802 research outputs found
Is demagnetization an efficient optimization method?
Demagnetization, commonly employed to study ferromagnets, has been proposed
as the basis for an optimization tool, a method to find the ground state of a
disordered system. Here we present a detailed comparison between the ground
state and the demagnetized state in the random field Ising model, combing exact
results in and numerical solutions in . We show that there are
important differences between the two states that persist in the thermodynamic
limit and thus conclude that AC demagnetization is not an efficient
optimization method.Comment: 2 pages, 1 figur
New variable stars discovered in the fields of three Galactic open clusters using the VVV Survey
This project is a massive near-infrared (NIR) search for variable stars in
highly reddened and obscured open cluster (OC) fields projected on regions of
the Galactic bulge and disk. The search is performed using photometric NIR data
in the -, - and - bands obtained from the Vista Variables in the
V\'ia L\'actea (VVV) Survey. We performed in each cluster field a variability
search using Stetson's variability statistics to select the variable
candidates. Later, those candidates were subjected to a frequency analysis
using the Generalized Lomb-Scargle and the Phase Dispersion Minimization
algorithms. The number of independent observations range between 63 and 73. The
newly discovered variables in this study, 157 in total in three different known
OCs, are classified based on their light curve shapes, periods, amplitudes and
their location in the corresponding color-magnitude and
color-color diagrams. We found 5 possible Cepheid stars which,
based on the period-luminosity relation, are very likely type II Cepheids
located behind the bulge. Among the newly discovered variables, there are
eclipsing binaries, Scuti, as well as background RR Lyrae stars. Using
the new version of the Wilson & Devinney code as well as the "Physics Of
Eclipsing Binaries" (PHOEBE) code, we analyzed some of the best eclipsing
binaries we discovered. Our results show that these studied systems turn out to
be ranging from detached to double-contact binaries, with low eccentricities
and high inclinations of approximately . Their surface temperatures
range between K and K.Comment: 34 pages, 10 figures, 7 tables. Accepted for publication in New
Astronom
Impact of a Rapid Diluted Energy Density on the halo mass function
We study dark energy cosmological models, extensions of the standard model of
particles, characterized by having an extra relativistic energy density at very
early times, and that rapidly dilute after a phase transition occurs. These
models generate well localized features (or bumps) in the matter power spectrum
for modes crossing the horizon around and before the phase transition epoch.
This is because the presence of the additional energy component enhances the
growth of matter fluctuations during the radiation dominated epoch. Instead of
considering a particular model, we focus on a parametric family of Gaussian
bumps in the matter power spectrum, which otherwise would be a CDM
one. We study the evolution of such bump cosmologies and their effects in the
halo mass function and halo power spectrum using N-body simulations, the
halo-model based HMcode method, and the peak background split framework. The
bumps are subject to different nonlinear effects that become physically well
understood, and from them we are able to predict that the most distinctive
features will show up for intermediate halo masses . Out of this range, we expect halos are not
significantly affected regardless of the location of the primordial bump in the
matter power spectrum. Our analytical results are accurate and in very
satisfactory agreement with the simulated data.Comment: 17 pages, 7 figure
Small quenches and thermalization
We study the expectation values of observables and correlation functions at
long times after a global quantum quench. Our focus is on metallic (“gapless”)
fermionic many-body models and small quenches. The system is prepared in an
eigenstate of an initial Hamiltonian, and the time evolution is performed with
a final Hamiltonian which differs from the initial one in the value of one
global parameter. We first derive general relations between time-averaged
expectation values of observables as well as correlation functions and those
obtained in an eigenstate of the final Hamiltonian. Our results are valid to
linear and quadratic order in the quench parameter g and generalize prior
insights in several essential ways. This allows us to develop a phenomenology
for the thermalization of local quantities up to a given order in g. Our
phenomenology is put to a test in several case studies of one-dimensional
models representative of four distinct classes of Hamiltonians: quadratic
ones, effectively quadratic ones, those characterized by an extensive set of
(quasi-) local integrals of motion, and those for which no such set is known
(and believed to be nonexistent). We show that for each of these models, all
observables and correlation functions thermalize to linear order in g. The
more local a given quantity, the longer the linear behavior prevails when
increasing g. Typical local correlation functions and observables for which
the term O(g) vanishes thermalize even to order g2. Our results show that
lowest-order thermalization of local observables is an ubiquitous phenomenon
even in models with extensive sets of integrals of motion
Subharmonic stochastic synchronization and resonance in neuronal systems.
4 pages, 4 figures.-- PACS: 87.19.La, 05.40.Ca, 87.10.+eWe study the response of a model neuron, driven simultaneously by noise and at least two weak periodic signals. We focus on signals with frequencies components kf0,(k+1)f0,…(k+n)f0 with k>1. The neuron’s output is a sequence of pulses spaced at random interpulse intervals. We find an optimum input noise intensity for which the output pulses are spaced ∼1/f0, i.e., there is a stochastic resonance (SR) at a frequency missing in the input. Even higher noise intensities uncover additional, but weaker, resonances at frequencies present in the input. This is a different form of SR whereby the most robust resonance is the one enhancing a frequency, which is absent in the input, and which is not possible to recover via any linear processing. This can be important in understanding sensory systems including the neuronal mechanism for perception of complex tones.This work was supported by Spanish Ministerio de Ciencia y Technologia, Proyecto CONOCE contract BFM2000-1106, and by NIH-HL62543.Peer reviewe
Inflammatory and neuropathic pain animals exhibit distinct responses to innocuous thermal and motoric challenges
Most current methods for assessing pain in animals are based on reflexive measures and require constant interaction between the observer and the animal. Here we explore two new fully automated methods to quantify the impact of pain on the overall behavior of the organism. Both methods take advantage of the animals' natural preference for a dark environment. We used a box divided into two compartments: dark and bright. In the motoric operant task, "AngleTrack", one end of the box was raised so that the animals had to climb uphill to go from the light to the dark compartment. In the thermal operant task, "ThermalTrack", the floor of the dark compartment was heated to a given temperature, while the light compartment remained at 25°C. Rats were individually placed in the light box and their crossing between chambers monitored automatically for 30 minutes. The angle of the box, or the temperature of the dark compartment, was altered to challenge the animals' natural preference. We test the hypothesis that different models of pain (inflammatory or neuropathic) can be differentiated based on performance on these devices. Three groups of rats were tested at five different challenge levels on both tasks: 1) normal, 2) neuropathic injury pain (Spared Nerve Injury), and 3) inflammatory pain (intraplantar injection of Carrageenan). We monitored the position of the animals as well as their rate of switching between compartments. We find significant differences between the three groups and between the challenge levels both in their average position with respect to time, and in their switching rates. This suggests that the angle-track and thermal-track may be useful in assessing automatically the global impact of different types of pain on behavior
Massive open star clusters using the VVV survey, I: presentation of the data and description of the approach
Context. The ESO Public Survey “VISTA Variables in the Vía Láctea” (VVV) provides deep multi-epoch infrared observations for unprecedented 562 sq. degrees of the Galactic bulge, and adjacent regions of the disk. Aims. The VVV observations will foster the construction of a sample of Galactic star clusters with reliable and homogeneously derived physical parameters (e.g., age, distance, and mass, etc.). In this first paper in a series, the methodology employed to establish cluster parameters for the envisioned database are elaborated upon by analysing four known young open clusters: Danks 1, Danks 2, RCW 79, and DBS 132. The analysis offers a first glimpse of the information that can be gleaned from the VVV observations for clusters in the final database. Methods. Wide-field, deep JHKs VVV observations, combined with new infrared spectroscopy, are employed to constrain fundamental parameters for a subset of clusters. Results. Results are inferred from VVV near-infrared photometry and numerous low resolution spectra (typically more than 10 per cluster). The high quality of the spectra and the deep wide–field VVV photometry enables us to precisely and independently determine the characteristics of the clusters studied, which we compare to previous determinations. An anomalous reddening law in the direction of the Danks clusters is found, specifically E(J − H)/E(H − Ks) = 2.20 ± 0.06, which exceeds published values for the inner Galaxy. The G305 star forming complex, which includes the Danks clusters, lies beyond the Sagittarius-Carina spiral arm and occupies the Centaurus arm. Finally, the first deep infrared colour-magnitude diagram of RCW 79 is presented, which reveals a sizeable pre-main sequence population. A list of candidate variable stars in G305 region is reported. Conclusions. This study demonstrates the strength of the dataset and methodology employed, and constitutes the first step of a broader study which shall include reliable parameters for a sizeable number of poorly characterised and/or newly discovered clusters.Fil: Chené, A.-N.. Universidad de Valparaíso; ChileFil: Borissova, J.. Universidad de Valparaíso; ChileFil: Clarke, J. R. A.. Universidad de Valparaíso; ChileFil: Bonatto, C.. Universidade Federal do Rio Grande do Sul; BrasilFil: Majaess, D. J.. Saint Marys University; CanadáFil: Moni Bidin, C.. Universidad de Concepción; ChileFil: Sale, S. E.. Universidad de Valparaíso; ChileFil: Mauro, F.. Universidad de Concepción; ChileFil: Kurtev, R.. Universidad de Valparaíso; ChileFil: Baume, Gustavo Luis. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Feinstein Baigorri, Carlos. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Ivanov, V. D.. European Southern Observatory; ChileFil: Geisler, Doug. Universidad de Concepción; ChileFil: Catelan, M.. The Milky Way Millennium Nucleus; ChileFil: Minniti, Dante. The Milky Way Millennium Nucleus; ChileFil: Lucas, P.. University Of Hertfordshire; Reino UnidoFil: de Grijs, R.. Peking University; ChinaFil: Kumar, M. S. N.. Centro de Astrofisica da Universidade do Porto; Portuga
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