1,819 research outputs found
Effects of Magnetic Fields on the Diskoseismic Modes of Accreting Black Holes
The origin of the rapid quasi-periodic variabilities observed in a number of
accreting black hole X-ray binaries is not understood. It has been suggested
that these variabilities are associated with diskoseismic oscillation modes of
the black hole accretion disk. In particular, in a disk with no magnetic field,
the so-called g-modes (inertial oscillations) can be self-trapped at the inner
region of the disk due to general relativistic effects. Real accretion disks,
however, are expected to be turbulent and contain appreciable magnetic fields.
We show in this paper that even a weak magnetic field (with the magnetic energy
much less than the thermal energy) can modify or "destroy" the self-trapping
zone of disk g-modes, rendering their existence questionable in realistic black
hole accretion disks. The so-called corrugation modes (c-modes) are also
strongly affected when the poloidal field approaches equal-partition. On the
other hand, acoustic oscillations (p-modes), which do not have vertical
structure, are not affected qualitatively by the magnetic field, and therefore
may survive in a turbulent, magnetic disk.Comment: 21 pages, 5 figures, accepted for publication in Ap
Low- instabilities in differentially rotating proto-neutron stars with magnetic fields
Recent hydrodynamical simulations have shown that differentially rotating
neutron stars formed in core-collapse supernovae may develop global
non-axisymmetric instabilities even when (the ratio of the rotational
kinetic energy to the gravitational potential energy ) is relatively
small (less than 0.1). Such low- instability can give rise to efficient
gravitational wave emission from the proto-neutron star. We investigate how
this instability is affected by magnetic fields using a cylindrical stellar
model. Wave absorption at the corotation resonance plays an important role in
facilitating the hydrodynamic low- instability. In the presence of a
toroidal magnetic field, the corotation resonance is split into two magnetic
resonances where wave absorptions take place. We show that the toroidal
magnetic field suppresses the low- instability when the total magnetic
energy is of order or larger, corresponding to toroidal
fields of a few G or stronger. Although poloidal magnetic
fields do not influence the instability directly, they can affect the
instability by generating toroidal fields through linear winding of the initial
poloidal field and magneto-rotational instability. We show that an initial
poloidal field with strength as small as G may suppress the
low- instability.Comment: 12 pages, 6 figures; submitted to MNRA
Dynamics of the Innermost Accretion Flows Around Compact Objects: Magnetosphere-Disc Interface, Global Oscillations and Instabilities
We study global non-axisymmetric oscillation modes and instabilities in
magnetosphere- disc systems, as expected in neutron star X-ray binaries and
possibly also in accreting black hole systems. Our two-dimensional
magnetosphere-disc model consists of a Keplerian disc in contact with an
uniformly rotating magnetosphere with low plasma density. Two types of global
overstable modes exist in such systems, the interface modes and the disc
inertial-acoustic modes. We examine various physical effects and parameters
that influence the properties of these oscillation modes, particularly their
growth rates, including the magnetosphere field configuration, the velocity and
density contrasts across the magnetosphere-disc interface, the rotation profile
(with Newtonian or General Relativistic potential), the sound speed and
magnetic field of the disc. The interface modes are driven unstable by
Rayleigh-Taylor and Kelvin-Helmholtz in- stabilities, but can be stabilized by
the toroidal field (through magnetic tension) and disc differential rotation
(through finite vorticity). General relativity increases their growth rates by
modifying the disc vorticity outside the magnetosphere boundary. The interface
modes may also be affected by wave absorption associated with corotation
resonance in the disc. In the presence of a magnetosphere, the
inertial-acoustic modes are effectively trapped at the innermost region of the
relativistic disc just outside the interface. They are driven unstable by wave
absorption at the corotation resonance, but can be stabilized by modest disc
magnetic fields. The overstable oscillation modes studied in this paper have
characteristic properties that make them possible candidates for the
quasi-periodic oscillations observed in X-ray binaries.Comment: 18 pages, 9 figures, MNRAS accepte
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Wnts Enhance Neurotrophin-Induced Neuronal Differentiation in Adult Bone-Marrow-Derived Mesenchymal Stem Cells via Canonical and Noncanonical Signaling Pathways
Wnts were previously shown to regulate the neurogenesis of neural stem or progenitor cells. Here, we explored the underlying molecular mechanisms through which Wnt signaling regulates neurotrophins (NTs) in the NT-induced neuronal differentiation of human mesenchymal stem cells (hMSCs). NTs can increase the expression of Wnt1 and Wnt7a in hMSCs. However, only Wnt7a enables the expression of synapsin-1, a synaptic marker in mature neurons, to be induced and triggers the formation of cholinergic and dopaminergic neurons. Human recombinant (hr)Wnt7a and general neuron makers were positively correlated in a dose- and time-dependent manner. In addition, the expression of synaptic markers and neurites was induced by Wnt7a and lithium, a glycogen synthase kinase-3β inhibitor, in the NT-induced hMSCs via the canonical/β-catenin pathway, but was inhibited by Wnt inhibitors and frizzled-5 (Frz5) blocking antibodies. In addition, hrWnt7a triggered the formation of cholinergic and dopaminergic neurons via the non-canonical/c-jun N-terminal kinase (JNK) pathway, and the formation of these neurons was inhibited by a JNK inhibitor and Frz9 blocking antibodies. In conclusion, hrWnt7a enhances the synthesis of synapse and facilitates neuronal differentiation in hMSCS through various Frz receptors. These mechanisms may be employed widely in the transdifferentiation of other adult stem cells
Hypothermic manipulation of bone cement can extend the handling time during vertebroplasty
BACKGROUND: Polymethylmethacrylate (PMMA) is commonly used for clinical applications. However, the short handling time increases the probability of a surgeon missing the crucial period in which the cement maintains its ideal viscosity for a successful injection. The aim of this article was to illustrate the effects a reduction in temperature would have on the cement handling time during percutaneous vertebroplasty. METHODS: The injectability of bone cement was assessed using a cement compressor. By twisting the compressor, the piston transmits its axial load to the plunger, which then pumps the bone cement out. The experiments were categorized based on the different types of hypothermic manipulation that were used. In group I (room temperature, sham group), the syringes were kept at 22°C after mixing the bone cement. In group 2 (precooling the bone cement and the container), the PMMA powder and liquid, as well as the beaker, spatula, and syringe, were stored in the refrigerator (4°C) overnight before mixing. In group 3 (ice bath cooling), the syringes were immediately submerged in ice water after mixing the bone cement at room temperature. RESULTS: The average liquid time, paste time, and handling time were 5.1 ± 0.7, 3.4 ± 0.3, and 8.5 ± 0.8 min, respectively, for group 1; 9.4 ± 1.1, 5.8 ± 0.5, and 15.2 ± 1.2 min, respectively, for group 2; and 83.8 ± 5.2, 28.8 ± 6.9, and 112.5 ± 11.3 min, respectively, for group 3. The liquid and paste times could be increased through different cooling methods. In addition, the liquid time (i.e. waiting time) for ice bath cooling was longer than for that of the precooling method (p < 0.05). CONCLUSIONS: Both precooling (i.e. lowering the initial temperature) and ice bath cooling (i.e. lowering the surrounding temperature) can effectively slow polymerization. Precooling is easy for clinical applications, while ice bath cooling might be more suitable for multiple-level vertebroplasty. Clinicians can take advantage of the improved injectability without any increased cost
Corotational Instability, Magnetic Resonances and Global Inertial-Acoustic Oscillations in Magnetized Black-Hole Accretion Discs
Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic
modes) trapped in the inner-most region of hydrodynamic accretion discs around
black holes, are plausible candidates for high-frequency quasi-periodic
oscillations (QPOs) observed in a number of accreting black-hole systems. These
modes are subject to global instabilities due to wave absorption at the
corotation resonance (where the wave pattern frequency equals the
disc rotation rate ), when the fluid vortensity,
(where and are the radial
epicyclic frequency and disc surface density, respectively), has a positive
gradient. We investigate the effects of disc magnetic fields on the wave
absorption at corotation and the related wave super-reflection of the
corotation barrier, and on the overstability of disc p-modes. For discs with a
pure toroidal field, the corotation resonance is split into two magnetic
resonances, where the wave frequency in the corotating frame of the fluid,
\tomega=\omega-m\Omega, matches the slow magnetosonic wave frequency.
Significant wave energy/angular momentum absorption occurs at both magnetic
resonances, but with opposite signs. The combined effect of the two magnetic
resonances is to reduce the super-reflection and the growth rate of the
overstable p-modes. We show that even a subthermal toroidal field may suppress
the overstability of hydrodynamic (B=0) p-modes. For accretion discs with mixed
(toroidal and vertical) magnetic fields, two additional Alfven resonances
appear, where \tomega matches the local Alfven wave frequency. They further
reduce the growth rate of p-modes. Our results suggest that in order for the
non-axisymmetric p-modes to be a viable candidate for the observed
high-frequency QPOs, the disc magnetic field must be appreciably subthermal, or
other mode excitation mechanisms are at work.Comment: 21 pages, 11 figures, MNRAS accepte
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