5,297 research outputs found
Crumbling under Pressure
In order for an organism to maintain its form, it must be able to withstand physical perturbation, including the pull of gravity. A recent study in Nature from Porazinski and colleagues (2015) suggests that mechanisms promoting tissue tension are critical to resist the Earth’s downward pull
Actomyosin Pulsing in Tissue Integrity Maintenance during Morphogenesis
The actomyosin cytoskeleton is responsible for many changes in cell and tissue shape. For a long time, the actomyosin cytoskeleton has been known to exhibit dynamic contractile behavior. Recently, discrete actomyosin assembly/disassembly cycles have also been observed in cells. These so-called actomyosin pulses have been observed in a variety of contexts, including cell polarization and division, and in epithelia, where they occur during tissue contraction, folding, and extension. In epithelia, evidence suggests that actomyosin pulsing, and more generally, actomyosin turnover, is required to maintain tissue integrity during contractile processes. This review explores possible functions for pulsing in the many instances during which pulsing has been observed, and also highlights proposed molecular mechanisms that drive pulsing
Driving spiral arms in the circumstellar disks of HD 100546 and HD 141569A
With 2D hydrodynamical simulations of disks perturbed externally by stars,
brown dwarfs or planets we investigate possible scenarios that can account for
the spiral structure in circumstellar disks. We consider two scenarios, spiral
structure driven by an external bound planet or low mass star and that excited
by a previous stellar close encounter or flyby. We find that both scenarios
produce morphology similar to that observed in the outer disks of HD 141569A
and HD 100546; moderately open 2-armed outer spiral structure. The outer
two-armed spiral structure observed in the disk of HD 141569A is qualitatively
reproduced with tidal perturbations from its companion binary HD 141569B,C on a
prograde orbit near periapse. Our simulation accounts for the outer spiral
arms, but is less successful than the secular model of Augereau and Papaloizou
at matching the lopsidedness or asymmetry of the disk edge at 300AU. The disk
has been previously truncated by the tidal force from the binary. A bound
object (stellar or planetary) is unlikely to explain the spiral structure in HD
100546. A co-eval planet or brown dwarf in the disk of sufficient mass to
account for the amplitude of the spiral structure would be detectable in NICMOS
and STIS images, however existing images reveal no such object. A previous
encounter could explain the observed structure, provided that the encounter
occurred less than a few thousand year ago. The object responsible for causing
the spiral structure must then be within a few arcminutes of the star. However,
the USNO-B proper motion survey reveals no candidate object. Moreover, the
probability that a field star encountered HD 100546 in the past few thousand
years is very low.Comment: accepted to A
Tides and Overtides in Long Island Sound
Using observations obtained by acoustic Doppler profilers and coastal water level recorders, we describe the vertical and horizontal structure of the currents and sea level due to the principal tidal constituents in Long Island Sound, a shallow estuary in southern New England. As expected, the observations reveal that M2 is the dominant constituent in both sea surface and velocity at all depths and sites. We also find evidence that the vertical structure of the M2 tidal current ellipse parameters vary with the seasonal evolution of vertical stratification at some sites. By comparing our estimates of the vertical structure of the M2 amplitudes to model predictions, we demonstrate that both uniform and vertically variable, time invariant eddy viscosities are not consistent with our measurements in the Sound. The current records from the western Sound contain significant overtides at the M4 and M6 frequencies with amplitudes and phases that are independent of depth. Though the M4 amplitude decreases to the west in proportion to M2, the M6 amplifies. Since the dynamics that generate overtides also produce tidal residuals, this provides a sensitive diagnostic of the performances of numerical circulation models. We demonstrate that the observed along-Sound structure of the amplitude of the M4 and M6 overtides is only qualitatively consistent with the predictions of a nonlinear, laterally averaged layer model forced by a mean flow and sea level at the boundaries. Since neither the vertical structure of the principal tidal constituent nor the pattern of horizontal variation of the largest overtides can be explained using well established models, we conclude that they are fundamentally inadequate and should no longer be used for more than a basic qualitative understanding, and even then should be used with caution. We provide comprehensive tables of the tidal current parameters to facilitate the critical evaluation of future models of the circulation in the Sound
The MHD Kelvin-Helmholtz Instability III: The Role of Sheared Magnetic Field in Planar Flows
We have carried out simulations of the nonlinear evolution of the
magnetohydrodynamic (MHD) Kelvin-Helmholtz (KH) instability for compressible
fluids in -dimensions, extending our previous work by Frank et al
(1996) and Jones \etal (1997). In the present work we have simulated flows in
the x-y plane in which a ``sheared'' magnetic field of uniform strength
``smoothly'' rotates across a thin velocity shear layer from the z direction to
the x direction, aligned with the flow field. We focus on dynamical evolution
of fluid features, kinetic energy dissipation, and mixing of the fluid between
the two layers, considering their dependence on magnetic field strength for
this geometry. The introduction of magnetic shear can allow a Cat's Eye-like
vortex to form, even when the field is stronger than the nominal linear
instability limit given above. For strong fields that vortex is asymmetric with
respect to the preliminary shear layer, however, so the subsequent dissipation
is enhanced over the uniform field cases of comparable field strength. In fact,
so long as the magnetic field achieves some level of dynamical importance
during an eddy turnover time, the asymmetries introduced through the magnetic
shear will increase flow complexity, and, with that, dissipation and mixing.
The degree of the fluid mixing between the two layers is strongly influenced by
the magnetic field strength. Mixing of the fluid is most effective when the
vortex is disrupted by magnetic tension during transient reconnection, through
local chaotic behavior that follows.Comment: 14 pages including 9 figures (4 figures in degraded jpg format), full
paper with original quality figures available via anonymous ftp at
ftp://canopus.chungnam.ac.kr/ryu/mhdkh2d.uu, to appear in The Astrophysical
Journa
The Formation of Crystalline Dust in AGB Winds from Binary Induced Spiral Shocks
As stars evolve along the Asymptotic Giant Branch, strong winds are driven
from the outer envelope. These winds form a shell, which may ultimately become
a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at
the presence of a binary companion. Some post-Asymptotic Giant Branch objects
are surrounded by torii of crystalline dust, but there is no generally accepted
mechanism for annealing the amorphous grains in the wind to crystals. In this
Letter, we show that the shaping of the wind by a binary companion is likely to
lead to the formation of crystalline dust in the orbital plane of the binary.Comment: Submitted to ApJ
On the Planet and the Disk of CoKuTau/4
Spitzer observations of the young star CoKuTau/4 reveal a disk with a 10 AU
hole that is most likely caused by a newly formed planet. Assuming that the
planet opened a gap in the viscous disk, we estimate that the planet mass is
greater than 0.1 Jupiter masses. This estimate depends on a lower limit to the
disk viscosity derived from the time scale needed to accrete the inner disk,
creating the now detectable hole. The planet migration time scale must at least
modestly exceed the time for the spectrally inferred hole to clear. The
proximity of the planet to the disk edge implied by our limits suggests that
the latter is perturbed by the nearby planet and may exhibit a spiral pattern
rotating with the planet. This pattern might be resolved with current ground
based mid-infrared cameras and optical cameras on the Hubble Space Telescope.
The required sub-Myr planet formation may challenge core accretion formation
models. However, we find that only if the planet mass is larger than about 10
Jupiter masses, allowing for a high enough surface density without inducing
migration, would formation by direct gravitational instability be possible.Comment: Submitted to ApJ
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