47,669 research outputs found
A Note on the Relativistic Covariance of the Cyclic Relations
It is shown that the Evans-Vigier modified electrodynamics is compatible with
the Relativity Theory.Comment: ReVTeX file, 14pp., no figure
Condensation for a fixed number of independent random variables
A family of m independent identically distributed random variables indexed by
a chemical potential \phi\in[0,\gamma] represents piles of particles. As \phi
increases to \gamma, the mean number of particles per site converges to a
maximal density \rho_c<\infty. The distribution of particles conditioned on the
total number of particles equal to n does not depend on \phi (canonical
ensemble). For fixed m, as n goes to infinity the canonical ensemble measure
behave as follows: removing the site with the maximal number of particles, the
distribution of particles in the remaining sites converges to the grand
canonical measure with density \rho_c; the remaining particles concentrate
(condensate) on a single site.Comment: 6 page
The Magellanic Bridge cluster NGC 796: Deep optical AO imaging reveals the stellar content and initial mass function of a massive open cluster
NGC 796 is a massive young cluster located 59 kpc from us in the diffuse
intergalactic medium of the 1/5-1/10 Magellanic Bridge, allowing to
probe variations in star formation and stellar evolution processes as a
function of metallicity in a resolved fashion, providing a link between
resolved studies of nearby solar-metallicity and unresolved distant metal-poor
clusters located in high-redshift galaxies. In this paper, we present adaptive
optics H imaging of NGC 796 (at 0.5", which is ~0.14 pc at the
cluster distance) along with optical spectroscopy of two bright members to
quantify the cluster properties. Our aim is to explore if star formation and
stellar evolution varies as a function of metallicity by comparing the
properties of NGC 796 to higher metallicity clusters. We find from isochronal
fitting of the cluster main sequence in the colour-magnitude diagram an age of
20 Myr. Based on the cluster luminosity function, we derive a
top-heavy stellar initial mass function (IMF) with a slope =
1.990.2, hinting at an metallicity and/or environmental dependence of the
IMF which may lead to a top-heavy IMF in the early Universe. Study of the
H emission line stars reveals that Classical Be stars constitute a
higher fraction of the total B-type stars when compared with similar clusters
at greater metallicity, providing some support to the chemically homogeneous
theory of stellar evolution. Overall, NGC 796 has a total estimated mass of
990 , and a core radius of 1.40.3 pc which classifies
it as a massive young open cluster, unique in the diffuse interstellar medium
of the Magellanic Bridge.Comment: Accepted for publication in the Astrophysical Journal. Contains 14
pages, 11 figures, and 3 table
Hard rod gas with long-range interactions: Exact predictions for hydrodynamic properties of continuum systems from discrete models
One-dimensional hard rod gases are explicitly constructed as the limits of
discrete systems: exclusion processes involving particles of arbitrary length.
Those continuum many-body systems in general do not exhibit the same
hydrodynamic properties as the underlying discrete models. Considering as
examples a hard rod gas with additional long-range interaction and the
generalized asymmetric exclusion process for extended particles (-ASEP),
it is shown how a correspondence between continuous and discrete systems must
be established instead. This opens up a new possibility to exactly predict the
hydrodynamic behaviour of this continuum system under Eulerian scaling by
solving its discrete counterpart with analytical or numerical tools. As an
illustration, simulations of the totally asymmetric exclusion process
(-TASEP) are compared to analytical solutions of the model and applied to
the corresponding hard rod gas. The case of short-range interaction is treated
separately.Comment: 19 pages, 8 figure
On the Ehrenfest theorem of quantum mechanics
We give a mathematically rigorous derivation of Ehrenfest's equations for the
evolution of position and momentum expectation values, under general and
natural assumptions which include atomic and molecular Hamiltonians with
Coulomb interactions.Comment: To appear in J. Math. Phy
Axisymmetric Self-Similar Equilibria of Self-Gravitating Isothermal Systems
All axisymmetric self-similar equilibria of self-gravitating, rotating,
isothermal systems are identified by solving the nonlinear Poisson equation
analytically. There are two families of equilibria: (1) Cylindrically symmetric
solutions in which the density varies with cylindrical radius as R^(-alpha),
with 0 <= alpha <= 2. (2) Axially symmetric solutions in which the density
varies as f(theta)/r^2, where `r' is the spherical radius and `theta' is the
co-latitude. The singular isothermal sphere is a special case of the latter
class with f(theta)=constant. The axially symmetric equilibrium configurations
form a two-parameter family of solutions and include equilibria which are
surprisingly asymmetric with respect to the equatorial plane. The asymmetric
equilibria are, however, not force-free at the singular points r=0, infinity,
and their relevance to real systems is unclear. For each hydrodynamic
equilibrium, we determine the phase-space distribution of the collisionless
analog.Comment: 13 pages, 7 figures, uses emulateapj.sty. Submitted to Ap
Neuropilins 1 and 2 mediate neointimal hyperplasia and re-endothelialization following arterial injury
AIMS: Neuropilins 1 and 2 (NRP1 and NRP2) play crucial roles in endothelial cell migration contributing to angiogenesis and vascular development. Both NRPs are also expressed by cultured vascular smooth muscle cells (VSMCs) and are implicated in VSMC migration stimulated by PDGF-BB, but it is unknown whether NRPs are relevant for VSMC function in vivo. We investigated the role of NRPs in the rat carotid balloon injury model, in which endothelial denudation and arterial stretch induce neointimal hyperplasia involving VSMC migration and proliferation. METHODS AND RESULTS: NRP1 and NRP2 mRNAs and proteins increased significantly following arterial injury, and immunofluorescent staining revealed neointimal NRP expression. Down-regulation of NRP1 and NRP2 using shRNA significantly reduced neointimal hyperplasia following injury. Furthermore, inhibition of NRP1 by adenovirally overexpressing a loss-of-function NRP1 mutant lacking the cytoplasmic domain (ΔC) reduced neointimal hyperplasia, whereas wild-type (WT) NRP1 had no effect. NRP-targeted shRNAs impaired, while overexpression of NRP1 WT and NRP1 ΔC enhanced, arterial re-endothelialization 14 days after injury. Knockdown of either NRP1 or NRP2 inhibited PDGF-BB-induced rat VSMC migration, whereas knockdown of NRP2, but not NRP1, reduced proliferation of cultured rat VSMC and neointimal VSMC in vivo. NRP knockdown also reduced the phosphorylation of PDGFα and PDGFβ receptors in rat VSMC, which mediate VSMC migration and proliferation. CONCLUSION: NRP1 and NRP2 play important roles in the regulation of neointimal hyperplasia in vivo by modulating VSMC migration (via NRP1 and NRP2) and proliferation (via NRP2), independently of the role of NRPs in re-endothelialization
Design and simulation of InGaAs/AlAsSb quantum-cascade lasers for short wavelength emission
The design and simulation of an In-0.53Ga-0.47As/Al-0.56As-0.44Sb quantum-cascade laser emitting in the near infrared is presented. Designed using a self-consistent rate equation solver coupled with an energy balance rate equation, the proposed laser has a calculated population inversion of ~20% at 77 K and sufficient gain to achieve room-temperature laser emission at λ ~2.8 µm. Threshold currents in the range 4–8 kA/cm2 are estimated as the temperature increases from 77 K to 300 K. The output characteristics of the proposed laser are compared to an existing λ ~3.1 µm In-0.53Ga-0.47As/Al-0.56As-0.44Sb quantum-cascade structure presented in the literature
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