6,088 research outputs found
Dynamics of Protoplanetary Disks
Protoplanetary disks are quasi-steady structures whose evolution and
dispersal determine the environment for planet formation. I review the theory
of protoplanetary disk evolution and its connection to observations.
Substantial progress has been made in elucidating the physics of potential
angular momentum transport processes - including self-gravity,
magnetorotational instability, baroclinic instabilities, and magnetic braking -
and in developing testable models for disk dispersal via photoevaporation. The
relative importance of these processes depends upon the initial mass, size and
magnetization of the disk, and subsequently on its opacity, ionization state,
and external irradiation. Disk dynamics is therefore coupled to star formation,
pre-main-sequence stellar evolution, and dust coagulation during the early
stages of planet formation, and may vary dramatically from star to star. The
importance of validating theoretical models is emphasized, with the key
observations being those that probe disk structure on the scales, between 1 AU
and 10 AU, where theory is most uncertain.Comment: Annual Review of Astronomy and Astrophysics (2011). Final edited
version at
http://www.annualreviews.org/doi/abs/10.1146/annurev-astro-081710-102521
.High resolution versions of illustrations at
http://jila.colorado.edu/~pja/araa.htm
The effects of tidally induced disc structure on white dwarf accretion in intermediate polars
We investigate the effects of tidally induced asymmetric disc structure on
accretion onto the white dwarf in intermediate polars. Using numerical
simulation, we show that it is possible for tidally induced spiral waves to
propagate sufficiently far into the disc of an intermediate polar that
accretion onto the central white dwarf could be modulated as a result. We
suggest that accretion from the resulting asymmetric inner disc may contribute
to the observed X-ray and optical periodicities in the light curves of these
systems. In contrast to the stream-fed accretion model for these periodicities,
the tidal picture predicts that modulation can exist even for systems with
weaker magnetic fields where the magnetospheric radius is smaller than the
radius of periastron of the mass transfer stream. We also predict that
additional periodic components should exist in the emission from low mass ratio
intermediate polars displaying superhumps.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Resonant state expansion applied to planar waveguides
The resonant state expansion, a recently developed method in electrodynamics,
is generalized here to planar open optical systems with non-normal incidence of
light. The method is illustrated and verified on exactly solvable examples,
such as a dielectric slab and a Bragg reflector microcavity, for which explicit
analytic formulas are developed. This comparison demonstrates the accuracy and
convergence of the method. Interestingly, the spectral analysis of a dielectric
slab in terms of resonant states reveals an influence of waveguide modes in the
transmission. These modes, which on resonance do not couple to external light,
surprisingly do couple to external light for off-resonant excitation
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Modeling chemotaxis reveals the role of reversed phosphotransfer and a bi-functional kinase-phosphatase
Understanding how multiple signals are integrated in living cells to produce a balanced response is a major challenge in
biology. Two-component signal transduction pathways, such as bacterial chemotaxis, comprise histidine protein kinases
(HPKs) and response regulators (RRs). These are used to sense and respond to changes in the environment. Rhodobacter
sphaeroides has a complex chemosensory network with two signaling clusters, each containing a HPK, CheA. Here we
demonstrate, using a mathematical model, how the outputs of the two signaling clusters may be integrated. We use our
mathematical model supported by experimental data to predict that: (1) the main RR controlling flagellar rotation, CheY6, aided by its specific phosphatase, the bifunctional kinase CheA3, acts as a phosphate sink for the other RRs; and (2) a phosphorelay pathway involving CheB2 connects the cytoplasmic cluster kinase CheA3 with the polar localised kinase CheA2, and allows CheA3-P to phosphorylate non-cognate chemotaxis RRs. These two mechanisms enable the bifunctional kinase/phosphatase activity of CheA3 to integrate and tune the sensory output of each signaling cluster to produce a balanced response. The signal integration mechanisms identified here may be widely used by other bacteria, since like R. sphaeroides, over 50% of chemotactic bacteria have multiple cheA homologues and need to integrate signals from different
sources
Gauss Sums and Quantum Mechanics
By adapting Feynman's sum over paths method to a quantum mechanical system
whose phase space is a torus, a new proof of the Landsberg-Schaar identity for
quadratic Gauss sums is given. In contrast to existing non-elementary proofs,
which use infinite sums and a limiting process or contour integration, only
finite sums are involved. The toroidal nature of the classical phase space
leads to discrete position and momentum, and hence discrete time. The
corresponding `path integrals' are finite sums whose normalisations are derived
and which are shown to intertwine cyclicity and discreteness to give a finite
version of Kelvin's method of images.Comment: 14 pages, LaTe
Ultrafast (but Many-Body) Relaxation in a Low-Density Electron Glass
We present a study of the relaxation dynamics of the photoexcited
conductivity of the impurity states in the low-density electronic glass,
phosphorous-doped silicon Si:P. Using optical pump-terahertz probe spectroscopy
we find strongly temperature and fluence dependent glassy power-law relaxation
occurring over sub-ns time scales. Such behavior is in contrast to the much
longer time scales found in higher electron density glassy systems. We also
find evidence for both multi-particle relaxation mechanisms and/or coupling to
electronic collective modes and a low temperature quantum relaxational regime.Comment: 4 pages, 4 figures, Appeared in Phys. Rev. Let
Caddisflies (Insecta: Trichoptera) of Fringing Wetlands of the Laurentian Great Lakes
Fringing wetlands of the Laurentian Great Lakes are subject to natural processes, such as water-level fluctuation and wave-induced erosion, and to human alterations. In order to evaluate the quality of these wetlands over space and time, biological communities are often examined. Ideally, the groups of organisms selected for these evaluations should be resident in the wetlands themselves. Fish are often sampled, but many species are not truly resident, visiting wetlands on an occasional basis to feed or on a seasonal basis to breed. Aquatic vascular plants are perhaps the most common group selected for evaluation. However, in some cases, aquatic plants give a false impression by providing photosynthetic capabilities and structural infrastructure but having greatly diminished herbivore and carnivore communities
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