10,226 research outputs found
Particle Dispersion on Rapidly Folding Random Hetero-Polymers
We investigate the dynamics of a particle moving randomly along a disordered
hetero-polymer subjected to rapid conformational changes which induce
superdiffusive motion in chemical coordinates. We study the antagonistic
interplay between the enhanced diffusion and the quenched disorder. The
dispersion speed exhibits universal behavior independent of the folding
statistics. On the other hand it is strongly affected by the structure of the
disordered potential. The results may serve as a reference point for a number
of translocation phenomena observed in biological cells, such as protein
dynamics on DNA strands.Comment: 4 pages, 4 figure
Potts models in the continuum. Uniqueness and exponential decay in the restricted ensembles
In this paper we study a continuum version of the Potts model. Particles are
points in R^d, with a spin which may take S possible values, S being at least
3. Particles with different spins repel each other via a Kac pair potential. In
mean field, for any inverse temperature there is a value of the chemical
potential at which S+1 distinct phases coexist. For each mean field pure phase,
we introduce a restricted ensemble which is defined so that the empirical
particles densities are close to the mean field values. Then, in the spirit of
the Dobrushin Shlosman theory, we get uniqueness and exponential decay of
correlations when the range of the interaction is large enough. In a second
paper, we will use such a result to implement the Pirogov-Sinai scheme proving
coexistence of S+1 extremal DLR measures.Comment: 72 pages, 1 figur
Inflation from D3-brane motion in the background of D5-branes
We study inflation arising from the motion of a BPS D3-brane in the
background of a stack of k parallel D5-branes. There are two scalar fields in
this set up-- (i) the radion field R, a real scalar field, and (ii) a complex
tachyonic scalar field chi living on the world volume of the open string
stretched between the D3 and D5 branes. We find that inflation is realized by
the potential of the radion field, which satisfies observational constraints
coming from the Cosmic Microwave Background. After the radion becomes of order
the string length scale l_s, the dynamics is governed by the potential of the
complex scalar field. Since this field has a standard kinematic term, reheating
can be successfully realized by the mechanism of tachyonic preheating with
spontaneous symmetry breaking.Comment: 10 pages, 4 figures. Minor clarifications and references added.
Version to appear in Phys. Rev.
Time-dependent appearance of myofibroblasts in granulation tissue of human skin wounds
Human skin wounds (66) inflicted between 20 h and 7 months prior to biopsy were studied. In order to identify the type of cellular differentiation of the fibroblastic cells in the granulation tissue, alpha-smooth muscle actin and desmin were immunohistochemically localized. The value of any presumed time-dependent appearance and/or disappearance of positively stained cells was tested for the estimation of wound age. In skin specimens with a wound age less than 5 days (n =15) no typical granulation tissue had developed and no alpha-actin-positive myofibroblasts could be detected. The first appearance of positively reacting myofibroblasts was noted in a 5-day-old wound. In 57% of the lesions with a wound age between 5 and 31 days (25 out of 44 cases) typical granulation tissue formation was present and myofibroblasts with positive reaction for alpha-smooth muscle actin could be identified. Numerous positively reacting cells could generally be found in wounds aged between 16 and 31 days, but also in wounds less than 16 days old. In 29% of the cases with a wound age of more than 31 days (2 out of 7 cases) alpha-sma-positive myofibroblasts also occured. Fibroblastic cells positive for desmin could not be seen at all in our series. Our results demonstrate the appearance of alpha-sma-positive myofibroblasts with the initial formation of typical granulation tissue in human skin lesions as early as approximately 5 days after wounding. In contrast to recent experimental results these cells remained detectable in wounds aged more than 2 months in some cases. The immunohistochemical detection of actin-positive cells, therefore, demonstrates whether an unknown skin wound is aged approximately 5 days or more. Even though a time-dependent decrease of myofibroblasts in human granulation tissue after 31 days in human wounds seems probable, the extended presence (up to about 2 months) of these cells allows no further exact age determination of older wounds
New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae
We present 22 new (+3 confirmed) cataclysmic variables (CVs) in the non
core-collapsed globular cluster 47 Tucanae (47 Tuc). The total number of CVs in
the cluster is now 43, the largest sample in any globular cluster so far. For
the identifications we used near-ultraviolet (NUV) and optical images from the
Hubble Space Telescope, in combination with X-ray results from the Chandra
X-ray Observatory. This allowed us to build the deepest NUV CV luminosity
function of the cluster to date. We found that the CVs in 47 Tuc are more
concentrated towards the cluster center than the main sequence turnoff stars.
We compared our results to the CV populations of the core-collapsed globular
clusters NGC 6397 and NGC 6752. We found that 47 Tuc has fewer bright CVs per
unit mass than those two other clusters. That suggests that dynamical
interactions in core-collapsed clusters play a major role creating new CVs. In
47 Tuc, the CV population is probably dominated by primordial and old
dynamically formed systems. We estimated that the CVs in 47 Tuc have total
masses of approx. 1.4 M_sun. We also found that the X-ray luminosity function
of the CVs in the three clusters is bimodal. Additionally, we discuss a
possible double degenerate system and an intriguing/unclassified object.
Finally, we present four systems that could be millisecond pulsar companions
given their X-ray and NUV/optical colors. For one of them we present very
strong evidence for being an ablated companion. The other three could be CO- or
He-WDs.Comment: Published on MNRAS. 31 Pages, 23 Figures, 5 Tables. Minor changes
with respect to previous arXiv versio
Long-term observations of the pulsars in 47 Tucanae. I. A study of four elusive binary systems
For the past couple of decades, the Parkes radio telescope has been regularly
observing the millisecond pulsars in 47 Tucanae (47 Tuc). This long-term timing
program was designed to address a wide range of scientific issues related to
these pulsars and the globular cluster where they are located. In this paper,
the first of a series, we address one of these objectives: the characterization
of four previously known binary pulsars for which no precise orbital parameters
were known, namely 47 Tuc P, V, W and X (pulsars 47 Tuc R and Y are discussed
elsewhere). We determined the previously unknown orbital parameters of 47 Tuc V
and X and greatly improved those of 47 Tuc P and W. For pulsars W and X we
obtained, for the first time, full coherent timing solutions across the whole
data span, which allowed a much more detailed characterization of these
systems. 47 Tuc W, a well-known tight eclipsing binary pulsar, exhibits a large
orbital period variability, as expected for a system of its class. 47 Tuc X
turns out to be in a wide, extremely circular, 10.9-day long binary orbit and
its position is ~3.8 arcmin away from the cluster center, more than three times
the distance of any other pulsar in 47 Tuc. These characteristics make 47 Tuc X
a very different object with respect to the other pulsars of the cluster.Comment: Accepted for publication by MNRAS, 18 pages, 11 figure
Crescimento diamétrico de maçaranduba (Manilkara Huberi Chevalier) após a colheita da madeira.
Conhecer o crescimento das espĂ©cies de um povoamento florestal, principalmente daquelas de interesse econĂ´mico, Ă© de fundamental importância para o manejo florestal. Nesse sentido, avaliou-se o processo dinâmico do crescimento diamĂ©trico da população de maçaranduba (Manilkara huberi Chevalier), no perĂodo de 1981-1997, em uma área de 64 hectares localizada na Floresta Nacional do TapajĂłs, área experimental sob responsabilidade da Embrapa AmazĂ´nia Oriental. Os dados foram coletados em 36 parcelas permanentes de 0,25ha (50mx50m), divididas em 25 subparcelas de 10mx10m, onde foram medidas e identificadas todas as árvores com DAP (diâmetro a 1,30 do solo) e 5cm. A espĂ©cie teve, no perĂodo de 1981-1997, crescimento mĂ©dio em diâmetro de 0,39cm/ano. As árvores cujas copas receberam iluminação total cresceram 0,67cm/ano, superior Ă quelas que receberam iluminação parcial (0,58cm/ano) e Ă s que estavam totalmente sombreadas (0,26cm/ano). Portanto, as árvores necessitam de iluminação para acelerar seu crescimento. As árvores sem cipĂłs na copa cresceram em mĂ©dia 0,60cm/ano, enquanto que aquelas com cipĂłs cresceram 0,45cm/ano, indicando que a infestação de cipĂłs nas árvores afeta o seu crescimento.DisponĂvel tambĂ©m on-line
Mean first-passage time of surface-mediated diffusion in spherical domains
We present an exact calculation of the mean first-passage time to a target on
the surface of a 2D or 3D spherical domain, for a molecule alternating phases
of surface diffusion on the domain boundary and phases of bulk diffusion. The
presented approach is based on an integral equation which can be solved
analytically. Numerically validated approximation schemes, which provide more
tractable expressions of the mean first-passage time are also proposed. In the
framework of this minimal model of surface-mediated reactions, we show
analytically that the mean reaction time can be minimized as a function of the
desorption rate from the surface.Comment: to appear in J. Stat. Phy
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