6,165 research outputs found
On Fabry P\'erot Etalon based Instruments. I. The Isotropic Case
Here we assess the spectral and imaging properties of Fabry P\'erot etalons
when located in solar magnetographs. We discuss the chosen configuration
(collimated or telecentric) for both ideal and real cases. For the real cases,
we focus on the effects caused by the polychromatic illumination of the filter
by the irregularities in the optical thickness of the etalon and by deviations
from the ideal illumination in both setups. We first review the general
properties of Fabry P\'erots and we then address the different sources of
degradation of the spectral transmission profile. We review and extend the
general treatment of defects followed by different authors. We discuss the
differences between the point spread functions (PSFs) of the collimated and
telecentric configurations for both monochromatic and (real)
quasi-monochromatic illumination of the etalon. The PSF corresponding to
collimated mounts is shown to have a better performance, although it varies
from point to point due to an apodization of the image inherent to this
configuration. This is in contrast to the (perfect) telecentric case, where the
PSF remains constant but produces artificial velocities and magnetic field
signals because of its strong spectral dependence. We find that the unavoidable
presence of imperfections in the telecentrism produces a decrease of flux of
photons and a shift, a broadening and a loss of symmetrization of both the
spectral and PSF profiles over the field of view, thus compromising their
advantages over the collimated configuration. We evaluate these effects for
different apertures of the incident beam.Comment: 20 pages 22 figures 2 Appendice
Phase transition in a super superspin glass
We here confirm the occurrence of spin glass phase transition and extract
estimates of associated critical exponents of a highly monodisperse and densely
compacted system of bare maghemite nanoparticles. This system has earlier been
found to behave like an archetypal spin glass, with e.g. a sharp transition
from paramagnetic to non-equilibrium behavior, suggesting that this system
undergoes a spin-glass phase transition at a relatively high temperature,
140 K.Comment: 4 pages, 3 figure
Imbalance of p75(NTR)/TrkB protein expression in Huntington's disease: Implication for neuroprotective therapies
Neuroprotective therapies based on brain-derived neurotrophic factor (BDNF) administration have been proposed for Huntington's disease (HD) treatment. However, our group has recently reported reduced levels of TrkB in HD mouse models and HD human brain suggesting that besides a decrease on BDNF levels a reduction of TrkB expression could also contribute to diminished neurotrophic support in HD. BDNF can also bind to p75 neurotrophin receptor (p75(NTR)) modulating TrkB signaling. Therefore, in this study we have analyzed the levels of p75(NTR) in several HD models, as well as in HD human brain. Our data demonstrates a p75(NTR)/TrkB imbalance in the striatum of two different HD mouse models, Hdh(Q111/111) homozygous knockin mice and R6/1 mice that was also manifested in the putamen of HD patients. The imbalance between TrkB and p75(NTR) levels in a HD cellular model did not affect BDNF-mediated TrkB activation of prosurvival pathways but induced activation of apoptotic cascades as demonstrated by increased JNK phosphorylation. Moreover, BDNF failed to protect mutant huntingtin striatal cells transfected with p75(NTR) against NMDA-mediated excitotoxicity, which was associated with decreased Akt phosphorylation. Interestingly, lack of Akt activation following BDNF and NMDA treatment correlated with increased PP1 levels. Accordingly, pharmacological inhibition of PP1 by okadaic acid (OA) prevented mutant huntingtin striatal cell death induced by NMDA and BDNF. Altogether, our findings demonstrate that the p75(NTR)/TrkB imbalance induced by mutant huntingtin in striatal cells associated with the aberrant activity of PP1 disturbs BDNF neuroprotection likely contributing to increasing striatal vulnerability in HD. On the basis of this data we hypothesize that normalization of p75(NTR) and/or TrkB expression or their signaling will improve BDNF neuroprotective therapies in HD. Cell Death and Disease (2013) 4, e595; doi:10.1038/cddis.2013.116; published online 18 April 201
Electronic structure of the double perovskite Ba2Er(Nb,Sb)O 6
In this work, we present a detailed study of the structural and the electronic structure of the double perovskite Ba2Er(Nb,Sb)O 6. All calculations were performed with the Full-Potential Linear Augmented Plane Wave method (FP-LAPW) based on the Density Functional Theory (DFT). From the minimization of energy as a function of volume using the Murnaghan's state equation has been obtained the equilibrium lattice parameter and the bulk modulus of these compounds. The study of the electronic structure was based in the analysis of the electronic density of states (DOS), and the density of charge, showing that these compounds have a total magnetic moment of 3.0 ÎŒB per formula unit due to Er atoms.Fil: Gil Rebaza, Arles VĂctor. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de FĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FĂsica La Plata; ArgentinaFil: Deluque Toro, Crispulo Enrique. Universidad Popular del Cesar; ColombiaFil: TĂ©llez LandĂnez, D. A. Universidad Nacional de Colombia; ColombiaFil: Roa Rojas, J.. Universidad Nacional de Colombia; Colombi
Evershed clouds as precursors of moving magnetic features around sunspots
The relation between the Evershed flow and moving magnetic features (MMFs) is
studied using high-cadence, simultaneous spectropolarimetric measurements of a
sunspot in visible (630.2 nm) and near-infrared (1565 nm) lines. Doppler
velocities, magnetograms, and total linear polarization maps are calculated
from the observed Stokes profiles. We follow the temporal evolution of two
Evershed clouds that move radially outward along the same penumbral filament.
Eventually, the clouds cross the visible border of the spot and enter the moat
region, where they become MMFs. The flux patch farther from the sunspot has the
same polarity of the spot, while the MMF closer to it has opposite polarity and
exhibits abnormal circular polarization profiles. Our results provide strong
evidence that at least some MMFs are the continuation of the penumbral Evershed
flow into the moat. This, in turn, suggests that MMFs are magnetically
connected to sunspots.Comment: To appear in ApJ Letters, Vol 649, 2006 September 20 issu
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