106 research outputs found
On the effective velocity created by a point vortex in two-dimensional hydrodynamics
We complete previous investigations on the statistics of velocity
fluctuations arising from a random distribution of point vortices in
two-dimensional hydrodynamics. We show that, on a statistical sense, the
velocity created by a point vortex is shielded by cooperative effects on a
distance , the inter-vortex separation. For , the ``effective'' velocity decays as instead of the ordinary
law recovered for . These results are similar to those
obtained by Agekyan [Sov. Astron. 5 (1962) 809] in his investigations on the
fluctuations of the gravitational field. They give further support to our
previous observation that the statistics of velocity fluctuations are
(marginally) dominated by the contribution of the nearest neighbor.Comment: Submitted to Phys. Rev.
The nearest young moving groups
The latest results in the research of forming planetary systems have led
several authors to compile a sample of candidates for searching for planets in
the vicinity of the sun. Young stellar associations are indeed excellent
laboratories for this study, but some of them are not close enough to allow the
detection of planets through adaptive optics techniques. However, the existence
of very close young moving groups can solve this problem. Here we have compiled
the members of the nearest young moving groups, as well as a list of new
candidates from our catalogue of late-type stars possible members of young
stellar kinematic groups, studying their membership through spectroscopic and
photometric criteria.Comment: Latex file with 16 pages, 4 figures. Available at
http://www.ucm.es/info/Astrof/invest/actividad/skg/skg_sag.html Accepted for
publication in: The Astrophysical Journal (ApJ
Statistics of the gravitational force in various dimensions of space: from Gaussian to Levy laws
We discuss the distribution of the gravitational force created by a
Poissonian distribution of field sources (stars, galaxies,...) in different
dimensions of space d. In d=3, it is given by a Levy law called the Holtsmark
distribution. It presents an algebraic tail for large fluctuations due to the
contribution of the nearest neighbor. In d=2, it is given by a marginal
Gaussian distribution intermediate between Gaussian and Levy laws. In d=1, it
is exactly given by the Bernouilli distribution (for any particle number N)
which becomes Gaussian for N>>1. Therefore, the dimension d=2 is critical
regarding the statistics of the gravitational force. We generalize these
results for inhomogeneous systems with arbitrary power-law density profile and
arbitrary power-law force in a d-dimensional universe
Kinematic Control of the Inertiality of ICRS Catalogs
We perform a kinematic analysis of the Hipparcos and TRC proper motions of
stars by using a linear Ogorodnikov-Milne model. All of the distant (r more
than 0.2 kpc) stars of the Hipparcos catalog have been found to rotate around
the Galactic y axis with an angular velocity of -0.36 +/- 0.09 mas/year. One of
the causes of this rotation may be an uncertainty in the lunisolar precession
constant adopted when constructing the ICRS. In this case? the correction to
the IAU (1976) lunisolar precession constant in longitude is shown to be -3.26
+/- 0.10 mas/yr. Based on the TRC catalog, we have determined the main Oort
constants: A = 14.9 +/- 1.0 and B = -10.8 +/- 0.3 km/s/kpc. The component of
the model that describes the rotation of all TRC stars around the Galactic y
axis is nonzero for all magnitudes, My= -0.86 +/- 0.11 mas/yr.Comment: 10 pages, 4 figures, 2 table
Luminescence spectra and kinetics of disordered solid solutions
We have studied both theoretically and experimentally the luminescence spectra and kinetics of crystalline, disordered solid solutions after pulsed excitation. First, we present the model calculations of the steady-state luminescence band shape caused by recombination of excitons localized in the wells of random potential induced by disorder. Classification of optically active tail states of the main exciton band into two groups is proposed. The majority of the states responsible for the optical absorption corresponds to the group of extended states belonging to the percolation cluster, whereas only a relatively small group of âradiativeâ states forms the steady-state luminescence band. The continuum percolation theory is applied to distinguish the âradiativeâ localized states, which are isolated in space and have no ways for nonradiative transitions along the tail states. It is found that the analysis of the exciton-phonon interaction gives the information about the character of the localization of excitons. We have shown that the model used describes quite well the experimental cw spectra of CdS(1âc)Sec and ZnSe(1âc)Tec solid solutions. Further, the experimental results are presented for the temporal evolution of the luminescence band. It is shown that the changes of band shape with time come from the interplay of population dynamics of extended states and spatially isolated âradiativeâ states. Finally, the measurements of the decay of the spectrally integrated luminescence intensity at long delay times are presented. It is shown that the observed temporal behavior can be described in terms of relaxation of separated pairs followed by subsequent exciton formation and radiative recombination. Electron tunneling processes are supposed to be responsible for the luminescence in the long-time limit at excitation below the exciton mobility edge. At excitation by photons with higher energies the diffusion of electrons can account for the observed behavior of the luminescence
Statistical mechanics of two-dimensional vortices and stellar systems
The formation of large-scale vortices is an intriguing phenomenon in
two-dimensional turbulence. Such organization is observed in large-scale
oceanic or atmospheric flows, and can be reproduced in laboratory experiments
and numerical simulations. A general explanation of this organization was first
proposed by Onsager (1949) by considering the statistical mechanics for a set
of point vortices in two-dimensional hydrodynamics. Similarly, the structure
and the organization of stellar systems (globular clusters, elliptical
galaxies,...) in astrophysics can be understood by developing a statistical
mechanics for a system of particles in gravitational interaction as initiated
by Chandrasekhar (1942). These statistical mechanics turn out to be relatively
similar and present the same difficulties due to the unshielded long-range
nature of the interaction. This analogy concerns not only the equilibrium
states, i.e. the formation of large-scale structures, but also the relaxation
towards equilibrium and the statistics of fluctuations. We will discuss these
analogies in detail and also point out the specificities of each system.Comment: Chapter of the forthcoming "Lecture Notes in Physics" volume:
``Dynamics and Thermodynamics of Systems with Long Range Interactions'', T.
Dauxois, S. Ruffo, E. Arimondo, M. Wilkens Eds., Lecture Notes in Physics
Vol. 602, Springer (2002
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