1,145 research outputs found
Using Chemistry to Unveil the Kinematics of Starless Cores: Complex Radial Motions in Barnard 68
We present observations of 13CO, C18O, HCO+, H13CO+, DCO+ and N2H+ line
emission towards the Barnard 68 starless core. The line profiles are
interpreted using a chemical network coupled with a radiative transfer code in
order to reconstruct the radial velocity profile of the core. Our observations
and modeling indicate the presence of complex radial motions, with the inward
motions in the outer layers of the core but outward motions in the inner part,
suggesting radial oscillations. The presence of such oscillation would imply
that B68 is relatively old, typically one order of magnitude older than the age
inferred from its chemical evolution and statistical core lifetimes. Our study
demonstrates that chemistry can be used as a tool to constrain the radial
velocity profiles of starless cores.Comment: 12 pages, 3 figures, to appear in the Astrophysical Journal Letter
Dynamics of liquid crystalline domains in magnetic field
We study microscopic single domains nucleating and growing within the
coexistence region of the Isotropic (I) and Nematic (N) phases in magnetic
field. By rapidly switching on the magnetic field the time needed to align the
nuclei of sufficiently large size is measured, and is found to decrease with
the square of the magnetic field. When the field is removed the disordering
time is observed to last on a longer time scale. The growth rate of the nematic
domains at constant temperature within the coexistence region is found to
increase when a magnetic field is applied.Comment: 10 pages, 5 figures, unpublishe
Frank's constant in the hexatic phase
Using video-microscopy data of a two-dimensional colloidal system the
bond-order correlation function G6 is calculated and used to determine the
temperature-dependence of both the orientational correlation length xi6 in the
isotropic liquid phase and the Frank constant F_A in the hexatic phase. F_A
takes the value 72/pi at the hexatic to isotropic liquid phase transition and
diverges at the hexatic to crystal transition as predicted by the KTHNY-theory.
This is a quantitative test of the mechanism of breaking the orientational
symmetry by disclination unbinding
Observation of the critical regime near Anderson localization of light
Diffusive transport is among the most common phenomena in nature [1].
However, as predicted by Anderson [2], diffusion may break down due to
interference. This transition from diffusive transport to localization of waves
should occur for any type of classical or quantum wave in any media as long as
the wavelength becomes comparable to the transport mean free path [3].
The signatures of localization and those of absorption, or bound states, can
however be similar, such that an unequivocal proof of the existence of wave
localization in disordered bulk materials is still lacking. Here we present
measurements of time resolved non-classical diffusion of visible light in
strongly scattering samples, which cannot be explained by absorption, sample
geometry or reduction in transport velocity. Deviations from classical
diffusion increase strongly with decreasing as expected for a phase
transition. This constitutes an experimental realization of the critical regime
in the approach to Anderson localization.Comment: 5 pages, 4 figure
In vivo imaging of the central and peripheral effects of sleep deprivation and suprachiasmatic nuclei lesion on PERIOD-2 protein in mice.
STUDY OBJECTIVES: That sleep deprivation increases the brain expression of various clock genes has been well documented. Based on these and other findings we hypothesized that clock genes not only underlie circadian rhythm generation but are also implicated in sleep homeostasis. However, long time lags have been reported between the changes in the clock gene messenger RNA levels and their encoded proteins. It is therefore crucial to establish whether also protein levels increase within the time frame known to activate a homeostatic sleep response. We report on the central and peripheral effects of sleep deprivation on PERIOD-2 (PER2) protein both in intact and suprachiasmatic nuclei-lesioned mice.
DESIGN: In vivo and in situ PER2 imaging during baseline, sleep deprivation, and recovery.
SETTINGS: Mouse sleep-recording facility.
PARTICIPANTS: Per2::Luciferase knock-in mice.
INTERVENTIONS: N/A.
MEASUREMENTS AND RESULTS: Six-hour sleep deprivation increased PER2 not only in the brain but also in liver and kidney. Remarkably, the effects in the liver outlasted those observed in the brain. Within the brain the increase in PER2 concerned the cerebral cortex mainly, while leaving suprachiasmatic nuclei (SCN) levels unaffected. Against expectation, sleep deprivation did not increase PER2 in the brain of arrhythmic SCN-lesioned mice because of higher PER2 levels in baseline. In contrast, liver PER2 levels did increase in these mice similar to the sham and partially lesioned controls.
CONCLUSIONS: Our results stress the importance of considering both sleep-wake dependent and circadian processes when quantifying clock-gene levels. Because sleep deprivation alters PERIOD-2 in the brain as well as in the periphery, it is tempting to speculate that clock genes constitute a common pathway mediating the shared and well-known adverse effects of both chronic sleep loss and disrupted circadian rhythmicity on metabolic health
Nonlocal elastic compliance for soft solids: theory, simulations, and experiments
The nonlocal elastic response function is crucial for understanding many properties of soft solids. This may be obtained by measuring strain-strain autocorrelation functions. We use computer simulations as well as video microscopy data of superparamagnetic colloids to obtain these correlations for two-dimensional triangular solids. Elastic constants and elastic correlation lengths are extracted by analyzing the correlation functions. We show that to explain our observations displacement fluctuations in a soft solid need to contain affine (strain) as well as nonaffine components
Elastic Behavior of a Two-dimensional Crystal near Melting
Using positional data from video-microscopy we determine the elastic moduli
of two-dimensional colloidal crystals as a function of temperature. The moduli
are extracted from the wave-vector-dependent normal mode spring constants in
the limit and are compared to the renormalized Young's modulus of the
KTHNY theory. An essential element of this theory is the universal prediction
that Young's modulus must approach at the melting temperature. This is
indeed observed in our experiment.Comment: 4 pages, 3 figure
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