14,847 research outputs found
London dispersion forces without density distortion: a path to first principles inclusion in density functional theory
We analyse a path to construct density functionals for the dispersion
interaction energy from an expression in terms of the ground state densities
and exchange-correlation holes of the isolated fragments. The expression is
based on a constrained search formalism for a supramolecular wavefunction that
is forced to leave the diagonal of the many-body density matrix of each
fragment unchanged, and is exact for the interaction between one-electron
densities. We discuss several aspects: the needed features a density functional
approximation for the exchange-correlation holes of the monomers should have,
the optimal choice of the one-electron basis needed (named "dispersals"), and
the functional derivative with respect to monomer density variations.Comment: 12 pages, 4 figure
Designing Professional Learning Tasks for Mathematics Learning Trajectories
In this paper, we present an emerging set of learning conjectures and design principles to be used in the development of professional learning tasks that support elementary teachers’ learning of mathematics learning trajectories. We outline our theoretical perspective on teacher knowledge of learning trajectories, review the literature concerning mathematics professional learning tasks, offer a set of initial conjectures about teacher learning of learning trajectories, and articulate a set of principles to guide the design of tasks. We conclude with an example of one learning trajectory professional learning task taken from our current research project
Fingerprints of giant planets in the photospheres of Herbig stars
Around 2% of all A stars have photospheres depleted in refractory elements.
This is hypothesized to arise from a preferential accretion of gas rather than
dust, but the specific processes and the origin of the material -- circum- or
interstellar -- are not known. The same depletion is seen in 30% of young,
disk-hosting Herbig Ae/Be stars. We investigate whether the chemical
peculiarity originates in a circumstellar disk. Using a sample of systems for
which both the stellar abundances and the protoplanetary disk structure are
known, we find that stars hosting warm, flaring group I disks typically have
Fe, Mg and Si depletions of 0.5 dex compared to the solar-like abundances of
stars hosting cold, flat group II disks. The volatile, C and O, abundances in
both sets are identical. Group I disks are generally transitional, having
radial cavities depleted in millimetre-sized dust grains, while those of group
II are usually not. Thus we propose that the depletion of heavy elements
emerges as Jupiter-like planets block the accretion of part of the dust, while
gas continues to flow towards the central star. We calculate gas to dust ratios
for the accreted material and find values consistent with models of disk
clearing by planets. Our results suggest that giant planets of ~0.1 to 10 M_Jup
are hiding in at least 30% of Herbig Ae/Be disks.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
On Eta-Einstein Sasakian Geometry
We study eta-Einstein geometry as a class of distinguished Riemannian metrics
on contact metric manifolds. In particular, we use a previous solution of the
Calabi problem for Sasakian geometry to prove the existence of eta-Einstein
structures on many different compact manifolds, including exotic spheres. We
also relate these results to the existence of Einstein-Weyl structures.Comment: 31 pages, minor changes made, to appear in Commun. Math. Phy
Geometry and Mechanics of Thin Growing Bilayers
We investigate how thin sheets of arbitrary shapes morph under the isotropic
in-plane expansion of their top surface, which may represent several stimuli
such as nonuniform heating, local swelling and differential growth. Inspired by
geometry, an analytical model is presented that rationalizes how the shape of
the disk influences morphing, from the initial spherical bending to the final
isometric limit. We introduce a new measure of slenderness that
describes a sheet in terms of both thickness and plate shape. We find that the
mean curvature of the isometric state is three fourth's the natural curvature,
which we verify by numerics and experiments. We finally investigate the
emergence of a preferred direction of bending in the isometric state, guided by
numerical analyses. The scalability of our model suggests that it is suitable
to describe the morphing of sheets spanning several orders of magnitude.Comment: 5 pages, 4 figure
X-ray Spectroscopy of the Contact Binary VW Cephei
Short-period binaries represent extreme cases in the generation of stellar
coronae via a rotational dynamo. Such stars are important for probing the
origin and nature of coronae in the regimes of rapid rotation and activity
saturation. VW Cep (P=0.28 d) is a relatively bright, partially eclipsing, and
very active object. Light curves made from Chandra/HETGS data show flaring and
rotational modulation, but no eclipses. Velocity modulation of emission lines
indicates that one component dominates the X-ray emission. The emission measure
is highly structured, having three peaks. Helium-like triplet lines give
electron densities of about 3.0E+10 - 18.0E+10 /cm^3. We conclude that the
corona is predominantly on the polar regions of the primary star and compact.Comment: Accepted for publication in the Astropysical Journal, 23 June 2006;
22 pages, 15 figure
- …