4,246 research outputs found
Exploring Halo Substructure with Giant Stars IV: The Extended Structure of the Ursa Minor Dwarf Spheroidal
We present a large area photometric survey of the Ursa Minor dSph. We
identify UMi giant star candidates extending to ~3 deg from the center of the
dSph. Comparison to previous catalogues of stars within the tidal radius of UMi
suggests that our photometric luminosity classification is 100% accurate. Over
a large fraction of the survey area, blue horizontal branch stars associated
with UMi can also be identified. The spatial distribution of both the UMi giant
stars and the BHB stars are remarkably similar, and a large fraction of both
samples of stars are found outside the tidal radius of UMi. An isodensity
contour map of the stars within the tidal radius of UMi reveals two
morphological peculiarities: (1) The highest density of dSph stars is offset
from the center of symmetry of the outer isodensity contours. (2) The overall
shape of the outer contours appear S-shaped. We find that previously determined
King profiles with ~50' tidal radii do not fit well the distribution of our UMi
stars. A King profile with a larger tidal radius produces a reasonable fit,
however a power law with index -3 provides a better fit for radii > 20'. The
existence of UMi stars at large distances from the core of the galaxy, the
peculiar morphology of the dSph within its tidal radius, and the shape of its
surface density profile all suggest that UMi is evolving significantly due to
the tidal influence of the Milky Way. However, the photometric data on UMi
stars alone does not allow us to determine if the candidate extratidal stars
are now unbound or if they remain bound to the dSph within an extended dark
matter halo. (Abridged)Comment: accepted by AJ, 32 pages, 15 figures, emulateapj5 styl
A spatial data handling system for retrieval of images by unrestricted regions of user interest
The Intelligent Data Management (IDM) project at NASA/Goddard Space Flight Center has prototyped an Intelligent Information Fusion System (IIFS), which automatically ingests metadata from remote sensor observations into a large catalog which is directly queryable by end-users. The greatest challenge in the implementation of this catalog was supporting spatially-driven searches, where the user has a possible complex region of interest and wishes to recover those images that overlap all or simply a part of that region. A spatial data management system is described, which is capable of storing and retrieving records of image data regardless of their source. This system was designed and implemented as part of the IIFS catalog. A new data structure, called a hypercylinder, is central to the design. The hypercylinder is specifically tailored for data distributed over the surface of a sphere, such as satellite observations of the Earth or space. Operations on the hypercylinder are regulated by two expert systems. The first governs the ingest of new metadata records, and maintains the efficiency of the data structure as it grows. The second translates, plans, and executes users' spatial queries, performing incremental optimization as partial query results are returned
Simulating Hard Rigid Bodies
Several physical systems in condensed matter have been modeled approximating
their constituent particles as hard objects. The hard spheres model has been
indeed one of the cornerstones of the computational and theoretical description
in condensed matter. The next level of description is to consider particles as
rigid objects of generic shape, which would enrich the possible phenomenology
enormously. This kind of modeling will prove to be interesting in all those
situations in which steric effects play a relevant role. These include biology,
soft matter, granular materials and molecular systems. With a view to
developing a general recipe for event-driven Molecular Dynamics simulations of
hard rigid bodies, two algorithms for calculating the distance between two
convex hard rigid bodies and the contact time of two colliding hard rigid
bodies solving a non-linear set of equations will be described. Building on
these two methods, an event-driven molecular dynamics algorithm for simulating
systems of convex hard rigid bodies will be developed and illustrated in
details. In order to optimize the collision detection between very elongated
hard rigid bodies, a novel nearest-neighbor list method based on an oriented
bounding box will be introduced and fully explained. Efficiency and performance
of the new algorithm proposed will be extensively tested for uniaxial hard
ellipsoids and superquadrics. Finally applications in various scientific fields
will be reported and discussed.Comment: 36 pages, 17 figure
Neotectonics of the Sumatran fault, Indonesia
The 1900-km-long, trench-parallel Sumatran fault accommodates a significant amount of the right-lateral component of oblique convergence between the Eurasian and Indian/Australian plates from 10°N to 7°S. Our detailed map of the fault, compiled from topographic maps and stereographic aerial photographs, shows that unlike many other great strike-slip faults, the Sumatran fault is highly segmented. Cross-strike width of step overs between the 19 major subaerial segments is commonly many kilometers. The influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry. Geomorphic offsets along the fault range as high as ~20 km and may represent the total offset across the fault. If this is so, other structures must have accommodated much of the dextral component of oblique convergence during the past few million years. Our analysis of stretching of the forearc region, near the southern tip of Sumatra, constrains the combined dextral slip on the Sumatran and Mentawai faults to be no more than 100 km in the past few million years. The shape and location of the Sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere. Nonetheless, active volcanic centers of the Sumatran volcanic arc have not influenced noticeably the geometry of the active Sumatran fault. On the basis of its geologic history and pattern of deformation, we divide the Sumatran plate margin into northern, central and southern domains. We support previous proposals that the geometry and character of the subducting Investigator fracture zone are affecting the shape and evolution of the Sumatran fault system within the central domain. The southern domain is the most regular. The Sumatran fault there comprises six right-stepping segments. This pattern indicates that the overall trend of the fault deviates 4° clockwise from the slip vector between the two blocks it separates. The regularity of this section and its association with the portion of the subduction zone that generated the giant (M_w 9) earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike-slip faulting and unusually large subduction earthquakes
Event-Driven Simulation of the Dynamics of Hard Ellipsoids
We introduce a novel algorithm to perform event-driven simulations of hard
rigid bodies of arbitrary shape, that relies on the evaluation of the geometric
distance. In the case of a monodisperse system of uniaxial hard ellipsoids,we
perform molecular dynamics simulations varying the aspect-ratio X0 and the
packing fraction phi. We evaluate the translational Dtrans and the rotational
Drot diffusion coefficient and the associated isodiffusivity lines in the
phi-X0 plane. We observe a decoupling of the translational and rotational
dynamics which generates an almost perpendicular crossing of the Dtrans and
Drot isodiffusivity lines. While the self intermediate scattering function
exhibits stretched relaxation, i.e. glassy dynamics, only for large phi and X0
about equals to 1, the second order orientational correlator C2(t) shows
stretching only for large and small X0 values. We discuss these findings in the
context of a possible pre-nematic order driven glass transition.Comment: Proceedings of IWCS2007 Sendai (Japan
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