6,991 research outputs found
Angular Momentum Transport in Particle and Fluid Disks
We examine the angular momentum transport properties of disks composed of
macroscopic particles whose velocity dispersions are externally enhanced
(``stirred''). Our simple Boltzmann equation model serves as an analogy for
unmagnetized fluid disks in which turbulence may be driven by thermal
convection. We show that interparticle collisions in particle disks play the
same role as fluctuating pressure forces and viscous dissipation in turbulent
disks: both transfer energy in random motions associated with one direction to
those associated with another, and convert kinetic energy into heat. The
direction of angular momentum transport in stirred particle and fluid disks is
determined by the direction of external stirring and by the properties of the
collision term in the Boltzmann equation (or its analogue in the fluid
problem). In particular, our model problem yields inward transport for
vertically or radially stirred disks, provided collisions are suitably
inelastic; the transport is outwards in the elastic limit. Numerical
simulations of hydrodynamic turbulence driven by thermal convection find inward
transport; this requires that fluctuating pressure forces do little to no work,
and is analogous to an externally stirred particle disk in which collisions are
highly inelastic.Comment: 15 pages; final version accepted by ApJ; minor changes, some
clarificatio
Shifts of the nuclear resonance in the vortex lattice in YBaCuO
The NMR and NQR spectra of Cu in the CuO plane of
YBaCuO in the superconducting state are discussed in terms of the
phenomenological theory of Ginzburg-Landau type extended to lower temperatures.
We show that the observed spectra, Kumagai {\em et al.}, PRB {\bf 63}, 144502
(2001), can be explained by a standard theory of the Bernoulli potential with
the charge transfer between CuO planes and CuO chains assumed.Comment: 11 pages 7 figure
Spectral Energy Distributions of T Tauri Stars With Passive Circumstellar Disks
We derive hydrostatic, radiative equilibrium models for passive disks
surrounding T Tauri stars. Each disk is encased by an optically thin layer of
superheated dust grains. This layer re-emits directly to space about half the
stellar energy it absorbs. The other half is emitted inward and regulates the
interior temperature of the disk. The heated disk flares. As a consequence, it
absorbs more stellar radiation, especially at large radii, than a flat disk
would. The portion of the spectral energy distribution contributed by the disk
is fairly flat throughout the thermal infrared. At fixed frequency, the
contribution from the surface layer exceeds that from the interior by about a
factor 3 and is emitted at more than an order of magnitude greater radius.
Spectral features from dust grains in the superheated layer appear in emission
if the disk is viewed nearly face-on.Comment: 29 LaTeX pages w/ 10 eps. figures, aaspp4.sty, final version with few
minor stylistic alterations and 1 content change (section 4.1.1 on GM Aur and
non-zero inclination
Apsidal Alignment in Upsilon Andromedae
One of the parameters fitted by Doppler radial velocity measurements of
extrasolar planetary systems is omega, the argument of pericenter of a given
planet's orbit referenced to the plane of the sky. Curiously, the omega's of
the outer two planets orbiting Upsilon Andromedae are presently nearly
identical: Delta-omega = omega_D - omega_C = 4.8 deg +/- 4.8 deg (1 sigma).
This observation is least surprising if planets C and D occupy orbits that are
seen close to edge-on (sin i_C, sin i_D > 0.5) and whose mutual inclination
Theta does not exceed 20 deg. In this case, planets C and D inhabit a secular
resonance in which Delta-omega librates about 0 deg with an amplitude of 30 deg
and a period of 4000 yr. The resonant configuration spends about one-third of
its time with |Delta-omega| 40 deg, either Delta-omega
circulates or the system is unstable. This instability is driven by the Kozai
mechanism which couples the eccentricity of planet C to Theta to drive the
former quantity to values approaching unity. Our expectation that Theta < 20
deg suggests that planets C and D formed in a flattened, circumstellar disk,
and may be tested by upcoming astrometric measurements with the FAME satellite.Comment: Refereed version, accepted by AJ, to appear in September 2001 issu
Excitation of Orbital Eccentricities of Extrasolar Planets by Repeated Resonance Crossings
Orbits of known extrasolar planets that are located outside the tidal
circularization regions of their parent stars are often substantially
eccentric. By contrast, planetary orbits in our Solar System are approximately
circular, reflecting planet formation within a nearly axisymmetric, circumsolar
disk. We propose that orbital eccentricities may be generated by divergent
orbital migration of two planets in a viscously accreting circumstellar disk.
The migration is divergent in the sense that the ratio of the orbital period of
the outer planet to that of the inner planet grows. As the period ratio
diverges, the planets traverse, but are not captured into, a series of
mean-motion resonances that amplify their orbital eccentricities in rough
inverse proportion to their masses. Strong viscosity gradients in
protoplanetary disks offer a way to reconcile the circular orbits of Solar
System gas giants with the eccentric orbits of currently known extrasolar
planets.Comment: Final revised version, accepted by ApJ Letters. Includes discussion
from the community at larg
Long-term culture captures injury-repair cycles of colonic stem cells
The colonic epithelium can undergo multiple rounds of damage and repair, often in response to excessive inflammation. The responsive stem cell that mediates this process is unclear, in part because of a lack of in vitro models that recapitulate key epithelial changes that occur in vivo during damage and repair. Here, we identify a Hop
Orientational order on curved surfaces - the high temperature region
We study orientational order, subject to thermal fluctuations, on a fixed
curved surface. We derive, in particular, the average density of zeros of
Gaussian distributed vector fields on a closed Riemannian manifold. Results are
compared with the density of disclination charges obtained from a Coulomb gas
model. Our model describes the disordered state of two dimensional objects with
orientational degrees of freedom, such as vector ordering in Langmuir
monolayers and lipid bilayers above the hexatic to fluid transition.Comment: final version, 13 Pages, 2 figures, uses iopart.cl
First Release of Gauss-Legendre Sky Pixelization (GLESP) software package for CMB analysis
We report the release of the Gauss--Legendre Sky Pixelization (GLESP)
software package version 1.0. In this report we present the main features and
functions for processing and manipulation of sky signals. Features for CMB
polarization is underway and to be incorporated in a future release. Interested
readers can visit http://www.glesp.nbi.dk (www.glesp.nbi.dk) and register for
receiving the package
Joule Heating and Current-Induced Instabilities in Magnetic Nanocontacts
We consider the electrical current through a magnetic point contact in the
limit of a strong inelastic scattering of electrons. In this limit local Joule
heating of the contact region plays a decisive role in determining the
transport properties of the point contact. We show that if an applied constant
bias voltage exceeds a critical value, the stationary state of the system is
unstable, and that periodic, non-harmonic oscillations in time of both the
electrical current through the contact and the local temperature in the contact
region develop spontaneously. Our estimations show that the necessary
experimental conditions for observing such oscillations with characteristic
frequencies in the range Hz can easily be met. We also show a
possibility to manipulate upon the magnetization direction of a magnetic grain
coupled through a point contact to a bulk ferromagnetic by exciting the
above-mentioned thermal-electric oscillations.Comment: 9 pages, 6 figures, submitted to Physical Review
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