17,869 research outputs found
Anomalous optical absorption in a random system with scale-free disorder
We report on an anomalous behavior of the absorption spectrum in a
one-dimensional lattice with long-range-correlated diagonal disorder with a
power-like spectrum in the form S(k) ~ 1/k^A. These type of correlations give
rise to a phase of extended states at the band center, provided A is larger
than a critical value A_c. We show that for A < A_c the absorption spectrum is
single-peaked, while an additional peak arises when A > A_c, signalling the
occurrence of the Anderson transition. The peak is located slightly below the
low-energy mobility edge, providing a unique spectroscopic tool to monitor the
latter. We present qualitative arguments explaining this anomaly.Comment: 4 pages, 4 postscript figures, uses revtex
Renormalized coordinate approach to the thermalization process
We consider a particle in the harmonic approximation coupled linearly to an
environment. modeled by an infinite set of harmonic oscillators. The system
(particle--environment) is considered in a cavity at thermal equilibrium. We
employ the recently introduced notion of renormalized coordinates to
investigate the time evolution of the particle occupation number. For
comparison we first present this study in bare coordinates. For a long ellapsed
time, in both approaches, the occupation number of the particle becomes
independent of its initial value. The value of ocupation number of the particle
is the physically expected one at the given temperature. So we have a Markovian
process, describing the particle thermalization with the environment. With
renormalized coordinates no renormalization procedure is required, leading
directly to a finite result.Comment: 16 pages, LATEX, 2 figure
A spectroscopic investigation of the O-type star population in four Cygnus OB associations. II. Determination of the fundamental parameters
Aims. Having established the binary status of nineteen O-type stars located
in four Cygnus OB associations, we now determine their fundamental parameters
to constrain their properties and their evolutionary status. We also
investigate their surface nitrogen abundances, which we compare with other
results from the literature obtained for galactic O-type stars. Methods. Using
optical spectra collected for each object in our sample and some UV data from
the archives, we apply the CMFGEN atmosphere code to determine their main
properties. For the binary systems, we have disentangled the components to
obtain their individual spectra and investigate them as if they were single
stars. Results. We find that the distances of several presumably single O-type
stars seem poorly constrained because their luminosities are not in agreement
with the "standard" luminosities of stars with similar spectral types. The ages
of these O-type stars are all less than 7 Myrs. Therefore, the ages of these
stars agree with those, quoted in the literature, of the four associations,
except for CygOB8 for which the stars seem older than the association itself.
However, we point out that the distance of certain stars is debatable relative
to values found in the literature. The N content of these stars put in
perspective with N contents of several other galactic O-type stars seems to
draw the same five groups as found in the "Hunter" diagram for the O and B-type
stars in the LMC even though their locations are obviously different. We
determine mass-loss rates for several objects from the Halpha line and UV
spectra. Finally, we confirm the "mass discrepancy" especially for O stars with
masses smaller than 30 Msun. .Comment: 11 pages, and 26 pages of Appendix. A&A in pres
Phase resolved X-ray spectroscopy of HDE228766: Probing the wind of an extreme Of+/WNLha star
HDE228766 is a very massive binary system hosting a secondary component,
which is probably in an intermediate evolutionary stage between an Of
supergiant and an WN star. The wind of this star collides with the wind of its
O8 II companion, leading to relatively strong X-ray emission. Measuring the
orbital variations of the line-of-sight absorption toward the X-ray emission
from the wind-wind interaction zone yields information on the wind densities of
both stars. X-ray spectra have been collected at three key orbital phases to
probe the winds of both stars. Optical photometry has been gathered to set
constraints on the orbital inclination of the system. The X-ray spectra reveal
prominent variations of the intervening column density toward the X-ray
emission zone, which are in line with the expectations for a wind-wind
collision. We use a toy model to set constraints on the stellar wind parameters
by attempting to reproduce the observed variations of the relative fluxes and
wind optical depths at 1 keV. The lack of strong optical eclipses sets an upper
limit of about 68 degrees on the orbital inclination. The analysis of the
variations of the X-ray spectra suggests an inclination in the range 54 - 61
degrees and indicates that the secondary wind momentum ratio exceeds that of
the primary by at least a factor 5. Our models further suggest that the bulk of
the X-ray emission arises from the innermost region of the wind interaction
zone, which is from a region whose outer radius, as measured from the secondary
star, lies between 0.5 and 1.5 times the orbital separation
Oceanic terranes of S-Central America - 200 Million years of accretion history recorded on the W-edge of the Caribbean Plate
Vacuum energies due to delta-like currents: simulating classical objects along branes with arbitrary codimensions
In this paper we investigate the vacuum energies of several models of quantum
fields interacting with static external currents (linear couplings)
concentrated along parallel branes with an arbitrary number of codimensions. We
show that we can simulate the presence of static charges distributions as well
as the presence of classical static dipoles in any dimension for massive and
massless fields. We also show that we can produce confining potentials with
massless self interacting scalar fields as well as long range anisotropic
potentials.Comment: 18 latex page
Mathematical Model of Easter Island Society Collapse
In this paper we consider a mathematical model for the evolution and collapse
of the Easter Island society, starting from the fifth century until the last
period of the society collapse (fifteen century). Based on historical reports,
the available primary sources consisted almost exclusively on the trees. We
describe the inhabitants and the resources as an isolated system and both
considered as dynamic variables. A mathematical analysis about why the
structure of the Easter Island community collapse is performed. In particular,
we analyze the critical values of the fundamental parameters driving the
interaction humans-environment and consequently leading to the collapse. The
technological parameter, quantifying the exploitation of the resources, is
calculated and applied to the case of other extinguished civilization (Cop\'an
Maya) confirming, with a sufficiently precise estimation, the consistency of
the adopted model.Comment: 9 pages, 1 figure, final version published on EuroPhysics Letter
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