2,421 research outputs found
Scaling law for the heating of solar coronal loops
We report preliminary results from a series of numerical simulations of the
reduced magnetohydrodynamic equations, used to describe the dynamics of
magnetic loops in active regions of the solar corona. A stationary velocity
field is applied at the photospheric boundaries to imitate the driving action
of granule motions.
A turbulent stationary regime is reached, characterized by a broadband power
spectrum and heating rate levels compatible with the
heating requirements of active region loops. A dimensional analysis of the
equations indicates that their solutions are determined by two dimensionless
parameters: the Reynolds number and the ratio between the Alfven time and the
photospheric turnover time. From a series of simulations for different values
of this ratio, we determine how the heating rate scales with the physical
parameters of the problem, which might be useful for an observational test of
this model.Comment: 12 pages, 4 figures. Astrophysical Journal Letters (in press
Cauchy-characteristic Evolution of Einstein-Klein-Gordon Systems: The Black Hole Regime
The Cauchy+characteristic matching (CCM) problem for the scalar wave equation
is investigated in the background geometry of a Schwarzschild black hole.
Previously reported work developed the CCM framework for the coupled
Einstein-Klein-Gordon system of equations, assuming a regular center of
symmetry. Here, the time evolution after the formation of a black hole is
pursued, using a CCM formulation of the governing equations perturbed around
the Schwarzschild background. An extension of the matching scheme allows for
arbitrary matching boundary motion across the coordinate grid. As a proof of
concept, the late time behavior of the dynamics of the scalar field is
explored. The power-law tails in both the time-like and null infinity limits
are verified.Comment: To appear in Phys. Rev. D, 9 pages, revtex, 5 figures available at
http://www.astro.psu.edu/users/nr/preprints.htm
Enhancing nonlinear interactions by the superposition of plasmonic lattices on ê•(2)-nonlinear photonic crystals
Plasmonic structures have been revealed as efficient units to enhance localized nonlinear phenomena generated at dielectric-metal interfaces. However, their effect on the nonlinear interactions provided by quasi-phase matching processes in ê•(2) modulated dielectric crystals have been scarcely addressed, mainly due to the complexity in manufacturing appropriate periodic plasmonic structures overlying the ê•(2) dielectric structure. Here, by a simple method we have fabricated a periodic structure based on the combination of two commensurate lattices: a periodic lattice of chains of Ag nanoparticles and a periodic lattice of ê•(2)-modulation based on a ferroelectric domains structure. The hybrid system supports multiple surface plasmon lattice resonances (SLRs) at the technologically relevant NIR spectral region, which yield the enhancement of the nonlinear diffraction pattern generated by the ê•(2) structure. The superposition of the plasmonic and the ê•(2)-modulation lattice results in a 20-fold enhancement of the directional SHG due to the excitation of SLRs by the interacting waves involved in the nonlinear process. The results are obtained in lithium niobate, a widely used crystal in optoelectronics, and demonstrate the potential of the approach to design integrated solid-state platforms for on-chip optical steering, multiplexing or quantum technologiesThis work has been supported by the Spanish Government
(Contracts MAT2016-76106-R and PID2019-108257GB-I00/
AEI/10.13039/501100011033 and MarÃa de Maeztu “Pro gramme for Units of Excellence in R&D CEX2018-000805-M)
and Comunidad de Madrid (Grant SI1/PJI/2019-00105
Cauchy-characteristic Evolution of Einstein-Klein-Gordon Systems
A Cauchy-characteristic initial value problem for the Einstein-Klein-Gordon
system with spherical symmetry is presented. Initial data are specified on the
union of a space-like and null hypersurface. The development of the data is
obtained with the combination of a constrained Cauchy evolution in the interior
domain and a characteristic evolution in the exterior, asymptotically flat
region. The matching interface between the space-like and characteristic
foliations is constructed by imposing continuity conditions on metric,
extrinsic curvature and scalar field variables, ensuring smoothness across the
matching surface. The accuracy of the method is established for all ranges of
, most notably, with a detailed comparison of invariant observables
against reference solutions obtained with a calibrated, global, null algorithm.Comment: Submitted to Phys. Rev. D, 16 pages, revtex, 7 figures available at
http://nr.astro.psu.edu:8080/preprints.htm
Cementing proteins provide extra mechanical stabilization to viral cages
The study of virus shell stability is key not only for gaining insights into viral biological cycles but also for using viral capsids in materials science. The strength of viral particles depends profoundly on their structural changes occurring during maturation, whose final step often requires the specific binding of ‘decoration’ proteins (such as gpD in bacteriophage lambda) to the viral shell. Here we characterize the mechanical stability of gpD-free and gpD-decorated bacteriophage lambda capsids. The incorporation of gpD into the lambda shell imparts a major mechanical reinforcement that resists punctual deformations. We further interrogate lambda particle stability with molecular fatigue experiments that resemble the sub-lethal Brownian collisions of virus shells with macromolecules in crowded environments. Decorated particles are especially robust against collisions of a few kBT (where kB is the Boltzmann’s constant and T is the temperature ~300 K), which approximate those anticipated from molecular insults in the environmentWe acknowledge the MINECO of Spain (PIB2010US-00233, FIS3011-29493, Consolider CSD2010-00024, CAMprojectNo.S3009/MAT-1467), and the US National Science Foundation (MCB-1158107) for their financial support of this researc
LPS-Induced Genes in Intestinal Tissue of the Sea Cucumber Holothuria glaberrima
Metazoan immunity is mainly associated with specialized cells that are directly involved with the immune response. Nevertheless, both in vertebrates and invertebrates other organs might respond to immune activation and participate either directly or indirectly in the ongoing immune process. However, most of what is known about invertebrate immunity has been restricted to immune effector cells and little information is available on the immune responses of other tissues or organs. We now focus on the immune reactions of the intestinal tissue of an echinoderm. Our study employs a non-conventional model, the echinoderm Holothuria glaberrima, to identify intestinal molecules expressed after an immune challenge presented by an intra-coelomic injection of lipopolysaccharides (LPS). The expression profiles of intestinal genes expressed differentially between LPS-injected animals and control sea water-injected animals were determined using a custom-made Agilent microarray with 7209 sea cucumber intestinal ESTs. Fifty (50) unique sequences were found to be differentially expressed in the intestine of LPS-treated sea cucumbers. Seven (7) of these sequences represented homologues of known proteins, while the remaining (43) had no significant similarity with any protein, EST or RNA database. The known sequences corresponded to cytoskeletal proteins (Actin and alpha-actinin), metabolic enzymes (GAPDH, Ahcy and Gnmt), metal ion transport/metabolism (major yolk protein) and defense/recognition (fibrinogen-like protein). The expression pattern of 11 genes was validated using semi-quantitative RT-PCR. Nine of these corroborated the microarray results and the remaining two showed a similar trend but without statistical significance. Our results show some of the molecular events by which the holothurian intestine responds to an immune challenge and provide important information to the study of the evolution of the immune response
Effect of bile on the lipid composition and surface properties of bifidobacteria
Aim: The changes produced on the bacterial surface of Bifidobacteria cells when they are grown in bile were compared with those provoked by bile added to bacteria grown in the absence of bile.
Methods and Results: The adhesive properties, the zeta potential and the lipid composition of Bifidobacterial strains, isolated from human faeces and grown in MRS medium, were determined. Bacteria grown in MRS with bile showed a loss of adherence and autoaggregation in correlation with a decrease in the surface hydrophobicity in comparison to those grown in MRS without bile, concomitant with the absence of two glycolipids, the increase of sugar content and minor changes in fatty acid composition. The surface changes caused by bile shock on bacteria grown in bile-free medium were much less pronounced and, in addition, no effect on the lipid composition was apparent.
Conclusions: The comparison of the results indicates that bile action on surface properties is related to metabolic changes.
Significance and Impact of the Study: Long-term exposure of bacteria to bile may cause metabolic changes affecting their adhesive properties irreversibly. This may be taken as a criterion to define the probiotic properties of different strains.Facultad de Ciencias ExactasCentro de Investigación y Desarrollo en CriotecnologÃa de Alimento
Effect of bile on the lipid composition and surface properties of bifidobacteria
Aim: The changes produced on the bacterial surface of Bifidobacteria cells when they are grown in bile were compared with those provoked by bile added to bacteria grown in the absence of bile.
Methods and Results: The adhesive properties, the zeta potential and the lipid composition of Bifidobacterial strains, isolated from human faeces and grown in MRS medium, were determined. Bacteria grown in MRS with bile showed a loss of adherence and autoaggregation in correlation with a decrease in the surface hydrophobicity in comparison to those grown in MRS without bile, concomitant with the absence of two glycolipids, the increase of sugar content and minor changes in fatty acid composition. The surface changes caused by bile shock on bacteria grown in bile-free medium were much less pronounced and, in addition, no effect on the lipid composition was apparent.
Conclusions: The comparison of the results indicates that bile action on surface properties is related to metabolic changes.
Significance and Impact of the Study: Long-term exposure of bacteria to bile may cause metabolic changes affecting their adhesive properties irreversibly. This may be taken as a criterion to define the probiotic properties of different strains.Facultad de Ciencias ExactasCentro de Investigación y Desarrollo en CriotecnologÃa de Alimento
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