7,780 research outputs found
Cored Apple Bipolarity : A Global Instability to Convection in Radial Accretion?
We propose that the prevalence of bipolarity in Young Stellar Objects is due
to the fine tuning that is required for spherical accretion of an ambient
medium onto a central node.It is shown that there are two steady modes that are
more likely than radial accretion, each of which is associated with a
hyperbolic central point in the meridional stream lines, and consequently with
either an equatorial inflow and an axial ejection or vice versa. In each case
the stream lines pass through a thick accretion torus, which is better thought
of as a standing pressure wave rather than as a relatively inert Keplerian
structure.We base our arguments on a simple analytic example,which is topologi
cally generic,wherein each bipolarmode is created by the rebound of accreting
matter under the action of the thermal,magnetic,turbulent and centrifugal
pressures created in the flow. In both bipolar modes the presence of non-zero
angular momentum implies axial regions wherein the pressure is first reduced
below the value at infinity and then becomes negative, where the solution fails
because rotating material can not enter this region without suction.The model
thus has empty stems where the activity of the central source must dominate.So
the basic engine of the bipolar flow discussed here is simply the rebound of
freely falling material from a thick pressure disc into an axial low pressure
region.The low mass,high velocity outflow must be produced in this region by an
additional mechanism. This is reminiscent of the cored apple structure observed
recently in the very young bipolar source VLA 1623.Comment: PostScript, 10 page
Hydrodynamics of compressible superfluids in confined geometries
We present a study of the hydrodynamics of compressible superfluids in
confined geometries. We use a perturbative procedure in terms of the
dimensionless expansion parameter where is the typical speed of
the flow and the speed of sound. A zero value of this parameter
corresponds to the incompressible limit. We apply the procedure to two specific
problems: the case of a trapped superfluid with a gaussian profile of the local
density, and that of a superfluid confined in a rotating obstructed cylinder.
We find that the corrections due to finite compressibility which are, as
expected, negligible for liquid He, are important but amenable to the
perturbative treatment for typical ultracold atomic systems.Comment: 17 pages, including 7 figures. To appear in Journ. Phys.
Dislocation Mobility and Anomalous Shear Modulus Effect in He Crystals
We calculate the dislocation glide mobility in solid He within a model
that assumes the existence of a superfluid field associated with dislocation
lines. Prompted by the results of this mobility calculation, we study within
this model the role that such a superfluid field may play in the motion of the
dislocation line when a stress is applied to the crystal. To do this, we relate
the damping of dislocation motion, calculated in the presence of the assumed
superfluid field, to the shear modulus of the crystal. As the temperature
increases, we find that a sharp drop in the shear modulus will occur at the
temperature where the superfluid field disappears. We compare the drop in shear
modulus of the crystal arising from the temperature dependence of the damping
contribution due to the superfluid field, to the experimental observation of
the same phenomena in solid He and find quantitative agreement. Our results
indicate that such a superfluid field plays an important role in dislocation
pinning in a clean solid He at low temperatures and in this regime may
provide an alternative source for the unusual elastic phenomena observed in
solid He.Comment: 17 pages, 2 figures. To appear in JLT
Ising Model on a random network with annealed or quenched disorder
We study the equilibrium properties of an Ising model on a disordered random
network where the disorder can be quenched or annealed. The network consists of
four-fold coordinated sites connected via variable length one-dimensional
chains. Our emphasis is on nonuniversal properties and we consider the
transition temperature and other equilibrium thermodynamic properties,
including those associated with one dimensional fluctuations arising from the
chains. We use analytic methods in the annealed case, and a Monte Carlo
simulation for the quenched disorder. Our objective is to study the difference
between quenched and annealed results with a broad random distribution of
interaction parameters. The former represents a situation where the time scale
associated with the randomness is very long and the corresponding degrees of
freedom can be viewed as frozen, while the annealed case models the situation
where this is not so. We find that the transition temperature and the entropy
associated with one dimensional fluctuations are always higher for quenched
disorder than in the annealed case. These differences increase with the
strength of the disorder up to a saturating value. We discuss our results in
connection to physical systems where a broad distribution of interaction
strengths is present.Comment: 11 pages including 9 figures. To appear in Phys. Rev.
Superfluid Field response to Edge dislocation motion
We study the dynamic response of a superfluid field to a moving edge
dislocation line to which the field is minimally coupled. We use a dissipative
Gross-Pitaevskii equation, and determine the initial conditions by solving the
equilibrium version of the model. We consider the subsequent time evolution of
the field for both glide and climb dislocation motion and analyze the results
for a range of values of the constant speed of the moving dislocation. We
find that the type of motion of the dislocation line is very important in
determining the time evolution of the superfluid field distribution associated
with it. Climb motion of the dislocation line induces increasing asymmetry, as
function of time, in the field profile, with part of the probability being, as
it were, left behind. On the other hand, glide motion has no effect on the
symmetry properties of the superfluid field distribution. Damping of the
superfluid field due to excitations associated with the moving dislocation line
occurs in both cases.Comment: 10 pages 7 figures. To appear in Phys. Rev
Angular Dependence of the Superconducting Transition Temperature in Ferromagnet-Superconductor-Ferromagnet Trilayers
The superconducting transition temperature, , of a ferromagnet (F) -
superconductor (S) - ferromagnet trilayer depends on the mutual orientation of
the magnetic moments of the F layers. This effect has been previously observed
in F/S/F systems as a difference between parallel and antiparallel
configurations of the F layers. Here we report measurements of in
CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments
of the CuNi ferromagnets. The observed angular dependence of is in
qualitative agreement with a F/S proximity theory that accounts for the odd
triplet component of the condensate predicted to arise for non-collinear
orientation of the magnetic moments of the F layers.Comment: 4 + \epsilon pages including 4 figures. To appear in Phys. Rev. Let
Angular dependence of superconductivity in superconductor / spin valve heterostructures
We report measurements of the superconducting transition temperature, ,
in CoO/Co/Cu/Co/Nb multilayers as a function of the angle between the
magnetic moments of the Co layers. Our measurements reveal that
is a nonmonotonic function, with a minimum near . Numerical
self-consistent solutions of the Bogoliubov - de Gennes equations
quantitatively and accurately describe the behavior of as a function of
and layer thicknesses in these superconductor / spin-valve
heterostructures. We show that experimental data and theoretical evidence agree
in relating to enhanced penetration of the triplet component of
the condensate into the Co/Cu/Co spin valve in the maximally noncollinear
magnetic configuration.Comment: 9 pages, 9 figure
Hernia discal traumática en un hurón
La hernia discal es una enfermedad bien definida y estudiada en perros y gatos. En estas especies se han descrito procesos degenerativos que llevan al prolapso del disco, o de su núcleo pulposo, hacia el canal medular, incluso en ausencia de traumatismos. En el caso del hurón doméstico (Mustela putorius furo) no se han descrito cambios degenerativos en el disco qu epuedan predisponer al prolapso, pero las hernias discales son relativamente frecuentes como consecuencia de traumatismos graves. En este artículo se describe un caso de hernia discal en un hurón producida a partir de una caída, su diagnóstico a través de una mielografía, su resolución quirúrgica y la recuperación del paciente tras la cirugía, poniendo especial énfasis en las dificultades diagnósticas y terapéuticas que plantea esta especie, así como las diferencias que presenta cuandose compara perros y gatos.
Theory of pixel lensing towards M31 I: the density contribution and mass of MACHOs
POINT-AGAPE is an Anglo-French collaboration which is employing the Isaac
Newton Telescope (INT) to conduct a pixel-lensing survey towards M31. In this
paper we investigate what we can learn from pixel-lensing observables about the
MACHO mass and fractional contribution in M31 and the Galaxy for the case of
spherically-symmetric near-isothermal haloes. We employ detailed pixel-lensing
simulations which include many of the factors which affect the observables. For
a maximum MACHO halo we predict an event rate in V of up to 100 per season for
M31 and 40 per season for the Galaxy. However, the Einstein radius crossing
time is generally not measurable and the observed full-width half-maximum
duration provides only a weak tracer of lens mass. Nonetheless, we find that
the near-far asymmetry in the spatial distribution of M31 MACHOs provides
significant information on their mass and density contribution. We present a
likelihood estimator for measuring the fractional contribution and mass of both
M31 and Galaxy MACHOs which permits an unbiased determination to be made of
MACHO parameters, even from data-sets strongly contaminated by variable stars.
If M31 does not have a significant population of MACHOs in the mass range
0.001-1 Solar masses strong limits will result from the first season of INT
observations. Simulations based on currently favoured density and mass values
indicate that, after three seasons, the M31 MACHO parameters should be
constrained to within a factor four uncertainty in halo fraction and an order
of magnitude uncertainty in mass (90% confidence). Interesting constraints on
Galaxy MACHOs may also be possible. For a campaign lasting ten years,
comparable to the lifetime of current LMC surveys, reliable estimates of MACHO
parameters in both galaxies should be possible. (Abridged)Comment: 21 pages, 14 figures. Submitted to MNRA
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