203 research outputs found
Nonaxisymmetric Magnetorotational Instability in Proto-Neutron Stars
We investigate the stability of differentially rotating proto-neutron stars
(PNSs) with a toroidal magnetic field. Stability criteria for nonaxisymmetric
MHD instabilities are derived using a local linear analysis. PNSs are expected
to have much stronger radial shear in the rotation velocity compared to normal
stars. We find that nonaxisymmetric magnetorotational instability (NMRI) with a
large azimuthal wavenumber is dominant over the kink mode () in
differentially rotating PNSs. The growth rate of the NMRI is of the order of
the angular velocity which is faster than that of the kink-type
instability by several orders of magnitude. The stability criteria are
analogous to those of the axisymmetric magnetorotational instability with a
poloidal field, although the effects of leptonic gradients are considered in
our analysis. The NMRI can grow even in convectively stable layers if the
wavevectors of unstable modes are parallel to the restoring force by the
Brunt-V\"ais\"al\"a oscillation. The nonlinear evolution of NMRI could amplify
the magnetic fields and drive MHD turbulence in PNSs, which may lead to
enhancement of the neutrino luminosity.Comment: 24pages, 7figures, Accepted for publication in the Astrophysical
Journal (December 12, 2005
On the Saturation of the Magnetorotational Instability via Parasitic Modes
We investigate the stability of incompressible, exact, non-ideal
magnetorotational (MRI) modes against parasitic instabilities. Both
Kelvin-Helmholtz and tearing-mode parasitic instabilities may occur in the
dissipative regimes accessible to current numerical simulations. We suppose
that a primary MRI mode saturates at an amplitude such that its fastest
parasite has a growth rate comparable to its own. The predicted alpha parameter
then depends critically on whether the fastest primary and parasitic modes fit
within the computational domain and whether non-axisymmetric parasitic modes
are allowed. Hence even simulations that resolve viscous and resistive scales
may not saturate properly unless the numerical domain is large enough to allow
the free evolution of both MRI and parasitic modes. To minimally satisfy these
requirements in simulations with vertical background fields, the vertical
extent of the domain should accommodate the fastest growing MRI mode while the
radial and azimuthal extents must be twice as large. The fastest parasites have
horizontal wavelengths roughly twice as long as the vertical wavelength of the
primary.Comment: 5 pages, 2 figures, uses emulateapj. Modified text in order to
address referee's comments and suggestions. References added. ApJ Letters
accepte
Relativistic magnetic reconnection at X-type neutral points
Relativistic effects in the oscillatory damping of magnetic disturbances near
two-dimensional X-points are investigated. By taking into account displacement
current, we study new features of extremely magnetized systems, in which the
Alfv\'en velocity is almost the speed of light. The frequencies of the
least-damped mode are calculated using linearized relativistic MHD equations
for wide ranges of the Lundquist number S and the magnetization parameter
. These timescales approach constant values in the large resistive
limit: the oscillation time becomes a few times the light crossing time,
irrespective of , and the decay time is proportional to and
therefore is longer for a highly magnetized system.Comment: 6 pages, 4 figure
Sustained Magnetorotational Turbulence in Local Simulations of Stratified Disks with Zero Net Magnetic Flux
We examine the effects of density stratification on magnetohydrodynamic
turbulence driven by the magnetorotational instability in local simulations
that adopt the shearing box approximation. Our primary result is that, even in
the absence of explicit dissipation, the addition of vertical gravity leads to
convergence in the turbulent energy densities and stresses as the resolution
increases, contrary to results for zero net flux, unstratified boxes. The ratio
of total stress to midplane pressure has a mean of ~0.01, although there can be
significant fluctuations on long (>~50 orbit) timescales. We find that the time
averaged stresses are largely insensitive to both the radial or vertical aspect
ratio of our simulation domain. For simulations with explicit dissipation, we
find that stratification extends the range of Reynolds and magnetic Prandtl
numbers for which turbulence is sustained. Confirming the results of previous
studies, we find oscillations in the large scale toroidal field with periods of
~10 orbits and describe the dynamo process that underlies these cycles.Comment: 13 pages, 18 figures, submitted to Ap
Salivary TNFα levels in groups of subjects with rheumatoid arthritis and chronic periodontitis
Rhaponticum acaule (L) DC essential oil: chemical composition, in vitro antioxidant and enzyme inhibition properties
Background: α-glucosidase is a therapeutic target for diabetes mellitus (DM) and α-glucosidase inhibitors play a
vital role in the treatments for the disease. Furthermore, xanthine oxidase (XO) is a key enzyme that catalyzes
hypoxanthine and xanthine to uric acid which at high levels can lead to hyperuricemia which is an important cause
of gout. Pancreatic lipase (PL) secreted into the duodenum plays a key role in the digestion and absorption of fats.
For its importance in lipid digestion, PL represents an attractive target for obesity prevention.
Methods: The flowers essential oil of Rhaponticum acaule (L) DC (R. acaule) was characterized using gas
chromatography-mass spectrometry (GC-MS). The antioxidant activities of R. acaule essential oil (RaEO) were also
determined using 2,2’-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS), reducing power,
phosphomolybdenum, and DNA nicking assays. The inhibitory power of RaEO against α-glucosidase, xanthine
oxidase and pancreatic lipase was evaluated. Enzyme kinetic studies using Michaelis-Menten and the derived
Lineweaver-Burk (LB) plots were performed to understand the possible mechanism of inhibition exercised by the
components of this essential oil.
Results: The result revealed the presence of 26 compounds (97.4%). The main constituents include germacrene D
(49.2%), methyl eugenol (8.3%), (E)-β-ionone (6.2%), β-caryophyllene (5.7%), (E,E)-α-farnesene (4.2%),
bicyclogermacrene (4.1%) and (Z)-α-bisabolene (3.7%). The kinetic inhibition study showed that the essential oil
demonstrated a strong α-glucosidase inhibiton and it was a mixed inhibitor. On the other hand, our results
evidenced that this oil exhibited important xanthine oxidase inhibitory effect, behaving as a non-competitive
inhibitor. The essential oil inhibited the turkey pancreatic lipase, with maximum inhibition of 80% achieved at
2 mg/mL. Furthermore, the inhibition of turkey pancreatic lipase by RaEO was an irreversible one.
Conclusion: The results revealed that the RaEO is a new promising potential source of antioxidant compounds,
endowed with good practical applications for human health.
Keywords: α-glucosidase, Antioxidant activity, Chemical composition, Pancreatic lipase inhibition, Rhaponticum
acaule essential oil, Xanthine oxidase
The Effect of Age on the Gingival Crevicular Fluid Composition During Experimental Gingivitis. A Pilot Study
Preconditioning-induced ischemic tolerance: a window into endogenous gearing for cerebroprotection
Ischemic tolerance defines transient resistance to lethal ischemia gained by a prior sublethal noxious stimulus (i.e., preconditioning). This adaptive response is thought to be an evolutionarily conserved defense mechanism, observed in a wide variety of species. Preconditioning confers ischemic tolerance if not in all, in most organ systems, including the heart, kidney, liver, and small intestine. Since the first landmark experimental demonstration of ischemic tolerance in the gerbil brain in early 1990's, basic scientific knowledge on the mechanisms of cerebral ischemic tolerance increased substantially. Various noxious stimuli can precondition the brain, presumably through a common mechanism, genomic reprogramming. Ischemic tolerance occurs in two temporally distinct windows. Early tolerance can be achieved within minutes, but wanes also rapidly, within hours. Delayed tolerance develops in hours and lasts for days. The main mechanism involved in early tolerance is adaptation of membrane receptors, whereas gene activation with subsequent de novo protein synthesis dominates delayed tolerance. Ischemic preconditioning is associated with robust cerebroprotection in animals. In humans, transient ischemic attacks may be the clinical correlate of preconditioning leading to ischemic tolerance. Mimicking the mechanisms of this unique endogenous protection process is therefore a potential strategy for stroke prevention. Perhaps new remedies for stroke are very close, right in our cells
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