17,728 research outputs found
Formulating a State Approach to Professional Development
When viewed from the perspective of an entire state\u27s needs, the challenges of designing professional development to meet the requirements of the federal No Child Left Behind legislation of 2001 are daunting. In Oklahoma, the concerns about delivering to rural and urban populations which contain a variety of underserved populations are further complicated by the differences in the way science and mathematics are structured as disciplines. We describe two model programs, one in science and one in mathematics, which take much different approaches. However, the programs have three common elements that make them highly successful. Each program engages teachers strongly, seeks to change learning by altering both teachers\u27 behavior and content knowledge, and is continuously reflective
Direct Investigation of Superparamagnetism in Co Nanoparticle Films
A direct probe of superparamagnetism was used to determine the complete
anisotropy energy distribution of Co nanoparticle films. The films were
composed of self-assembled lattices of uniform Co nanoparticles 3 nm or 5 nm in
diameter, and a variable temperature scanning-SQUID microscope was used to
measure temperature-induced spontaneous magnetic noise in the samples. Accurate
measurements of anisotropy energy distributions of small volume samples will be
critical to magnetic optimization of nanoparticle devices and media.Comment: 4 pages, 4 figures. Submitted to Physical Review Letter
On the Maximum Mass of Accreting Primordial Supermassive Stars
Supermassive primordial stars are suspected to be the progenitors of the most
massive quasars at z~6. Previous studies of such stars were either unable to
resolve hydrodynamical timescales or considered stars in isolation, not in the
extreme accretion flows in which they actually form. Therefore, they could not
self-consistently predict their final masses at collapse, or those of the
resulting supermassive black hole seeds, but rather invoked comparison to
simple polytropic models. Here, we systematically examine the birth, evolution
and collapse of accreting non-rotating supermassive stars under accretion rates
of 0.01-10 solar masses per year, using the stellar evolution code KEPLER. Our
approach includes post-Newtonian corrections to the stellar structure and an
adaptive nuclear network, and can transition to following the hydrodynamic
evolution of supermassive stars after they encounter the general relativistic
instability. We find that this instability triggers the collapse of the star at
masses of 150,000-330,000 solar masses for accretion rates of 0.1-10 solar
masses per year, and that the final mass of the star scales roughly
logarithmically with the rate. The structure of the star, and thus its
stability against collapse, is sensitive to the treatment of convection, and
the heat content of the outer accreted envelope. Comparison with other codes
suggests differences here may lead to small deviations in the evolutionary
state of the star as a function of time, that worsen with accretion rate. Since
the general relativistic instability leads to the immediate death of these
stars, our models place an upper limit on the masses of the first quasars at
birth.Comment: 5 pages, 4 figures. Accepted ApJ letter
The Evolution of Supermassive Population III Stars
Supermassive primordial stars forming in atomically-cooled halos at are currently thought to be the progenitors of the earliest quasars
in the Universe. In this picture, the star evolves under accretion rates of
yr until the general relativistic instability
triggers its collapse to a black hole at masses of .
However, the ability of the accretion flow to sustain such high rates depends
crucially on the photospheric properties of the accreting star, because its
ionising radiation could reduce or even halt accretion. Here we present new
models of supermassive Population III protostars accreting at rates yr, computed with the GENEVA stellar evolution code
including general relativistic corrections to the internal structure. We use
the polytropic stability criterion to estimate the mass at which the collapse
occurs, which has been shown to give a lower limit of the actual mass at
collapse in recent hydrodynamic simulations. We find that at accretion rates
higher than yr the stars evolve as red, cool
supergiants with surface temperatures below K towards masses
, and become blue and hot, with surface temperatures above K,
only for rates yr. Compared to previous
studies, our results extend the range of masses and accretion rates at which
the ionising feedback remains weak, reinforcing the case for direct collapse as
the origin of the first quasars
On the Rotation of Supermassive Stars
Supermassive stars born from pristine gas in atomically-cooled haloes are
thought to be the progenitors of supermassive black holes at high redshifts.
However, the way they accrete their mass is still an unsolved problem. In
particular, for accretion to proceed, a large amount of angular momentum has to
be extracted from the collapsing gas. Here, we investigate the constraints
stellar evolution imposes on this angular momentum problem. We present an
evolution model of a supermassive Population III star including simultaneously
accretion and rotation. We find that, for supermassive stars to form by
accretion, the accreted angular momentum has to be about 1% of the Keplerian
angular momentum. This tight constraint comes from the -limit, at
which the combination of radiation pressure and centrifugal force cancels
gravity. It implies that supermassive stars are slow rotators, with a surface
velocity less than 10-20% of their first critical velocity, at which the
centrifugal force alone cancels gravity. At such low velocities, the
deformation of the star due to rotation is negligible
Brain natriuretic peptide and NT-proBNP levels reflect pulmonary artery systolic pressure in trekkers at high altitude.
Our objective was to evaluate the utility of the natriuretic peptides BNP (brain natriuretic peptide) and NT-proBNP as markers of pulmonary artery systolic pressure (PASP) in trekkers ascending to high altitude (HA). 20 participants had BNP and NT-proBNP assayed and simultaneous echocardiographic assessment of PASP performed during a trek to 5150 m. PASP increased significantly (p=0.006) with ascent from 24+/-4 to 39+/-11 mm Hg at 5150 m. At 5150 m those with a PASP>/=40 mm Hg (n=8) (versus those with PASP/=400 pg/ml) rise in NT-proBNP at 5150 m (n=4) PASP was significantly higher: 45.9+/-7.5 vs. 32.2+/-6.2 mm Hg (p=0.015). BNP and NT-proBNP may reflect elevated PASP, a central feature of high altitude pulmonary oedema, at HA
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