3,780 research outputs found
Signal buffering in random networks of spiking neurons: microscopic vs. macroscopic phenomena
In randomly connected networks of pulse-coupled elements a time-dependent
input signal can be buffered over a short time. We studied the signal buffering
properties in simulated networks as a function of the networks state,
characterized by both the Lyapunov exponent of the microscopic dynamics and the
macroscopic activity derived from mean-field theory. If all network elements
receive the same signal, signal buffering over delays comparable to the
intrinsic time constant of the network elements can be explained by macroscopic
properties and works best at the phase transition to chaos. However, if only 20
percent of the network units receive a common time-dependent signal, signal
buffering properties improve and can no longer be attributed to the macroscopic
dynamics.Comment: 5 pages, 3 figure
Colossal Effects in Transition Metal Oxides Caused by Intrinsic Inhomogeneities
The influence of quenched disorder on the competition between ordered states
separated by a first-order transition is investigated. A phase diagram with
features resembling quantum-critical behavior is observed, even using classical
models. The low-temperature paramagnetic regime consists of coexisting ordered
clusters, with randomly oriented order parameters. Extended to manganites, this
state is argued to have a colossal magnetoresistance effect. A scale T* for
cluster formation is discussed. This is the analog of the Griffiths
temperature, but for the case of two competing orders, producing a strong
susceptibility to external fields. Cuprates may have similar features,
compatible with the large proximity effect of the very underdoped regime.Comment: 4 pages, 4 figure
Oscillations on the star Procyon
Stars are sphere of hot gas whose interiors transmit acoustic waves very
efficiently. Geologists learn about the interior structure of Earth by
monitoring how seismic waves propagate through it and, in a similar way, the
interior of a star can be probed using the periodic motions on the surface that
arise from such waves. Matthews et al. claim that the star Procyon does not
have acoustic surface oscillations of the strength predicted. However, we show
here, using ground-based spectroscopy, that Procyon is oscillating, albeit with
an amplitude that is only slightly greater than the noise level observed by
Matthews et al. using spaced-based photometry
Extrasolar planets and brown dwarfs around A-F type stars. II. A planet found with ELODIE around the F6V star HD 33564
We present here the detection of a planet orbiting around the F6V star HD
33564. The radial velocity measurements, obtained with the ELODIE echelle
spectrograph at the Haute-Provence Observatory, show a variation with a period
of 388 days. Assuming a primary mass of 1.25 Mo, the best Keplerian fit to the
data leads to a minimum mass of 9.1 MJup for the companion.Comment: 5 pages. Final version, accepted for publication (A&A). Some Spitzer
results on HD33564 (taken this year; not yet published), finally show that
the detection of IR excess around this star (by IRAS) is spuriou
Estimación de la función de distribución sobre poblaciones finitas mediante diseños muestrales bietápicos apropiados
Con el objeto de estimar la función de distribución de una variable de estudio sobre una población finita, se propone en este trabajo emplear el estimador de Horvitz-Thompson, lo que proporciona una estrategia muestral insesgada, siendo la varianza de dicho estimador una función real de variable real cuya minimización permite obtener diseños muestrales óptimos bajo diferentes criterios. En este trabajo empleamos la norma ||·||1 como criterio de optimización, minimizando la norma de la varianza, como función de la matriz del diseño muestral. De esta forma, suponiendo muestreo por conglomerados en dos etapas y considerando como dominio de búsqueda el conjunto de los diseños muestrales de tipo uniforme, en el sentido de ser iguales las probabilidades de inclusión de primer orden, se estudia la obtención de diseños muestrales adecuados para dicha estimación
Double stars with wide separations in the AGK3 - I. Components that are themselves spectroscopic binaries
Wide binaries are tracers of the gravity field of the Galaxy, but their study requires some caution. A large list of common proper motion stars selected from the third Astronomischen Gesellschaft Katalog (AGK3) was monitored with the CORAVEL (for COrrelation RAdial VELocities) spectrovelocimeter, in order to prepare a sample of physical binaries with very wide separations. 66 stars received special attention, since their radial velocities (RV) seemed to be variable. These stars were monitored over several years in order to derive the elements of their spectroscopic orbits. In addition, 10 of them received accurate RV measurements from the SOPHIE spectrograph of the T193 telescope at the Observatory of Haute-Provence. For deriving the orbital elements of double-lined spectroscopic binaries (SB2), a new method was applied, which assumed that the RV of blended measurements are linear combinations of the RV of the components. 13 SB2 orbits were thus calculated. The orbital elements were eventually obtained for 52 spectroscopic binaries (SB), two of them making a triple system. 40 SB received their first orbit and the orbital elements were improved for 10 others. In addition, 11 SB were discovered with very long periods for which the orbital parameters were not found. The median period of the 40 first orbits is 1 yr, and several SB should be resolved or should receive an astrometric orbit in future, providing the masses of the components. In addition, it appeared that HD 153252 has a close companion, which is a candidate brown dwarf with a minimum mass of 50 Jupiter masses. The final selection of wide binaries and the derivation of their statistical properties will be presented in a second pape
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