8,554 research outputs found
Red Runaways II: Low mass Hills stars in SDSS Stripe 82
Stars ejected from the Galactic centre can be used to place important
constraints on the Milky Way potential. Since existing hypervelocity stars are
too distant to accurately determine orbits, we have conducted a search for
nearby candidates using full three-dimensional velocities. Since the efficacy
of such studies are often hampered by deficiencies in proper motion catalogs,
we have chosen to utilize the reliable, high-precision SDSS Stripe 82 proper
motion catalog. Although we do not find any candidates which have velocities in
excess of the escape speed, we identify 226 stars on orbits that are consistent
with Galactic centre ejection. This number is significantly larger than what we
would expect for halo stars on radial orbits and cannot be explained by disk or
bulge contamination. If we restrict ourselves to metal-rich stars, we find 29
candidates with [Fe/H] > -0.8 dex and 10 with [Fe/H] > -0.6 dex. Their
metallicities are more consistent with what we expect for bulge ejecta, and so
we believe these candidates are especially deserving of further study. We have
supplemented this sample using our own radial velocities, developing an
algorithm to use proper motions for optimizing candidate selection. This
technique provides considerable improvement on the blind spectroscopic sample
of SDSS, being able to identify candidates with an efficiency around 20 times
better than a blind search.Comment: 13 pages, accepted for publication in Ap
Nearby Low-Mass Hypervelocity Stars
Hypervelocity stars are those that have speeds exceeding the escape speed and
are hence unbound from the Milky Way. We investigate a sample of low-mass
hypervelocity candidates obtained using data from the high-precision SDSS
Stripe 82 catalogue, which we have combined with spectroscopy from the 200-inch
Hale Telescope at Palomar Observatory. We find four good candidates, but
without metallicities it is difficult to pin-down their distances and therefore
total velocities. Our best candidate has a significant likelihood that it is
escaping the Milky Way for a wide-range of metallicities.Comment: 5 pages; Contribution to proceedings for "The Milky Way Unravelled by
Gaia" conference, Barcelona, Dec 201
LIFE-CYCLE COST ANALYSIS OF REINFORCED CONCRETE BRIDGES REHABILITATED WITH CFRP
The deterioration of highway bridges and structures and the cost of repairing, rehabilitating, or replacing deteriorated structures is a major issue for bridge owners. An aging infrastructure as well as the need to upgrade structural capacity for heavier trucks adds to problem. Life-cycle cost analysis (LCCA) is a useful tool for determining when the deployment of fiber-reinforced polymer (FRP) composite components is an economically viable alternative for rehabilitating deteriorated concrete bridges.
The use of LCCA in bridge design and rehabilitation has been limited. The use of LCCA for bridges on a project level basis has often been limited to the non-routine design of major bridges where the life-cycle cost model is customized.
LCCA has historically been deterministic. The deterministic analysis uses discrete values for inputs and is fairly simple and easy to do. It does not give any indication of risk, i.e. the probability that the input values used in the analysis and the resulting life-cycle cost will actually occur.
Probabilistic analysis accounts for uncertainty and variability in input variables. It requires more effort than a deterministic analysis because probability distribution functions are required, random sampling is used, and a large number of iterations of the life-cycle cost calculations are carried out. The data needed is often not available.
The significance of this study lies in its identification of the parameters that had the most influence on life-cycle costs of concrete bridge and how those parameters interacted. The parameters are: (1) Time to construct the new bridge; (2) traffic volume under bridge (when applicable); (3) value of time for cars; and (4) delay time under the bridge during new bridge construction (when applicable). Using these parameters the analyst can now “simulate” a probabilistic analysis by using the deterministic approach and reducing the number of iterations. This study also extended the use of LCCA to bridge rehabilitations and to bridges with low traffic volumes. A large number of bridges in the United States have low traffic volumes. For the highway bridge considered in the parametric study, rehabilitation using FRP had a lower life-cycle cost when compared to the new bridge alternative
Remote Sensing Information Sciences Research Group: Santa Barbara Information Sciences Research Group, year 4
Information Sciences Research Group (ISRG) research continues to focus on improving the type, quantity, and quality of information which can be derived from remotely sensed data. Particular focus in on the needs of the remote sensing research and application science community which will be served by the Earth Observing System (EOS) and Space Station, including associated polar and co-orbiting platforms. The areas of georeferenced information systems, machine assisted information extraction from image data, artificial intelligence and both natural and cultural vegetation analysis and modeling research will be expanded
Remote sensing information sciences research group
Research conducted under this grant was used to extend and expand existing remote sensing activities at the University of California, Santa Barbara in the areas of georeferenced information systems, matching assisted information extraction from image data and large spatial data bases, artificial intelligence, and vegetation analysis and modeling. The research thrusts during the past year are summarized. The projects are discussed in some detail
Remote Sensing Information Sciences Research Group, year four
The needs of the remote sensing research and application community which will be served by the Earth Observing System (EOS) and space station, including associated polar and co-orbiting platforms are examined. Research conducted was used to extend and expand existing remote sensing research activities in the areas of georeferenced information systems, machine assisted information extraction from image data, artificial intelligence, and vegetation analysis and modeling. Projects are discussed in detail
The Inability of Ambipolar Diffusion to set a Characteristic Mass Scale in Molecular Clouds
We investigate the question of whether ambipolar diffusion (ion-neutral
drift) determines the smallest length and mass scale on which structure forms
in a turbulent molecular cloud. We simulate magnetized turbulence in a mostly
neutral, uniformly driven, turbulent medium, using a three-dimensional,
two-fluid, magnetohydrodynamics (MHD) code modified from Zeus-MP. We find that
substantial structure persists below the ambipolar diffusion scale because of
the propagation of compressive slow MHD waves at smaller scales. Contrary to
simple scaling arguments, ambipolar diffusion thus does not suppress structure
below its characteristic dissipation scale as would be expected for a classical
diffusive process. We have found this to be true for the magnetic energy,
velocity, and density. Correspondingly, ambipolar diffusion leaves the clump
mass spectrum unchanged. Ambipolar diffusion appears unable to set a
characteristic scale for gravitational collapse and star formation in turbulent
molecular clouds.Comment: 16 pages, 5 figures. ApJ accepte
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