6,843 research outputs found
Instantons and seven-branes in type IIB superstring theory
Instanton and seven-brane solutions of type IIB supergravity carrying charges
in the Ramond-Ramond sector are constructed. The singular seven-brane has a
quantized \RR \ \lq magnetic' charge whereas its dual is the instanton, which
is non-singular in the string frame and has an associated {\it global} \lq
electric' charge. The product of these charges is constrained by a Dirac
quantization condition. The instanton has the form of a space-time wormhole in
the string frame, and is responsible for the non-conservation of the Noether
current.Comment: PHYZZX macros. 13 pages. Final version as it will appear in Phys.
Lett. B (typos corrected
The Value of Singularities
We point out that spacetime singularities play a useful role in gravitational
theories by eliminating unphysical solutions. In particular, we argue that any
modification of general relativity which is completely nonsingular cannot have
a stable ground state. This argument applies both to classical extensions of
general relativity, and to candidate quantum theories of gravity.Comment: 5 pages, no figures; a few clarifying comments adde
Healthcare worker competencies for disaster training
BACKGROUND: Although training and education have long been accepted as integral to disaster preparedness, many currently taught practices are neither evidence-based nor standardized. The need for effective evidence-based disaster training of healthcare staff at all levels, including the development of standards and guidelines for training in the multi-disciplinary health response to major events, has been designated by the disaster response community as a high priority. We describe the application of systematic evidence-based consensus building methods to derive educational competencies and objectives in criteria-based preparedness and response relevant to all hospital healthcare workers. METHODS: The conceptual development of cross-cutting competencies incorporated current evidence through a systematic consensus building process with the following steps: (1) review of peer-reviewed literature on relevant content areas and educational theory; (2) structured review of existing competencies, national level courses and published training objectives; (3) synthesis of new cross-cutting competencies; (4) expert panel review; (5) refinement of new competencies and; (6) development of testable terminal objectives for each competency using similar processes covering requisite knowledge, attitudes, and skills. RESULTS: Seven cross-cutting competencies were developed: (1) Recognize a potential critical event and implement initial actions; (2) Apply the principles of critical event management; (3) Demonstrate critical event safety principles; (4) Understand the institutional emergency operations plan; (5) Demonstrate effective critical event communications; (6) Understand the incident command system and your role in it; (7) Demonstrate the knowledge and skills needed to fulfill your role during a critical event. For each of the cross-cutting competencies, comprehensive terminal objectives are described. CONCLUSION: Cross-cutting competencies and objectives developed through a systematic evidence-based consensus building approach may serve as a foundation for future hospital healthcare worker training and education in disaster preparedness and response
Iridium Cyclooctene Complex Forms a Hyperpolarization Transfer Catalyst Before Converting to a Binuclear C-H Bond Activation Product Responsible for Hydrogen Isotope Exchange
[IrCl(COE)2]2 ( 1 ) reacts with pyridine and H2 to form crystallo-graphically characterized IrCl(H)2(COE)(py)2 ( 2 ). 2 undergoes pyridine loss to form 16-electron IrCl(H)2(COE)(py) (3) with equivalent hydride ligands. When this reaction is studied with parahydrogen, 1 efficiently achieves the hyperpolarization of free pyridine (and nicotinamide, nicotine, 5-aminopyrimidine and 3,5-lutudine) via signal amplification by reversible exchange (SABRE) and hence reflects a simple and readily available precatayst for this process. 2 reacts further over 48 hrs at 298 K to form crystallographically characterized (Cl)(H)(py)(ÎŒ-Cl)(ÎŒ-H)(Îș-ÎŒ-NC5H4)Ir(H)(py)2 (4). This dimer is shown to be active in the hydrogen isotope exchange process that is used in radiophar-maceutical preparations. Furthermore, while [Ir(H)2(COE)(py)3]PF6 ( 6 ) forms on addition of AgPF6 to 2 , its stability precludes its efficient involvement in SABRE
Achieving High Levels of NMR-Hyperpolarization in Aqueous Media With Minimal Catalyst Contamination via SABRE
Signal amplification by reversible exchange (SABRE) is shown to allow access to strongly enhanced 1H NMR signals in a range of substrates in aqueous media. In order to achieve this outcome phase-transfer-catalysis is exploited which leads to less than less than 1.5 x 10-6 mol dm-3 of the iridium catalyst in the aqueous phase. These observations reflect a compelling route to produce a saline based hyperpolarized bolus in just a few seconds for subsequent in vivo MRI monitoring. The new process has been called CAtalyst Separated Hyperpolarization via Signal Amplification By Reversible Exchange or CASH-SABRE. We illustrate this method for the substrates pyrazine, 5-methylpyrimidine, 4,6-d2-methyl nicotinate, 4,6-d2-nicotinamide and pyridazine achieving 1H signals gains of approximately 790-, 340-, 3000-, 260- and 380-fold per proton at 9.4 T at the time point where phase separation is complete
HST STIS spectroscopy of the triple nucleus of M31: two nested disks in Keplerian rotation around a Supermassive Black Hole
We present HST spectroscopy of the nucleus of M31 obtained with STIS. Spectra
taken around the CaT lines at 8500 see only the red giants in the double
bright- ness peaks P1 and P2. In contrast, spectra taken at 3600-5100 A are
sensitive to the tiny blue nucleus embedded in P2, the lower surface brightness
red nucleus. P2 has a K-type spectrum, but the embedded blue nucleus has an
A-type spectrum with strong Balmer absorption lines. Given the small likelihood
for stellar collisions, a 200 Myr old starburst appears to be the most
plausible origin of the blue nucleus. In stellar population, size, and velocity
dispersion, the blue nucleus is so different from P1 and P2 that we call it P3.
The line-of-sight velocity distributions of the red stars in P1+P2 strengthen
the support for Tremaine s eccentric disk model. The kinematics of P3 is
consistent with a circular stellar disk in Keplerian rotation around a
super-massive black hole with M_bh = 1.4 x 10^8 M_sun. The P3 and the P1+P2
disks rotate in the same sense and are almost coplanar. The observed velocity
dispersion of P3 is due to blurred rotation and has a maximum value of sigma =
1183+-201 km/s. The observed peak rotation velocity of P3 is V = 618+-81 km/s
at radius 0.05" = 0.19 pc corresponding to a circular rotation velocity at this
radius of ~1700 km/s. Any dark star cluster alternative to a black hole must
have a half-mass radius <= 0.03" = 0.11 pc. We show that this excludes clusters
of brown dwarfs or dead stars on astrophysical grounds.Comment: Astrophysical Journal, Sep 20, 2005, 21 pages including 20 figure
Iridium N-Heterocyclic Carbene Complexes as Efficient Catalysts for Magnetization Transfer from para-Hydrogen
Type IIB Orientifolds with NS-NS Antisymmetric Tensor Backgrounds
We consider six dimensional N=1 space-time supersymmetric Type IIB
orientifolds with non-zero untwisted NS-NS sector B-field. The B-field is
quantized due to the requirement that the Type IIB spectrum be left-right
symmetric. The presence of the B-field results in rank reduction of both 99 and
55 open string sector gauge groups. We point out that in some of the models
with non-zero B-field there are extra tensor multiplets in the Z_2 twisted
closed string sector, and we explain their origin in a simple example. Also,
the 59 open string sector states come with a multiplicity that depends on the
B-field. These two facts are in accord with anomaly cancellation requirements.
We point out relations between various orientifolds with and without the
B-field, and also discuss the F-theory duals of these models.Comment: 13 pages, revtex, minor misprints correcte
Testing magnetofrictional extrapolation with the Titov-D\'emoulin model of solar active regions
We examine the nonlinear magnetofrictional extrapolation scheme using the
solar active region model by Titov and D\'emoulin as test field. This model
consists of an arched, line-tied current channel held in force-free equilibrium
by the potential field of a bipolar flux distribution in the bottom boundary. A
modified version, having a parabolic current density profile, is employed here.
We find that the equilibrium is reconstructed with very high accuracy in a
representative range of parameter space, using only the vector field in the
bottom boundary as input. Structural features formed in the interface between
the flux rope and the surrounding arcade-"hyperbolic flux tube" and "bald patch
separatrix surface"-are reliably reproduced, as are the flux rope twist and the
energy and helicity of the configuration. This demonstrates that force-free
fields containing these basic structural elements of solar active regions can
be obtained by extrapolation. The influence of the chosen initial condition on
the accuracy of reconstruction is also addressed, confirming that the initial
field that best matches the external potential field of the model quite
naturally leads to the best reconstruction. Extrapolating the magnetogram of a
Titov-D\'emoulin equilibrium in the unstable range of parameter space yields a
sequence of two opposing evolutionary phases which clearly indicate the
unstable nature of the configuration: a partial buildup of the flux rope with
rising free energy is followed by destruction of the rope, losing most of the
free energy.Comment: 14 pages, 10 figure
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