7,345 research outputs found
The detection of extragalactic N: Consequences for nitrogen nucleosynthesis and chemical evolution
Detections of extragalactic N are reported from observations of the
rare hydrogen cyanide isotope HCN toward the Large Magellanic Cloud
(LMC) and the core of the (post-) starburst galaxy NGC 4945. Accounting for
optical depth effects, the LMC data from the massive star-forming region N113
infer a N ratio of 111 17, about twice the C
value. For the LMC star-forming region N159HW and for the central region of NGC
4945, N ratios are also 100. The N ratios
are smaller than all interstellar nitrogen isotope ratios measured in the disk
and center of the Milky Way, strongly supporting the idea that N is
predominantly of `primary' nature, with massive stars being its dominant
source. Although this appears to be in contradiction with standard stellar
evolution and nucleosynthesis calculations, it supports recent findings of
abundant N production due to rotationally induced mixing of protons into
the helium-burning shells of massive stars.Comment: 15 pages including one postscript figure, accepted for publication by
ApJ Letter, further comments: please contact Yi-nan Chi
Stability of Horava-Lifshitz Black Holes in the Context of AdS/CFT
The anti--de Sitter/conformal field theory (AdS/CFT) correspondence is a
powerful tool that promises to provide new insights toward a full understanding
of field theories under extreme conditions, including but not limited to
quark-gluon plasma, Fermi liquid and superconductor. In many such applications,
one typically models the field theory with asymptotically AdS black holes.
These black holes are subjected to stringy effects that might render them
unstable. Ho\v{r}ava-Lifshitz gravity, in which space and time undergo
different transformations, has attracted attentions due to its power-counting
renormalizability. In terms of AdS/CFT correspondence, Ho\v{r}ava-Lifshitz
black holes might be useful to model holographic superconductors with Lifshitz
scaling symmetry. It is thus interesting to study the stringy stability of
Ho\v{r}ava-Lifshitz black holes in the context of AdS/CFT. We find that
uncharged topological black holes in Ho\v{r}ava-Lifshitz theory are
nonperturbatively stable, unlike their counterparts in Einstein gravity, with
the possible exceptions of negatively curved black holes with detailed balance
parameter close to unity. Sufficiently charged flat black holes for
close to unity, and sufficiently charged positively curved black
holes with close to zero, are also unstable. The implication to the
Ho\v{r}ava-Lifshitz holographic superconductor is discussed.Comment: 15 pages, 6 figures. Updated version accepted by Phys. Rev. D, with
corrections to various misprints. References update
Anharmonicity Induced Resonances for Ultracold Atoms and their Detection
When two atoms interact in the presence of an anharmonic potential, such as
an optical lattice, the center of mass motion cannot be separated from the
relative motion. In addition to generating a confinement-induced resonance (or
shifting the position of an existing Feshbach resonance), the external
potential changes the resonance picture qualitatively by introducing new
resonances where molecular excited center of mass states cross the scattering
threshold. We demonstrate the existence of these resonances, give their
quantitative characterization in an optical superlattice, and propose an
experimental scheme to detect them through controlled sweeping of the magnetic
field.Comment: 6 pages, 5 figures; expanded presentatio
Dynamic Soil-Foundation-Structure Interaction Analyses of Large Caissons
Large cellular reinforced concrete caissons exist as foundations of major long-span bridges across waterways in many parts of the country. This study was conducted to evaluate the important factors affecting the seismic response of large caissons. The paper presents the results of equivalent linear and non-linear analyses performed for a typical caisson idealized based on the cellular caisson at Pier W3 of the West San Francisco Bay Bridge subject to ground motion with a peak rock acceleration of 0.6 g. This caisson is 38.7 m (127 fi) long by 22.9 m (75 ft) wide submerged in about 32.6 m (107 ft) of water. It is embedded in 33.5 m (110 fi) of soil deposits and is founded on rock. Equivalent linear 3-D and 2-D analyses conducted in the direction of the short axis (longitudinal) were performed using a modified version of computer program SASSI. The results of these 3-D and 2-D analyses are similar. Non-linear analyses were performed for 2-D models using computer program FLAC. The results indicate that side gapping, base lifting, interface sliding, and soil yielding reduce the earth pressure, base bearing stress, caisson shear and bending moment, and caisson motions. However, the frequency characteristics of the responses appear to be relatively unaffected
Effect of time delay on the onset of synchronization of the stochastic Kuramoto model
We consider the Kuramoto model of globally coupled phase oscillators with
time-delayed interactions, that is subject to the Ornstein-Uhlenbeck (Gaussian)
colored or the non-Gaussian colored noise. We investigate numerically the
interplay between the influences of the finite correlation time of noise
and the time delay on the onset of the synchronization process. Both
cases for identical and nonidentical oscillators had been considered. Among the
obtained results for identical oscillators is a large increase of the
synchronization threshold as a function of time delay for the colored
non-Gaussian noise compared to the case of the colored Gaussian noise at low
noise correlation time . However, the difference reduces remarkably for
large noise correlation times. For the case of nonidentical oscillators, the
incoherent state may become unstable around the maximum value of the threshold
(as a function of time delay) even at lower coupling strength values in the
presence of colored noise as compared to the noiseless case. We had studied the
dependence of the critical value of the coupling strength (the threshold of
synchronization) on given parameters of the stochastic Kuramoto model in great
details and presented results for possible cases of colored Gaussian and
non-Gaussian noises.Comment: 19 pages with 7 figure
The Role of Final State Interactions in Quasielastic Fe Reactions at large
A relativistic finite nucleus calculation using a Dirac optical potential is
used to investigate the importance of final state interactions [FSI] at large
momentum transfers in inclusive quasielastic electronuclear reactions. The
optical potential is derived from first-order multiple scattering theory and
then is used to calculate the FSI in a nonspectral Green's function doorway
approach. At intermediate momentum transfers excellent predictions of the
quasielastic Fe experimental data for the longitudinal response
function are obtained. In comparisons with recent measurements at ~GeV/c the theoretical calculations of give good agreement for
the quasielastic peak shape and amplitude, but place the position of the peak
at an energy transfer of about ~MeV higher than the data.Comment: 13 pages typeset using revtex 3.0 with 6 postscript figures in
accompanying uuencoded file; submitted to Phys. Rev.
Implementation of a closed-loop structural control system using wireless sensor networks
Wireless sensor networks have rapidly matured in recent years to offer data acquisition capabilities on par with those of traditional tethered data acquisition systems. Entire structural monitoring systems assembled from wireless sensors have proven to be low cost, easy to install, and accurate. However, the functionality of wireless sensors can be further extended to include actuation capabilities. Wireless sensors capable of actuating a structure could serve as building blocks of future generations of structural control systems. In this study, a wireless sensor prototype capable of data acquisition, computational analysis and actuation is proposed for use in a real-time structural control system. The performance of a wireless control system is illustrated using a full-scale structure controlled by a semi-active magnetorheological (MR) damper and a network of wireless sensors. One wireless sensor designated as a controller automates the task of collecting state data, calculating control forces, and issuing commands to the MR damper, all in real time. Additional wireless sensors are installed to measure the acceleration and velocity response of each system degree of freedom. Base motion is applied to the structure to simulate seismic excitations while the wireless control system mitigates inter-storey drift response of the structure. An optimal linear quadratic regulation solution is formulated for embedment within the computational cores of the wireless sensors. Copyright © 2007 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/60230/1/214_ftp.pd
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