithm. For $-1<E_0<1$, we show that the mean size of the clusters of (possibly) turned spins initially grows with the linear size of the lattice, $L$, but eventually saturates at a given lattice size $\widetilde{L}$, which depends on $E_0$. For $L>\widetilde{L}$, the Niedermayer algorithm is equivalent to the Metropolis one, i.e, they have the same dynamic exponent. For $E_0>1$, the autocorrelation time is always greater than for $E_0=1$ (Wolff) and, more important, it also grows faster than a power of $L$. Therefore, we show that the best choice of cluster algorithm is the Wolff one, when compared to the Nierdermayer generalization. We also obtain the dynamic behavior of the Wolff algorithm: although not conclusive, we propose a scaling law for the dependence of the autocorrelation time on $L$.Comment: Accepted for publication in Journal of Statistical Mechanics: Theory and Experimen

Dynamical behavior of the Niedermayer algorithm applied to Potts models

In this work we make a numerical study of the dynamic universality class of the Niedermayer algorithm applied to the two-dimensional Potts model with 2, 3, and 4 states. This algorithm updates clusters of spins and has a free parameter, $E_0$, which controls the size of these clusters, such that $E_0=1$ is the Metropolis algorithm and $E_0=0$ regains the Wolff algorithm, for the Potts model. For $-1<E_0<0$, only clusters of equal spins can be formed: we show that the mean size of the clusters of (possibly) turned spins initially grows with the linear size of the lattice, $L$, but eventually saturates at a given lattice size $\widetilde{L}$, which depends on $E_0$. For $L \geq \widetilde{L}$, the Niedermayer algorithm is in the same dynamic universality class of the Metropolis one, i.e, they have the same dynamic exponent. For $E_0>0$, spins in different states may be added to the cluster but the dynamic behavior is less efficient than for the Wolff algorithm ($E_0=0$). Therefore, our results show that the Wolff algorithm is the best choice for Potts models, when compared to the Niedermayer's generalization.Comment: 10 pages, 11 figures, to be published in Physica A. arXiv admin note: substantial text overlap with arXiv:1003.365

A Young Globular Cluster in the Galaxy NGC 6946

A globular cluster ~15 My old that contains 5x10^5 Msun of stars inside an 11 pc radius has been found in the nearby spiral galaxy NGC 6946, surrounded by clouds of dust and smaller young clusters inside a giant circular bubble 300 pc in radius. At the edge of the bubble is an arc of regularly-spaced clusters that could have been triggered during the bubble's formation. The region is at the end of a spiral arm, suggesting an origin by the asymmetric collapse of spiral arm gas. The globular is one of the nearest examples of a cluster that is similar to the massive old globulars in the Milky Way. We consider the energetics of the bubble and possible formation mechanisms for the globular cluster, including the coalescence of smaller clusters.Comment: 20 pages, 7 figures, accepted for Astrophysical Journal Vol 535, June 1 200

Safety and efficacy of retrograde cerebral perfusion as an adjunct for cerebral protection during surgery on the aortic arch

ObjectiveThe best adjunct for cerebral protection during aortic arch reconstruction remains controversial. Retrograde cerebral perfusion (RCP) as an adjunct to profound hypothermic circulatory arrest (PHCA) extends the tolerable period of brain ischemia by flushing emboli and air from the cerebral circulation while maintaining hypothermia. We examined our experience with RCP to determine its efficacy in patients undergoing complex arch reconstruction.MethodsWe retrospectively evaluated 879 patients undergoing arch reconstruction using RCP from July 1997 to March 2013. Perioperative risk factors were analyzed as predictors of neurologic injury and mortality. Survival for the type of arch reconstruction and for the interval of PHCA was calculated.ResultsOf the 879 patients, 671 underwent hemiarch and 208 total arch replacement. The mean age was 65 ± 13.3 years, and 61.6% were men. The total arch patients had longer mean periods of PHCA (39 vs 21 minutes, P < .001) and RCP (37 vs 19 minutes, P < .001). However, the incidence of transient neurologic dysfunction (3.0% vs 2.4%, P < .813) and permanent neurologic dysfunction (1.3% vs 1.9%, P < .519) was similar for both techniques. Mortality was greater in the hemiarch group (4.8% vs 0.5%, P < .003). Patients requiring >40 minutes of PHCA had outcomes similar to those requiring less. The 1-, 5-, and 10-year survival was similar, regardless of the procedure performed or interval of PHCA.ConclusionsRCP is a safe and effective adjunct for cerebral protection during arch surgery. Patients requiring more extensive arch reconstruction are not at greater risk of permanent neurologic dysfunction or perioperative mortality

Evolução da vegetação de um campo natural (excluído e pastejado) e auto-ecologia de plantas indesejáveis.

A dinâmica da vegaetação campestre, assim como a auto-ecologia de cinco espécies indesejáveis (Eupatorium buniifolium - "chirca", Erianthus angustifolius - "macega", Baccharis coridifolia - "mio-mio", B. trimera - "carqueja", Eryngium horridum - caraguaté) foram estudadas durante 5 anos, visando obter informações sobre a evolução da vegetação e dados fenológicos das espécies indesejáveis.bitstream/item/110556/1/EVOLUCAO-DA-VEGETACAO.pd