2,649 research outputs found
Dark matter, neutron stars and strange quark matter
We show that self-annihilating neutralino WIMP dark matter accreted onto
neutron stars may provide a mechanism to seed compact objects with long-lived
lumps of strange quark matter, or strangelets, for WIMP masses above a few GeV.
This effect may trigger a conversion of most of the star into a strange star.
We use an energy estimate for the long-lived strangelet based on the Fermi gas
model combined with the MIT bag model to set a new limit on the possible values
of the WIMP mass that can be especially relevant for subdominant species of
massive neutralinos.Comment: 5 pages, 2 figures, accepted for publication in Phys. Rev. Let
An Evaluation of Uranium-Series Dating of Fossil Echinoids from Southern California Pleistocene Marine Terraces
Fossil sea urchins (Strongylocentrotus) from Pleistocene marine terraces on the southern California Channel Islands have been dated by the uranium-series method in order to test the suitability of echinoids for dating marine terraces. Results indicate that urchin plates and spines do not behave as closed systems with respect to both uranium and thorium. Calculated ages based on these data do not agree with uranium-series ages (120,000 and 127,000 yrs) obtained previously from corals from the same localities. Thus, fossil sea urchins (Strongylocentrotus) are not considered suitable for uranium-series dating of Pleistocene marine terrace deposits
Voltage Stability Analysis of Grid-Connected Wind Farms with FACTS: Static and Dynamic Analysis
Recently, analysis of some major blackouts and failures of power system shows that voltage instability problem has been one of the main reasons of these disturbances and networks collapse. In this paper, a systematic approach to voltage stability analysis using various techniques for the IEEE 14-bus case study, is presented. Static analysis is used to analyze the voltage stability of the system under study, whilst the dynamic analysis is used to evaluate the performance of compensators. The static techniques used are Power Flow, VâP curve analysis, and QâV modal analysis. In this study, Flexible Alternating Current Transmission system (FACTS) devices- namely, Static Synchronous Compensators (STATCOMs) and Static Var Compensators (SVCs) - are used as reactive power compensators, taking into account maintaining the violated voltage magnitudes of the weak buses within the acceptable limits defined in ANSI C84.1. Simulation results validate that both the STATCOMs and the SVCs can be effectively used to enhance the static voltage stability and increasing network loadability margin. Additionally, based on the dynamic analysis results, it has been shown that STATCOMs have superior performance, in dynamic voltage stability enhancement, compared to SVCs
Tracing out the Northern Tidal Stream of the Sagittarius Dwarf Spheoridal Galaxy
The main aim of this paper is to report two new detections of tidal debris in
the northern stream of the Sagittarius dwarf galaxy located at 45 arcdeg and 55
arcdeg from the center of galaxy. Our observational approach is based on deep
color-magnitude diagrams, that provides accurate distances, surface brightness
and the properties of stellar population of the studied region of this tidal
stream. The derived distances for these tidal debris wraps are 45 kpc and 54
kpc respectively.We also confirm these detections with numerical simulations of
the Sagittarius dwarf plus the Milky Way. The model reproduces the present
position and velocity of the Sagittarius main body and presents a long tidal
stream formed by tidal interaction with the Milky Way potential. This model is
also in good agreement with the available observations of the Sagittarius tidal
stream. We also present a method for estimating the shape of the Milky Way halo
potential using numerical simulations. From our simulations we obtain an
oblateness of the Milky Way dark halo potential of 0.85, using the current
database of distances and radial velocities of the Sagittarius tidal stream.
The color-magnitude diagram of the apocenter of Sagittarius shows that this
region of the stream shares the complex star formation history observed in the
main body of the galaxy. We present the first evidence for a gradient in the
stellar population along the stream, possibly correlated with its different
pericenter passages. (abridged)Comment: 43 pages (including 15 figures; for high resolution color figures,
please contact [email protected]). Submitted to Ap
Synthesis of Spherical 4R Mechanism for Path Generation using Differential Evolution
The problem of path generation for the spherical 4R mechanism is solved using
the Differential Evolution algorithm (DE). Formulas for the spherical geodesics
are employed in order to obtain the parametric equation for the generated
trajectory. Direct optimization of the objective function gives the solution to
the path generation task without prescribed timing. Therefore, there is no need
to separate this task into two stages to make the optimization. Moreover, the
order defect problem can be solved without difficulty by means of manipulations
of the individuals in the DE algorithm. Two examples of optimum synthesis
showing the simplicity and effectiveness of this approach are included.Comment: Submitted to Mechanism and Machine Theor
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