5,483 research outputs found
R-local Delaunay inhibition model
Let us consider the local specification system of Gibbs point process with
inhib ition pairwise interaction acting on some Delaunay subgraph specifically
not con taining the edges of Delaunay triangles with circumscribed circle of
radius grea ter than some fixed positive real value . Even if we think that
there exists at least a stationary Gibbs state associated to such system, we do
not know yet how to prove it mainly due to some uncontrolled "negative"
contribution in the expression of the local energy needed to insert any number
of points in some large enough empty region of the space. This is solved by
introducing some subgraph, called the -local Delaunay graph, which is a
slight but tailored modification of the previous one. This kind of model does
not inherit the local stability property but satisfies s ome new extension
called -local stability. This weakened property combined with the local
property provides the existence o f Gibbs state.Comment: soumis \`{a} Journal of Statistical Physics 27 page
Orion revisited. II. The foreground population to Orion A
Following the recent discovery of a large population of young stars in front
of the Orion Nebula, we carried out an observational campaign with the DECam
wide-field camera covering ~10~deg^2 centered on NGC 1980 to confirm, probe the
extent of, and characterize this foreground population of pre-main-sequence
stars. We confirm the presence of a large foreground population towards the
Orion A cloud. This population contains several distinct subgroups, including
NGC1980 and NGC1981, and stretches across several degrees in front of the Orion
A cloud. By comparing the location of their sequence in various color-magnitude
diagrams with other clusters, we found a distance and an age of 380pc and
5~10Myr, in good agreement with previous estimates. Our final sample includes
2123 candidate members and is complete from below the hydrogen-burning limit to
about 0.3Msun, where the data start to be limited by saturation. Extrapolating
the mass function to the high masses, we estimate a total number of ~2600
members in the surveyed region. We confirm the presence of a rich, contiguous,
and essentially coeval population of about 2600 foreground stars in front of
the Orion A cloud, loosely clustered around NGC1980, NGC1981, and a new group
in the foreground of the OMC-2/3. For the area of the cloud surveyed, this
result implies that there are more young stars in the foreground population
than young stars inside the cloud. Assuming a normal initial mass function, we
estimate that between one to a few supernovae must have exploded in the
foreground population in the past few million years, close to the surface of
Orion A, which might be responsible, together with stellar winds, for the
structure and star formation activity in these clouds. This long-overlooked
foreground stellar population is of great significance, calling for a revision
of the star formation history in this region of the Galaxy.Comment: Accepted for publication in A&
Boltzmann and hydrodynamic description for self-propelled particles
We study analytically the emergence of spontaneous collective motion within
large bidimensional groups of self-propelled particles with noisy local
interactions, a schematic model for assemblies of biological organisms. As a
central result, we derive from the individual dynamics the hydrodynamic
equations for the density and velocity fields, thus giving a microscopic
foundation to the phenomenological equations used in previous approaches. A
homogeneous spontaneous motion emerges below a transition line in the
noise-density plane. Yet, this state is shown to be unstable against spatial
perturbations, suggesting that more complicated structures should eventually
appear.Comment: 4 pages, 3 figures, final versio
Simultaneous fine structure observation of wind and temperature profiles by the Arecibo 430-MHz radar and in situ measurements
A simultaneous campaign of balloon and radar measurements took place on March 14 to 16, 1984, above the Arecibo 430-MHz radar. This radar was operating with a vertical resolution of 150 m following two antenna beam directions: 15 deg. from the zenith, respectively, in the N-S and E-W directions. The main results concerning the comparison between the flight and simultaneous radar measurements obtained on March 15, 1984 are analyzed. The radar return power profile (S/N ratio in dB) exhibits maxima which are generally well correlated with step-like structures in the potential temperature profile. These structures are generally considered as the consequence of the mixing processes induced by the turbulence. A good correlation appears in the altitude range 12.5 to 19 km between wind shears induced by a wave structure observed in the meridional wind and the radar echo power maxima. This wave structure is characterized by a vertical wavelength of about 2.5 km, and a period in the range 30 to 40 hours. These characteristics are deduced from the twice daily rawinsonde data launched from the San Juan Airport by the National Weather Service. These results pointed out an example of the interaction between wave and turbulence in the upper troposphere and lower stratosphere. Turbulent layers are observed at locations where wind shears related to an internal inertia-gravity wave are maxima
Soft deformable self-propelled particles
In this work we investigate the collective behavior of self-propelled
particles that deform due to local pairwise interactions. We demonstrate that
this deformation alone can induce alignment of the velocity vectors. The onset
of collective motion is analyzed. Applying a Gaussian-core repulsion between
the particles, we find a transition to disordered non-collective motion under
compression. We here explain that this reflects the reentrant fluid behavior of
the general Gaussian-core model now applied to a self-propelled system.
Truncating the Gaussian potential can lead to cluster crystallization or more
disordered cluster states. For intermediate values of the Gaussian-core
potential we for the first time observe laning for deformable self-propelled
particles. Finally, without the core potential, but including orientational
noise, we connect our description to the Vicsek approach for self-propelled
particles with nematic alignment interactions.Comment: 6 pages, 7 figure
Dynamical Analysis of Nearby ClustErs. Automated astrometry from the ground: precision proper motions over wide field
The kinematic properties of the different classes of objects in a given
association hold important clues about its member's history, and offer a unique
opportunity to test the predictions of the various models of stellar formation
and evolution. DANCe (standing for Dynamical Analysis of Nearby ClustErs) is a
survey program aimed at deriving a comprehensive and homogeneous census of the
stellar and substellar content of a number of nearby (<1kpc) young (<500Myr)
associations. Whenever possible, members will be identified based on their
kinematics properties, ensuring little contamination from background and
foreground sources. Otherwise, the dynamics of previously confirmed members
will be studied using the proper motion measurements. We present here the
method used to derive precise proper motion measurements, using the Pleiades
cluster as a test bench. Combining deep wide field multi-epoch panchromatic
images obtained at various obervatories over up to 14 years, we derive accurate
proper motions for the sources present in the field of the survey. The datasets
cover ~80 square degrees, centered around the Seven Sisters. Using new tools,
we have computed a catalog of 6116907 unique sources, including proper motion
measurements for 3577478 of them. The catalogue covers the magnitude range
between i=12~24mag, achieving a proper motion accuracy <1mas/yr for sources as
faint as i=22.5mag. We estimate that our final accuracy reaches 0.3mas/yr in
the best cases, depending on magnitude, observing history, and the presence of
reference extragalactic sources for the anchoring onto the ICRS.Comment: Accepted for publication in A&
Linear and non linear response in the aging regime of the 1D trap model
We investigate the behaviour of the response function in the one dimensional
trap model using scaling arguments that we confirm by numerical simulations. We
study the average position of the random walk at time tw+t given that a small
bias h is applied at time tw. Several scaling regimes are found, depending on
the relative values of t, tw and h. Comparison with the diffusive motion in the
absence of bias allows us to show that the fluctuation dissipation relation is,
in this case, valid even in the aging regime.Comment: 5 pages, 3 figures, 3 references adde
Secular interactions between inclined planets and a gaseous disk
In a planetary system, a secular particle resonance occurs at a location
where the precession rate of a test particle (e.g. an asteroid) matches the
frequency of one of the precessional modes of the planetary system. We
investigate the secular interactions of a system of mutually inclined planets
with a gaseous protostellar disk that may contain a secular nodal particle
resonance. We determine the normal modes of some mutually inclined planet-disk
systems. The planets and disk interact gravitationally, and the disk is
internally subject to the effects of gas pressure, self-gravity, and turbulent
viscosity. The behavior of the disk at a secular resonance is radically
different from that of a particle, owing mainly to the effects of gas pressure.
The resonance is typically broadened by gas pressure to the extent that global
effects, including large-scale warps, dominate. The standard resonant torque
formula is invalid in this regime. Secular interactions cause a decay of the
inclination at a rate that depends on the disk properties, including its mass,
turbulent viscosity, and sound speed. For a Jupiter-mass planet embedded within
a minimum-mass solar nebula having typical parameters, dissipation within the
disk is sufficient to stabilize the system against tilt growth caused by
mean-motion resonances.Comment: 30 pages, 6 figures, to be published in The Astrophysical Journa
Real space analysis of inherent structures
We study a generalization of the one-dimensional disordered Potts model,
which exhibits glassy properties at low temperature. The real space properties
of inherent structures visited dynamically are analyzed through a decomposition
into domains over which the energy is minimized. The size of these domains is
distributed exponentially, defining a characteristic length scale which grows
in equilibrium when lowering temperature, as well as in the aging regime at a
given temperature. In the low temperature limit, this length can be interpreted
as the distance between `excited' domains within the inherent structures.Comment: 7 pages, 8 figures, final versio
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