46,504 research outputs found
Generation of Magnetic Field in the Pre-recombination Era
We study the possibility of generating magnetic fields during the evolution
of electron, proton, and photon plasma in the pre-recombination era. We show
that a small magnetic field can be generated in the second order of
perturbation theory for scalar modes with adiabatic initial conditions. The
amplitude of the field is \la 10^{-30} \rm G at the present epoch for scales
from sub-kpc to \ga 100 \rm Mpc.Comment: 8 page
Breakup coupling effects on near-barrier <sup>6</sup>Li, <sup>7</sup>Be and <sup>8</sup>B + <sup>58</sup>Ni elastic scattering compared
New data for near-barrier 6Li, 7Be and 8B + 58Ni elastic scattering enable a comparison of breakup coupling effects for these loosely-bound projectiles. Coupled Discretised Continuum Channels (CDCC) calculations suggest that the large total reaction cross sections for 8B + 58Ni are dominated by breakup at near-barrier energies, unlike 6Li and 7Be where breakup makes a small contribution. In spite of this, the CDCC calculations show a small coupling influence due to breakup for 8B, in contrast to the situation for 6Li and 7Be. An examination of the S matrices gives a clue to this counter-intuitive behaviour
How multiplicity determines entropy and the derivation of the maximum entropy principle for complex systems
The maximum entropy principle (MEP) is a method for obtaining the most likely
distribution functions of observables from statistical systems, by maximizing
entropy under constraints. The MEP has found hundreds of applications in
ergodic and Markovian systems in statistical mechanics, information theory, and
statistics. For several decades there exists an ongoing controversy whether the
notion of the maximum entropy principle can be extended in a meaningful way to
non-extensive, non-ergodic, and complex statistical systems and processes. In
this paper we start by reviewing how Boltzmann-Gibbs-Shannon entropy is related
to multiplicities of independent random processes. We then show how the
relaxation of independence naturally leads to the most general entropies that
are compatible with the first three Shannon-Khinchin axioms, the
(c,d)-entropies. We demonstrate that the MEP is a perfectly consistent concept
for non-ergodic and complex statistical systems if their relative entropy can
be factored into a generalized multiplicity and a constraint term. The problem
of finding such a factorization reduces to finding an appropriate
representation of relative entropy in a linear basis. In a particular example
we show that path-dependent random processes with memory naturally require
specific generalized entropies. The example is the first exact derivation of a
generalized entropy from the microscopic properties of a path-dependent random
process.Comment: 6 pages, 1 figure. To appear in PNA
Statistics of football dynamics
We investigate the dynamics of football matches. Our goal is to characterize
statistically the temporal sequence of ball movements in this collective sport
game, searching for traits of complex behavior. Data were collected over a
variety of matches in South American, European and World championships
throughout 2005 and 2006. We show that the statistics of ball touches presents
power-law tails and can be described by -gamma distributions. To explain
such behavior we propose a model that provides information on the
characteristics of football dynamics. Furthermore, we discuss the statistics of
duration of out-of-play intervals, not directly related to the previous
scenario.Comment: 7 page
Red-giant stars in eccentric binaries
The unparalleled photometric data obtained by NASA’s Kepler Space Telescope has led to improved understanding of red-giant stars and binary stars. We discuss the characterization of known eccentric system, containing a solar-like oscillating red-giant primary component. We also report several new binary systems that are candidates for hosting an oscillating companion. A powerful approach to study binary stars is to combine asteroseimic techniques with light curve fitting. Seismology allows us to deduce the properties of red giants. In addition, by modeling the ellipsoidal modulations we can constrain the parameters of the binary system. An valuable independent source are ground-bases, high-resolution spectrographs
Measuring cosmic magnetic fields by rotation measure-galaxy cross-correlations in cosmological simulations
Using cosmological MHD simulations of the magnetic field in galaxy clusters
and filaments we evaluate the possibility to infer the magnetic field strength
in filaments by measuring cross-correlation functions between Faraday Rotation
Measures (RM) and the galaxy density field. We also test the reliability of
recent estimates considering the problem of data quality and Galactic
foreground (GF) removal in current datasets. Besides the two self-consistent
simulations of cosmological magnetic fields based on primordial seed fields and
galactic outflows analyzed here, we also explore a larger range of models
scaling up the resulting magnetic fields of one of the simulations. We find
that, if an unnormalized estimator for the cross-correlation functions and a GF
removal procedure is used, the detectability of the cosmological signal is only
possible for future instruments (e.g. SKA and ASKAP). However, mapping of the
observed RM signal to the underlying magnetization of the Universe (both in
space and time) is an extremely challenging task which is limited by the
ambiguities of our model parameters, as well as to the weak response of the RM
signal in low density environments. Therefore, we conclude that current data
cannot constrain the amplitude and distribution of magnetic fields within the
large scale structure and a detailed theoretical understanding of the build up
and distribution of magnetic fields within the Universe will be needed for the
interpretation of future observations.Comment: 11 pages, 11 figures, comparation between RM data and simulations in
fig. 8, submited to MNRAS
The radio-infrared correlation in galaxies
The radio-infrared correlation was explained as a direct and linear
relationship between star formation and IR emission. However, one fact making
the IR-star formation linkage less obvious is that the IR emission consists of
at least two emission components, cold dust and warm dust. The cold dust
emission may not be directly linked to the young stellar population.
Furthermore, understanding the origin of the radio-IR correlation requires to
discriminate between the two main components of the radio continuum emission,
free-free and synchrotron emission. Here, we present a multi-scale study of the
correlation of IR with both the thermal and non-thermal (synchrotron)
components of the radio continuum emission from the nearby galaxies M33 and
M31.Comment: To appear in Highlights of Astronomy, Volume 15, XXVIIth IAU General
Assembly, August 200
The eight micron band of silicon monoxide in the expanding cloud around VY Canis Majoris
Observations of vibration-rotation transitions of silicon monoxide in VY CMa show that the lines originate in accelerating, expanding, and cool (600 K) layers of a circumstellar cloud at a distance of roughly 0.15 minutes from the central star. The central stellar velocity, as estimated from observed SiO P Cygni line profiles, is somewhat redshifted from the midpoint of the maser emission features. Most of the silicon is probably in the form of dust grains. The isotopic ratios of silicon are nearly terrestrial
The population of deformed bands in Cr by emission of Be from the S + Mg reaction
Using particle- coincidences we have studied the population of final
states after the emission of 2 -particles and of Be in nuclei
formed in S+Mg reactions at an energy of . The data were obtained in a setup
consisting of the GASP -ray detection array and the multidetector array
ISIS. Particle identification is obtained from the E and E signals of
the ISIS silicon detector telescopes, the Be being identified by the
instantaneous pile up of the E and E pulses. -ray decays of the
Cr nucleus are identified with coincidences set on 2 -particles
and on Be. Some transitions of the side-band with show
stronger population for Be emission relative to that of 2
-particles (by a factor ). This observation is interpreted as
due to an enhanced emission of Be into a more deformed nucleus.
Calculations based on the extended Hauser-Feshbach compound decay formalism
confirm this observation quantitatively.Comment: 17 pages, 9 figures accepted for publication in J. Phys.
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