931 research outputs found
Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks
In recent work we presented the first results of global general relativistic
magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion
disks around rotating black holes. The simulated tilted disks showed dramatic
differences from comparable untilted disks, such as asymmetrical accretion onto
the hole through opposing "plunging streams" and global precession of the disk
powered by a torque provided by the black hole. However, those simulations used
a traditional spherical-polar grid that was purposefully underresolved along
the pole, which prevented us from assessing the behavior of any jets that may
have been associated with the tilted disks. To address this shortcoming we have
added a block-structured "cubed-sphere" grid option to the Cosmos++ GRMHD code,
which will allow us to simultaneously resolve the disk and polar regions. Here
we present our implementation of this grid and the results of a small suite of
validation tests intended to demonstrate that the new grid performs as
expected. The most important test in this work is a comparison of identical
tilted disks, one evolved using our spherical-polar grid and the other with the
cubed-sphere grid. We also demonstrate an interesting dependence of the
early-time evolution of our disks on their orientation with respect to the grid
alignment. This dependence arises from the differing treatment of current
sheets within the disks, especially whether they are aligned with symmetry
planes of the grid or not.Comment: 15 pages, 11 figures, submitted to Ap
Z -> b\bar{b} Versus Dynamical Electroweak Symmetry Breaking involving the Top Quark
In models of dynamical electroweak symmetry breaking which sensitively
involve the third generation, such as top quark condensation, the effects of
the new dynamics can show up experimentally in Z->b\bar{b}. We compare the
sensitivity of Z->b\bar{b} and top quark production at the Tevatron to models
of the new physics. Z->b\bar{b} is a relatively more sensitive probe to new
strongly coupled U(1) gauge bosons, while it is generally less sensitive a
probe to new physics involving color octet gauge bosons as is top quark
production itself. Nonetheless, to accomodate a significant excess in
Z->b\bar{b} requires choosing model parameters that may be ruled out within run
I(b) at the Tevatron.Comment: LaTex file, 19 pages + 2 Figs., Fermilab-Pub-94/231-
Multi-scale Renormalisation Group Improvement of the Effective Potential
Using the renormalisation group and a conjecture concerning the perturbation
series for the effective potential, the leading logarithms in the effective
potential are exactly summed for scalar and Yukawa theories.Comment: 19 pages, DIAS STP 94-09. Expanded to check large N limit, typo's
corrected, to appear in Phys Rev
Simulations of Accretion Powered Supernovae in the Progenitors of Gamma Ray Bursts
Observational evidence suggests a link between long duration gamma ray bursts
(LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type
Ic supernovae in LGRB progenitors powered solely by accretion energy. We
present spherically-symmetric hydrodynamic simulations of the long-term
accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto
the central compact object, which we take to be a black hole. The simulations
were carried out with the adaptive mesh refinement code FLASH in one spatial
dimension and with rotation, an explicit shear viscosity, and convection in the
mixing length theory approximation. Once the accretion flow becomes
rotationally supported outside of the black hole, an accretion shock forms and
traverses the stellar envelope. Energy is carried from the central
geometrically thick accretion disk to the stellar envelope by convection.
Energy losses through neutrino emission and nuclear photodisintegration are
calculated but do not seem important following the rapid early drop of the
accretion rate following circularization. We find that the shock velocity,
energy, and unbound mass are sensitive to convective efficiency, effective
viscosity, and initial stellar angular momentum. Our simulations show that
given the appropriate combinations of stellar and physical parameters,
explosions with energies ~5x10^50 ergs, velocities 3000 km/s, and unbound
material masses >6 solar masses are possible in a rapidly rotating 16 solar
mass main sequence progenitor star. Further work is needed to constrain the
values of these parameters, to identify the likely outcomes in more plausible
and massive LRGB progenitors, and to explore nucleosynthetic implications.Comment: 20 Pages, 15 Figures, Accepted to Ap
Collapsar Accretion and the Gamma-Ray Burst X-Ray Light Curve
We present axisymmetric hydrodynamical simulations of the long-term accretion
of a rotating GRB progenitor star, a "collapsar," onto the central compact
object. The simulations were carried out with the adaptive mesh refinement code
FLASH in two spatial dimensions and with an explicit shear viscosity. The
evolution of the central accretion rate exhibits phases reminiscent of the long
GRB gamma-ray and X-ray light curve, which lends support to the proposal that
the luminosity is modulated by the central accretion rate. After a few tens of
seconds, an accretion shock sweeps outward through the star. The formation and
outward expansion of the accretion shock is accompanied with a sudden and rapid
power-law decline in the central accretion rate Mdot ~ t^{-2.8}, which
resembles the L_X ~ t^{-3} decline observed in the X-ray light curves. The
collapsed, shock-heated stellar envelope settles into a thick, low-mass
equatorial disk embedded within a massive, pressure-supported atmosphere. After
a few hundred seconds, the inflow of low-angular-momentum material in the axial
funnel reverses into an outflow from the surface of the thick disk. Meanwhile,
the rapid decline of the accretion rate slows down, or even settles a in steady
state with Mdot ~ 5x10^{-5} Msun/s, which resembles the "plateau" phase in the
X-ray light curve. While the duration of the "prompt" phase depends on the
resolution in our simulations, we provide an analytical model taking into
account neutrino losses that estimates the duration to be ~20 s. The model
suggests that the steep decline in GRB X-ray light curves is triggered by the
circularization of the infalling stellar envelope at radii where the virial
temperature is below ~10^{10} K, such that neutrino cooling shuts off and an
outward expansion of the accretion shock becomes imminent.Comment: 16 pages, 10 figure
Maternal neurofascin-specific autoantibodies bind to structures of the fetal nervous system during pregnancy, but have no long term effect on development in the rat
Neurofascin was recently reported as a target for axopathic autoantibodies in patients with multiple sclerosis (MS), a response that will exacerbate axonal pathology and disease severity in an animal model of multiple sclerosis. As transplacental transfer of maternal autoantibodies can permanently damage the developing nervous system we investigated whether intrauterine exposure to this neurofascin-specific response had any detrimental effect on white matter tract development. To address this question we intravenously injected pregnant rats with either a pathogenic anti-neurofascin monoclonal antibody or an appropriate isotype control on days 15 and 18 of pregnancy, respectively, to mimic the physiological concentration of maternal antibodies in the circulation of the fetus towards the end of pregnancy. Pups were monitored daily with respect to litter size, birth weight, growth and motor development. Histological studies were performed on E20 embryos and pups sacrificed on days 2, 10, 21, 32 and 45 days post partum. Results: Immunohistochemistry for light and confocal microscopy confirmed passively transferred anti-neurofascin antibody had crossed the placenta to bind to distinct structures in the developing cortex and cerebellum. However, this did not result in any significant differences in litter size, birth weight, or general physical development between litters from control mothers or those treated with the neurofascin-specific antibody. Histological analysis also failed to identify any neuronal or white matter tract abnormalities induced by the neurofascin-specific antibody. Conclusions: We show that transplacental transfer of circulating anti-neurofascin antibodies can occur and targets specific structures in the CNS of the developing fetus. However, this did not result in any pre- or post-natal abnormalities in the offspring of the treated mothers. These results assure that even if anti-neurofascin responses are detected in pregnant women with multiple sclerosis these are unlikely to have a negative effect on their children
Fibroblast growth factor signalling in multiple sclerosis: inhibition of myelination and induction of pro-inflammatory environment by FGF9
The failure of remyelination in multiple sclerosis is largely unexplained. Lindner et al. report that glial cells in demyelinating lesions show increased expression of fibroblast growth factor 9 (FGF9). This induces astrocyte-dependent responses that inhibit remyelination and stimulate expression of pro-inflammatory chemokines, supporting a feedback loop that amplifies disease activit
Indication for the disappearance of reactor electron antineutrinos in the Double Chooz experiment
The Double Chooz Experiment presents an indication of reactor electron
antineutrino disappearance consistent with neutrino oscillations. A ratio of
0.944 0.016 (stat) 0.040 (syst) observed to predicted events was
obtained in 101 days of running at the Chooz Nuclear Power Plant in France,
with two 4.25 GW reactors. The results were obtained from a single 10
m fiducial volume detector located 1050 m from the two reactor cores. The
reactor antineutrino flux prediction used the Bugey4 measurement as an anchor
point. The deficit can be interpreted as an indication of a non-zero value of
the still unmeasured neutrino mixing parameter \sang. Analyzing both the rate
of the prompt positrons and their energy spectrum we find \sang = 0.086
0.041 (stat) 0.030 (syst), or, at 90% CL, 0.015 \sang 0.16.Comment: 7 pages, 4 figures, (new version after PRL referee's comments
Fibroblast growth factor signalling in multiple sclerosis:inhibition of myelination and induction of pro-inflammatory environment by FGF9
Remyelination failure plays an important role in the pathophysiology of multiple sclerosis, but the underlying cellular and molecular mechanisms remain poorly understood. We now report actively demyelinating lesions in patients with multiple sclerosis are associated with increased glial expression of fibroblast growth factor 9 (FGF9), which we demonstrate inhibits myelination and remyelination in vitro. This inhibitory activity is associated with the appearance of multi-branched âpre-myelinatingâ MBP+/PLP+ oligodendrocytes that interact with axons but fail to assemble myelin sheaths; an oligodendrocyte phenotype described previously in chronically demyelinated multiple sclerosis lesions. This inhibitory activity is not due to a direct effect of FGF9 on cells of the oligodendrocyte lineage but is mediated by factors secreted by astrocytes. Transcriptional profiling and functional validation studies demonstrate that these include effects dependent on increased expression of tissue inhibitor of metalloproteinase-sensitive proteases, enzymes more commonly associated with extracellular matrix remodelling. Further, we found that FGF9 induces expression of Ccl2 and Ccl7, two pro-inflammatory chemokines that contribute to recruitment of microglia and macrophages into multiple sclerosis lesions. These data indicate glial expression of FGF9 can initiate a complex astrocyte-dependent response that contributes to two distinct pathogenic pathways involved in the development of multiple sclerosis lesions. Namely, induction of a pro-inflammatory environment and failure of remyelination; a combination of effects predicted to exacerbate axonal injury and loss in patients
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