500 research outputs found
Survival of the mm-cm size grain population observed in protoplanetary disks
Millimeter interferometry provides evidence for the presence of mm to cm size
"pebbles" in the outer parts of disks around pre-main-sequence stars. The
observations suggest that large grains are produced relatively early in disk
evolution (< 1 Myr) and remain at large radii for longer periods of time (5 to
10 Myr). Simple theoretical estimates of the radial drift time of solid
particles, however, imply that they would drift inward over a time scale of
less than 0.1 Myr. In this paper, we address this conflict between theory and
observation, using more detailed theoretical models, including the effects of
sedimentation, collective drag forces and turbulent viscosity. We find that,
although these effects slow down the radial drift of the dust particles, this
reduction is not sufficient to explain the observationally determined long
survival time of mm/cm-sized grains in protoplanetary disks. However, if for
some reason the gas to dust ratio in the disk is reduced by at least a factor
of 20 from the canonical value of 100 (for instance through photoevaporation of
the gas), then the radial drift time scales become sufficiently large to be in
agreement with observations.Comment: Accepted for publication in Astronomy and Astrophysic
Buckling instability in type-II superconductors with strong pinning
We predict a novel buckling instability in the critical state of thin type-II
superconductors with strong pinning. This elastic instability appears in high
perpendicular magnetic fields and may cause an almost periodic series of flux
jumps visible in the magnetization curve. As an illustration we apply the
obtained criteria to a long rectangular strip.Comment: Submitted to Phys. Rev. Let
3D MHD Simulations of Planet Migration in Turbulent Stratified Disks
We performed 3D MHD simulations of planet migration in stratified disks using
the Godunov code PLUTO, where the disk is turbulent due to the
magnetorotational instability. We study the migration for planets with
different planet-star mass ratios . In agreement with previous
studies, for the low-mass planet cases ( and ),
migration is dominated by random fluctuations in the torque. For a Jupiter-mass
planet for , we find a reduction of
the magnetic stress inside the orbit of the planet and around the gap region.
After an initial stage where the torque on the planet is positive, it reverses
and we recover migration rates similar to those found in disks where the
turbulent viscosity is modelled by an viscosity. For the
intermediate-mass planets ( and ) we
find a new and so far unexpected behavior. In some cases they experience
sustained and systematic outwards migration for the entire duration of the
simulation. For this case, the horseshoe region is resolved and torques coming
from the corotation region can remain unsaturated due to the stresses in the
disk. These stresses are generated directly by the magnetic field. The
magnitude of the horseshoe drag can overcome the negative Lindblad contribution
when the local surface density profile is flat or increasing outwards, which we
see in certain locations in our simulations due to the presence of a zonal
flow. The intermediate-mass planet is migrating radially outwards in locations
where there is a positive gradient of a pressure bump (zonal flow).Comment: Accepted for publication in Ap
Turbulence and Steady Flows in 3D Global Stratified MHD Simulations of Accretion Disks
We present full 2 Pi global 3-D stratified MHD simulations of accretion
disks. We interpret our results in the context of proto-planetary disks. We
investigate the turbulence driven by the magneto-rotational instability (MRI)
using the PLUTO Godunov code in spherical coordinates with the accurate and
robust HLLD Riemann solver. We follow the turbulence for more than 1500 orbits
at the innermost radius of the domain to measure the overall strength of
turbulent motions and the detailed accretion flow pattern. We find that regions
within two scale heights of the midplane have a turbulent Mach number of about
0.1 and a magnetic pressure two to three orders of magnitude less than the gas
pressure, while outside three scale heights the magnetic pressure equals or
exceeds the gas pressure and the turbulence is transonic, leading to large
density fluctuations. The strongest large-scale density disturbances are spiral
density waves, and the strongest of these waves has m=5. No clear meridional
circulation appears in the calculations because fluctuating radial pressure
gradients lead to changes in the orbital frequency, comparable in importance to
the stress gradients that drive the meridional flows in viscous models. The net
mass flow rate is well-reproduced by a viscous model using the mean stress
distribution taken from the MHD calculation. The strength of the mean turbulent
magnetic field is inversely proportional to the radius, so the fields are
approximately force-free on the largest scales. Consequently the accretion
stress falls off as the inverse square of the radius.Comment: Accepted for publication in Ap
Lipocalin 2 is protective against E. coli pneumonia
<p>Abstract</p> <p>Background</p> <p>Lipocalin 2 is a bacteriostatic protein that binds the siderophore enterobactin, an iron-chelating molecule produced by <it>Escherichia coli </it>(<it>E. coli</it>) that is required for bacterial growth. Infection of the lungs by <it>E. coli </it>is rare despite a frequent exposure to this commensal bacterium. Lipocalin 2 is an effector molecule of the innate immune system and could therefore play a role in hindering growth of <it>E. coli </it>in the lungs.</p> <p>Methods</p> <p>Lipocalin 2 knock-out and wild type mice were infected with two strains of <it>E. coli</it>. The lungs were removed 48 hours post-infection and examined for lipocalin 2 and MMP9 (a myeloid marker protein) by immunohistochemical staining and western blotting. Bacterial numbers were assessed in the lungs of the mice at 2 and 5 days after infection and mortality of the mice was monitored over a five-day period. The effect of administering ferrichrome (an iron source that cannot be bound by lipocalin 2) along with E.coli was also examined.</p> <p>Results</p> <p>Intratracheal installation of <it>E. coli </it>in mice resulted in strong induction of lipocalin 2 expression in bronchial epithelium and alveolar type II pneumocytes. Migration of myeloid cells to the site of infection also contributed to an increased lipocalin 2 level in the lungs. Significant higher bacterial numbers were observed in the lungs of lipocalin 2 knock-out mice on days 2 and 5 after infection with <it>E. coli </it>(p < 0.05). In addition, a higher number of <it>E. coli </it>was found in the spleen of surviving lipocalin 2 knock-out mice on day 5 post-infection than in the corresponding wild-type mice (p < 0.05). The protective effect against <it>E. coli </it>infection in wild type mice could be counteracted by the siderophore ferrichrome, indicating that the protective effect of lipocalin 2 depends on its ability to sequester iron.</p> <p>Conclusions</p> <p>Lipocalin 2 is important for protection of airways against infection by <it>E. coli</it>.</p
Interplay of dendritic avalanches and gradual flux penetration in superconducting MgB2 films
Magneto-optical imaging was used to study a zero-field-cooled MgB2 film at
9.6K where in a slowly increasing field the flux penetrates by abrupt formation
of large dendritic structures. Simultaneously, a gradual flux penetration takes
place, eventually covering the dendrites, and a detailed analysis of this
process is reported. We find an anomalously high gradient of the flux density
across a dendrite branch, and a peak value that decreases as the applied field
goes up. This unexpected behaviour is reproduced by flux creep simulations
based on the non-local field-current relation in the perpendicular geometry.
The simulations also provide indirect evidence that flux dendrites are formed
at an elevated local temperature, consistent with a thermo-magnetic mechanism
of the instabilityComment: 5 pages, 5 figures, submitted to Supercond. Sci. Techno
Trapping Solids at the Inner Edge of the Dead Zone: 3-D Global MHD Simulations
The poorly-ionized interior of the protoplanetary disk is the location where
dust coagulation processes may be most efficient. However even here,
planetesimal formation may be limited by the loss of solid material through
radial drift, and by collisional fragmentation of the particles. Our aim is to
investigate the possibility that solid particles are trapped at local pressure
maxima in the dynamically evolving disk. We perform the first 3-D global
non-ideal MHD calculations of the disk treating the turbulence driven by the
magneto-rotational instability. The domain contains an inner MRI-active region
near the young star and an outer midplane dead zone, with the transition
between the two modeled by a sharp increase in the magnetic diffusivity. The
azimuthal magnetic fields generated in the active zone oscillate over time,
changing sign about every 150 years. We thus observe the radial structure of
the `butterfly pattern' seen previously in local shearing-box simulations. The
mean magnetic field diffuses from the active zone into the dead zone, where the
Reynolds stress nevertheless dominates. The greater total accretion stress in
the active zone leads to a net reduction in the surface density, so that after
800 years an approximate steady state is reached in which a local radial
maximum in the midplane pressure lies near the transition radius. We also
observe the formation of density ridges within the active zone. The dead zone
in our models possesses a mean magnetic field, significant Reynolds stresses
and a steady local pressure maximum at the inner edge, where the outward
migration of planetary embryos and the efficient trapping of solid material are
possible.Comment: 17 pages, 30 *.ps files for figures. Accepted 16 November 2009 in A&
Planetesimal formation by sweep-up: How the bouncing barrier can be beneficial to growth
The formation of planetesimals is often accredited to collisional sticking of
dust grains. The exact process is unknown, as collisions between larger
aggregates tend to lead to fragmentation or bouncing rather than sticking.
Recent laboratory experiments have however made great progress in the
understanding and mapping of the complex physics involved in dust collisions.
We want to study the possibility of planetesimal formation using the results
from the latest laboratory experiments, particularly by including the
fragmentation with mass transfer effect, which might lead to growth even at
high impact velocities. We present a new experimentally and physically
motivated dust collision model capable of predicting the outcome of a collision
between two particles of arbitrary masses and velocities. It is used together
with a continuum dust-size evolution code that is both fast in terms of
execution time and able to resolve the dust well at all sizes, allowing for all
types of interactions to be studied without biases. We find that for the
general dust population, bouncing collisions prevent the growth above
millimeter-sizes. However, if a small number of cm-sized particles are
introduced, for example due to vertical mixing or radial drift, they can act as
a catalyst and start to sweep up the smaller particles. At a distance of 3 AU,
100-meter-sized bodies are formed on a timescale of 1 Myr. We conclude that
direct growth of planetesimals might be a possibility thanks to a combination
of the existence of a bouncing barrier and the fragmentation with mass transfer
effect. The bouncing barrier is here even beneficial, as it prevents the growth
of too many large particles that would otherwise only fragment among each
other, and creates a reservoir of small particles that can be swept up by
larger bodies. However, for this process to work, a few seeds of cm in size or
larger have to be introduced.Comment: 17 pages, 13 figures. Accepted for publication in Astronomy and
Astrophysic
Evidences of vortex curvature and anisotropic pinning in superconducting films by quantitative magneto-optics
We present the experimental observation of magnetic field line curvature at
the surface of a superconducting film by local quantitative magneto-optics. In
addition to the knowledge of the full induction field at the superconductor
surface yielding the quantitative observation of the flux line curvature, our
analysis method allows also local value measurements of the electrical current
density inside the sample. Thus, we study the interplay between the
electrodynamic constraints dictated by the film geometry and the pinning
properties of the superconductor. In particular, we investigate the anisotropic
vortex-pinning, due to columnar defects introduced by heavy ion irradiation, as
revealed in the local current density dependence on the vortex curvature during
magnetic flux diffusion inside the superconducting film.Comment: 12 pages, 11 figures, to be submitted to Phys. Rev.
Large Scale Azimuthal Structures Of Turbulence In Accretion Disks - Dynamo triggered variability of accretion
We investigate the significance of large scale azimuthal, magnetic and
velocity modes for the MRI turbulence in accretion disks. We perform 3D global
ideal MHD simulations of global stratified proto-planetary disk models. Our
domains span azimuthal angles of \pi/4, \pi/2, \pi and 2\pi. We observe up to
100% stronger magnetic fields and stronger turbulence for the restricted
azimuthal domain models \pi/2 and \pi/4 compared to the full 2\pi model. We
show that for those models, the Maxwell Stress is larger due to strong
axisymmetric magnetic fields, generated by the \alpha \Omega dynamo. Large
radial extended axisymmetric toroidal fields trigger temporal magnification of
accretion stress. All models display a positive dynamo-\alpha in the northern
hemisphere (upper disk). The parity is distinct in each model and changes on
timescales of 40 local orbits. In model 2\pi, the toroidal field is mostly
antisymmetric in respect to the midplane. The eddies of the MRI turbulence are
highly anisotropic. The major wavelengths of the turbulent velocity and
magnetic fields are between one and two disk scale heights. At the midplane, we
find magnetic tilt angles around 8-9 degree increasing up to 12-13 degree in
the corona. We conclude that an azimuthal extent of \pi is sufficient to
reproduce most turbulent properties in 3D global stratified simulations of
magnetised accretion disks.Comment: accepted for publication in Ap
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