13,314 research outputs found
Entropy driven key-lock assembly
The effective interaction between a sphere with an open cavity (lock) and a
spherical macroparticle (key), both immersed in a hard sphere fluid, is studied
by means of Monte Carlo simulations. As a result, a 2d map of the key-lock
effective interaction potential is constructed, which leads to the proposal of
a self-assembling mechanism: there exists trajectories through which the
key-lock pair could assemble avoiding trespassing potential barriers. Hence,
solely the entropic contribution can induce their self-assembling even in the
absence of attractive forces. This study points out the solvent contribution
within the underlying mechanisms of substrate-protein assembly/disassembly
processes, which are important steps of the enzyme catalysis and protein
mediated transport
Cosmology with moving dark energy and the CMB quadrupole
We study the consequences of a homogeneous dark energy fluid having a
non-vanishing velocity with respect to the matter and radiation large-scale
rest frames. We consider homogeneous anisotropic cosmological models with four
fluids (baryons, radiation, dark matter and dark energy) whose velocities can
differ from each other. Performing a perturbative calculation up to second
order in the velocities, we obtain the contribution of the anisotropies
generated by the fluids motion to the CMB quadrupole and compare with
observations. We also consider the exact problem for arbitrary velocities and
solve the corresponding equations numerically for different dark energy models.
We find that models whose equation of state is initially stiffer than
radiation, as for instance some tracking models, are unstable against velocity
perturbations, thus spoiling the late-time predictions for the energy
densities. In the case of scaling models, the contributions to the quadrupole
can be non-negligible for a wide range of initial conditions. We also consider
fluids moving at the speed of light (null fluids) with positive energy and show
that, without assuming any particular equation of state, they generically act
as a cosmological constant at late times. We find the parameter region for
which the models considered could be compatible with the measured (low)
quadrupole.Comment: 23 pages, 6 figures. Confidence intervals calculated from WMAP data,
new references and comments included. Final version to appear in PR
An infrared study of galactic OH/IR stars. I. An optical/near-IR atlas of the Arecibo sample
In this paper we present optical and near-infrared finding charts, accurate
astrometry (~1") and single-epoch near-infrared photometry for 371 IRAS
sources, 96% of those included in the so-called Arecibo sample of OH/IR stars
(Eder et al. 1988; Lewis et al. 1990a; Chengalur et al. 1993). The main
photometric properties of the stars in the sample are presented and discussed
as well as the problems found during the process of identification of the
optical/near-infrared counterparts. In addition, we also identify suitable
reference stars in each field to be used for differential photometry purposes
in the future.
We find that 39% of the sources (144 in number) have no optical counterpart,
8 of them being invisible even at near infrared wavelengths. The relative
distribution of sources with and without optical counterpart in the IRAS
two-colour diagram and their characteristic near infrared colours are
interpreted as the consequence of the increasing thickness of their
circumstellar shells. Among the objects not detected at near infrared
wavelengths four non-variable sources are proposed to be heavily obscured
post-AGB stars which have just very recently left the AGB. Eight additional
objects with unusually bright and/or blue near-infrared colours are identified
as candidate post-AGB stars and/or proto-planetary nebulae.Comment: 28 pages, 9 figures, for associated finding charts see:
http://www.edpsciences.org/articles/aa/full/2005/08/aa1709/FINDING_CHARTS/are
cibo_index.htm
Contour dynamics model for electric discharges
A contour dynamics model for electrical discharges is obtained and analyzed.
The model is deduced as the asymptotic limit of the minimal streamer model for
the propagation of electric discharges, in the limit of small electron
diffusion. The dispersion relation for a non planar 2-D discharge is
calculated. The development and propagation of finger-like patterns are studied
and their main features quantified.Comment: 4 pages, 2 fi
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