3,849 research outputs found
Fivebranes from gauge theory
We study theories with sixteen supercharges and a discrete energy spectrum.
One class of theories has symmetry group . They arise as truncations
of super Yang Mills. They include the plane wave matrix model, 2+1
super Yang Mills on and super Yang Mills on . We explain how to obtain their gravity duals in a unified way.
We explore the regions of the geometry that are relevant for the study of some
1/2 BPS and near BPS states. This leads to a class of two dimensional (4,4)
supersymmetric sigma models with non-zero flux, including a massive
deformed WZW model. We show how to match some features of the string spectrum
with the Yang Mills theory.
The other class of theories are also connected to super Yang
Mills and arise by making some of the transverse scalars compact. Their vacua
are characterized by a 2d Yang Mills theory or 3d Chern Simons theory. These
theories realize peculiar superpoincare symmetry algebras in 2+1 or 1+1
dimensions with "non-central" charges. We finally discuss gravity duals of
super Yang Mills on .Comment: 50+24 pages, 9 figures, latex. v2: typos corrected, references adde
2-D Circulation Control Airfoil Benchmark Experiments Intended for CFD Code Validation
A current NASA Research Announcement (NRA) project being conducted by Georgia Tech Research Institute (GTRI) personnel and NASA collaborators includes the development of Circulation Control (CC) blown airfoils to improve subsonic aircraft high-lift and cruise performance. The emphasis of this program is the development of CC active flow control concepts for both high-lift augmentation, drag control, and cruise efficiency. A collaboration in this project includes work by NASA research engineers, whereas CFD validation and flow physics experimental research are part of NASA s systematic approach to developing design and optimization tools for CC applications to fixed-wing aircraft. The design space for CESTOL type aircraft is focusing on geometries that depend on advanced flow control technologies that include Circulation Control aerodynamics. The ability to consistently predict advanced aircraft performance requires improvements in design tools to include these advanced concepts. Validation of these tools will be based on experimental methods applied to complex flows that go beyond conventional aircraft modeling techniques. This paper focuses on recent/ongoing benchmark high-lift experiments and CFD efforts intended to provide 2-D CFD validation data sets related to NASA s Cruise Efficient Short Take Off and Landing (CESTOL) study. Both the experimental data and related CFD predictions are discussed
Glutamate transporter EAAT2: a new target for the treatment of neurodegenerative diseases
Glutamate is the primary excitatory amino acid neurotransmitter in the CNS. The concentration of glutamate in the synaptic cleft is tightly controlled by interplay between glutamate release and glutamate clearance. Abnormal glutamate release and/or dysfunction of glutamate clearance can cause overstimulation of glutamate receptors and result in neuronal injury known as excitotoxicity. The glial glutamate transporter EAAT2 plays a major role in glutamate clearance. Dysfunction or reduced expression of EAAT2 has been documented in many neurodegenerative diseases. In addition, many studies in animal models of disease indicate that increased EAAT2 expression provides neuronal protection. Here, we summarize these studies and suggest that EAAT2 is a potential target for the prevention of excitotoxicity. EAAT2 can be upregulated by transcriptional or translational activation. We discuss current progress in the search for EAAT2 activators, which is a promising direction for the treatment of neurodegenerative diseases
The Megasecond Chandra X-Ray Visionary Project Observation of NGC 3115 (III): luminosity functions of LMXBs and dependence on stellar environments
We have studied the X-ray luminosity function (XLF) of low-mass X-ray
binaries (LMXBs) in the nearby lenticular galaxy NGC 3115, using the Megasecond
Chandra X-Ray Visionary Project Observation. With a total exposure time of ~1.1
Ms, we constructed the XLF down to a limiting luminosity of ~10^36 erg/s, much
deeper than typically reached for other early-type galaxies. We found
significant flattening of the overall LMXB XLF from dN/dL \propto
L^{-2.2\pm0.4} above 5.5x10^37 erg/s to dN/dL \propto L^{-1.0\pm0.1} below it,
though we could not rule out a fit with a higher break at ~1.6x10^38 erg/s. We
also found evidence that the XLF of LMXBs in globular clusters (GCs) is overall
flatter than that of field LMXBs. Thus our results for this galaxy do not
support the idea that all LMXBs are formed in GCs. The XLF of field LMXBs seems
to show spatial variation, with the XLF in the inner region of the galaxy being
flatter than that in the outer region, probably due to contamination of LMXBs
from undetected and/or disrupted GCs in the inner region. The XLF in the outer
region is probably the XLF of primordial field LMXBs, exhibiting dN/dL \propto
L^{-1.2\pm0.1} up to a break close to the Eddington limit of neutron star LMXBs
(~1.7x10^38 erg/s). The break of the GC LMXB XLF is lower, at ~1.1x10^37 erg/s.
We also confirm previous findings that the metal-rich/red GCs are more likely
to host LMXBs than the metal-poor/blue GCs, which is more significant for more
luminous LMXBs, and that more massive GCs are more likely to host LMXBs.Comment: 12 pages, 8 figures, accepted for publication in Ap
The Megasecond Chandra X-Ray Visionary Project Observation of NGC 3115 (II): properties of point sources
We have carried out an in-depth study of low-mass X-ray binaries (LMXBs)
detected in the nearby lenticular galaxy NGC 3115, using the Megasecond Chandra
X-Ray Visionary Project observation (total exposure time 1.1 Ms). In total we
found 136 candidate LMXBs in the field and 49 in globular clusters (GCs) above
2\sigma\ detection, with 0.3--8 keV luminosity L_X ~10^36-10^39 erg/s. Other
than 13 transient candidates, the sources overall have less long-term
variability at higher luminosity, at least at L_X > 2x10^37 erg/s. In order to
identify the nature and spectral state of our sources, we compared their
collective spectral properties based on single-component models (a simple power
law or a multicolor disk) with the spectral evolution seen in representative
Galactic LMXBs. We found that in the L_X versus photon index \Gamma_PL and L_X
versus disk temperature kT_MCD plots, most of our sources fall on a narrow
track in which the spectral shape hardens with increasing luminosity below
L_X~7x10^37 erg/s but is relatively constant (\Gamma_PL~1.5 or kT_MCD~1.5 keV)
above this luminosity, similar to the spectral evolution of Galactic neutron
star (NS) LMXBs in the soft state in the Chandra bandpass. Therefore we
identified the track as the NS LMXB soft-state track and suggested sources with
L_X7x10^37 erg/s
as Z sources. Ten other sources (five are transients) displayed significantly
softer spectra and are probably black hole X-ray binaries in the thermal state.
One of them (persistent) is in a metal-poor GC.Comment: 26 pages, 9 figures, four online tables, accepted for publication in
Ap
The non-dipolar magnetic fields of accreting T Tauri stars
Models of magnetospheric accretion on to classical T Tauri stars often assume
that stellar magnetic fields are simple dipoles. Recently published surface
magnetograms of BP Tau and V2129 Oph have shown, however, that their fields are
more complex. The magnetic field of V2129 Oph was found to be predominantly
octupolar. For BP Tau the magnetic energy was shared mainly between the dipole
and octupole field components, with the dipole component being almost four
times as strong as that of V2129 Oph. From the published surface maps of the
photospheric magnetic fields we extrapolate the coronal fields of both stars,
and compare the resulting field structures with that of a dipole. We consider
different models where the disc is truncated at, or well-within, the Keplerian
corotation radius. We find that although the structure of the surface magnetic
field is particularly complex for both stars, the geometry of the larger scale
field, along which accretion is occurring, is somewhat simpler. However, the
larger scale field is distorted close to the star by the stronger field
regions, with the net effect being that the fractional open flux through the
stellar surface is less than would be expected with a dipole magnetic field
model. Finally, we estimate the disc truncation radius, assuming that this
occurs where the magnetic torque from the stellar magnetosphere is comparable
to the viscous torque in the disc.Comment: 14 pages, 8 figures. Figures are reduced resolutio
Relationship among surgical volume, repair quality, and perioperative outcomes for repair of mitral insufficiency in a mitral valve reference center
ObjectiveAlthough it has been demonstrated that the repair rates and quality of the repair of mitral insufficiency are superior in mitral valve reference centers, it has not been studied whether an advantage exists for perioperative morbidity and mortality. We report 1 surgeon's evolution over 7 years, specifically considering the changes in perioperative morbidity and mortality.MethodsWe performed a retrospective review of 1054 patients who had undergone elective, day-of-surgery-admission mitral valve repair by a single surgeon (D.H.A.) at our institution from April 2005 to June 2012. The outcome variables studied were operative mortality (30-day or in-hospital mortality, if longer), length of stay, low cardiac output state after cardiopulmonary bypass, and major morbidity.ResultsThe overall operative mortality was 0.58%. Of the 1054 patients, 31% developed a low cardiac output state postoperatively and 6.52% experienced at least 1 of the composite morbidity events. Increased aortic crossclamp times were significantly and independently associated with a low cardiac output state, length of stay, and morbidity. When divided by service year, a statistically and clinically significant decrease was found in the aortic crossclamp time, despite an increase in the complexity of cases. The morbidity decreased concurrently with the decreases in crossclamp times.ConclusionsAs the number of mitral valve repairs performed each year by a single surgeon at a single institution increased, morbidity, including postoperative heart function and length of stay, decreased. This was demonstrated to occur in large part from a reduction in the aortic crossclamp times, despite an increase in the complexity of the procedures. This further demonstrates the value of reference centers for mitral valve surgery
Recollapse of the closed Tolman spacetimes
The closed-universe recollapse conjecture is studied for the spherically
symmetric spacetimes. It is proven that there exists an upper bound to the
lengths of timelike curves in any Tolman spacetime that possesses Cauchy
surfaces and whose energy density is positive. Furthermore, an explicit bound
is constructed from the initial data for such a spacetime.Comment: 25 pages, REVTeX, NCSU-MP-930
Compact Directional Microwave Antenna for Localized Heating
A directional, catheter-sized cylindrical antenna has been developed for localized delivery of microwave radiation for heating (and thus killing) diseased tissue without excessively heating nearby healthy tissue. By "localized" is meant that the antenna radiates much more in a selected azimuthal direction than in the opposite radial direction, so that it heats tissue much more on one side than it does on the opposite side. This antenna can be inserted using either a catheter or a syringe. A 2.4-mm prototype was tested, although smaller antennas are possible. Prior compact, cylindrical antennas designed for therapeutic localized hyperthermia do not exhibit such directionality; that is, they radiate in approximately axisymmetric patterns. Prior directional antennas designed for the same purpose have been, variously, (1) too large to fit within catheters or (2) too large, after deployment from catheters, to fit within the confines of most human organs. In contrast, the present antenna offers a high degree of directionality and is compact enough to be useable as a catheter in some applications
The Soft X-ray Lightcurves of Partially Eclipsed Stellar Flares
Most stellar flares' soft X-ray lightcurves possess a `typical' morphology,
which consists of a rapid rise followed by a slow exponential decay. However, a
study of 216 of the brightest flares on 161 pre-main sequence stars, observed
during the Chandra Orion-Ultradeep Project (COUP), showed that many flare
lightcurves depart from this typical morphology. While this can be attributed
to the superposition of multiple typical flares, we explore the possibility
that the time-variable eclipsing of flares by their host stars may also be an
important factor. We assume each flare is contained within a single, uniform
plasma density magnetic loop and specify the intrinsic variation of the flare's
emission measure with time. We consider rotational eclipse by the star itself,
but also by circumstellar discs and flare-associated prominences. Based on this
simple model, we generate a set of flares similar to those observed in the COUP
database. Many eclipses simply reduce the flare's maximum emission measure or
decay time. We conclude therefore that eclipses often pass undetected, but
usually have only a modest influence on the flare emission measure profile and
hence the derived loop lengths. We show that eclipsing can easily reproduce the
observed atypical flare morphologies. The number of atypical modelled flare
morphologies is however much less than that found in the COUP sample. The large
number of observed atypical flare morphologies, therefore, must be attributed
to other processes such as multiple flaring loops.Comment: 11 pages, 9 figure
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