15,878 research outputs found
Two novel classes of solvable many-body problems of goldfish type with constraints
Two novel classes of many-body models with nonlinear interactions "of
goldfish type" are introduced. They are solvable provided the initial data
satisfy a single constraint (in one case; in the other, two constraints): i.
e., for such initial data the solution of their initial-value problem can be
achieved via algebraic operations, such as finding the eigenvalues of given
matrices or equivalently the zeros of known polynomials. Entirely isochronous
versions of some of these models are also exhibited: i.e., versions of these
models whose nonsingular solutions are all completely periodic with the same
period.Comment: 30 pages, 2 figure
Accurate and automatic NOAA-AVHRR image navigation using a global contour matching approach
The problem of precise and automatic AVHRR image navigation is tractable in theory, but has proved to be somewhat difficult in practice. The authors' work has been motivated by the need for a fully automatic and operational navigation system capable of geo-referencing NOAA-AVHRR images with high accuracy and without operator supervision. The proposed method is based on the simultaneous use of an orbital model and a contour matching approach. This last process, relying on an affine transformation model, is used to correct the errors caused by inaccuracies in orbit modeling, nonzero value for the spacecraft's roll, pitch and yaw, errors due to inaccuracies in the satellite positioning and failures in the satellite internal clock. The automatic global contour matching process is summarized as follows: i) Estimation of the gradient energy map (edges) in the sensed image and detection of the cloudless (reliable) areas in this map. ii) Initialization of the affine model parameters by minimizing the Euclidean distance between the reference and sensed images objects. iii) Simultaneous optimization of all reference image contours on the sensed image by energy minimization in the domain of the global transformation parameters. The process is iterated in a hierarchical way, reducing the parameter searching space at each iteration. The proposed image navigation algorithm has proved to be capable of geo-referencing a satellite image within 1 pixel.Peer ReviewedPostprint (published version
Analytic estimates and topological properties of the weak stability boundary
The weak stability boundary (WSB) is the transition region of the phase space where the change from gravitational escape to ballistic capture occurs. Studies on this complicated region of chaotic motion aim to investigate its unique, fuel saving properties to enlarge the frontiers of low energy transfers. This “fuzzy stability” region is characterized by highly sensitive motion, and any analysis of it has been carried out almost exclusively using numerical methods. On the contrary this paper presents, for the planar circular restricted 3 body problem (PCR3BP), 1) an analytic definition of the WSB which is coherent with the known algorithmic definitions; 2) a precise description of the topology of the WSB; 3) analytic estimates on the “stable region” (nearby the smaller primary) whose boundary is, by definition, the WSB
Understanding the spiral structure of the Milky Way using the local kinematic groups
We study the spiral arm influence on the solar neighbourhood stellar
kinematics. As the nature of the Milky Way (MW) spiral arms is not completely
determined, we study two models: the Tight-Winding Approximation (TWA) model,
which represents a local approximation, and a model with self-consistent
material arms named PERLAS. This is a mass distribution with more abrupt
gravitational forces. We perform test particle simulations after tuning the two
models to the observational range for the MW spiral arm properties. We explore
the effects of the arm properties and find that a significant region of the
allowed parameter space favours the appearance of kinematic groups. The
velocity distribution is mostly sensitive to the relative spiral arm phase and
pattern speed. In all cases the arms induce strong kinematic imprints for
pattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no
substructure is induced close to corotation. The groups change significantly if
one moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The
appearance time of each group is different, ranging from 0 to more than 1 Gyr.
Recent spiral arms can produce strong kinematic structures. The stellar
response to the two potential models is significantly different near the Sun,
both in density and kinematics. The PERLAS model triggers more substructure for
a larger range of pattern speed values. The kinematic groups can be used to
reduce the current uncertainty about the MW spiral structure and to test
whether this follows the TWA. However, groups such as the observed ones in the
solar vicinity can be reproduced by different parameter combinations. Data from
velocity distributions at larger distances are needed for a definitive
constraint.Comment: 18 pages, 21 figures, 4 tables; acccepted for publication in MNRA
The formation of planetary disks and winds: an ultraviolet view
Planetary systems are angular momentum reservoirs generated during star
formation. This accretion process produces very powerful engines able to drive
the optical jets and the molecular outflows. A fraction of the engine energy is
released into heating thus the temperature of the engine ranges from the 3000K
of the inner disk material to the 10MK in the areas where magnetic reconnection
occurs. There are important unsolved problems concerning the nature of the
engine, its evolution and the impact of the engine in the chemical evolution of
the inner disk. Of special relevance is the understanding of the shear layer
between the stellar photosphere and the disk; this layer controls a significant
fraction of the magnetic field building up and the subsequent dissipative
processes ougth to be studied in the UV.
This contribution focus on describing the connections between 1 Myr old suns
and the Sun and the requirements for new UV instrumentation to address their
evolution during this period. Two types of observations are shown to be needed:
monitoring programmes and high resolution imaging down to, at least,
milliarsecond scales.Comment: Accepted for publication in Astrophysics and Space Science 9 figure
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