2,642 research outputs found
What sets the magnetic field strength and cycle period in solar-type stars?
Two fundamental properties of stellar magnetic fields have been determined by
observations for solar-like stars with different Rossby numbers (Ro), namely,
the magnetic field strength and the magnetic cycle period. The field strength
exhibits two regimes: 1) for fast rotation it is independent of Ro, 2) for slow
rotation it decays with Ro following a power law. For the magnetic cycle period
two regimes of activity, the active and inactive branches, also have been
identified. For both of them, the longer the rotation period, the longer the
activity cycle. Using global dynamo simulations of solar like stars with Rossby
numbers between ~0.4 and ~2, this paper explores the relevance of rotational
shear layers in determining these observational properties. Our results,
consistent with non-linear alpha^2-Omega dynamos, show that the total magnetic
field strength is independent of the rotation period. Yet at surface levels,
the origin of the magnetic field is determined by Ro. While for Ro<1 it is
generated in the convection zone, for Ro>1 strong toroidal fields are generated
at the tachocline and rapidly emerge towards the surface. In agreement with the
observations, the magnetic cycle period increases with the rotational period.
However, a bifurcation is observed for Ro~1, separating a regime where
oscillatory dynamos operate mainly in the convection zone, from the regime
where the tachocline has a predominant role. In the latter the cycles are
believed to result from the periodic energy exchange between the dynamo and the
magneto-shear instabilities developing in the tachocline and the radiative
interior.Comment: 43 pages, 14 figures, accepted for publication in The Astrophysical
Journa
Synergistically enhanced stability of laccase immobilized on synthesized silver nanoparticles with water-soluble polymers
"In Press, Accepted Manuscript, Available online 12 March 2017"Silver nanoparticles (AgNPs) were synthesized by citrate reduction method in the presence of polymers, poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA) and chitosan, used as stabilizing agents, and an oxidoreductase enzyme, laccase (Lac), with the goal of expanding the NPs antimicrobial action.
AgNPs were characterized by UV-visible spectrometry, dynamic light scattering and transmission electron microscopy. As protecting agents, PEG and PVA promoted the formation of spherical uniformly-shaped, small-sized, monodispersed AgNPs (≈ 20 nm). High Mw polymers were established as most effective in producing small-sized NPs. Chitosan's viscosity led to the formation of aggregates. Despite the decrease in Lac activity registered for the hybrid formulation, AgNPs-polymer-Lac, a significant augment in stability over time (up to 13 days, at 50 °C) was observed. This novel formulation displays improved synergistic performance over AgNPs-Lac or polymer-Lac conjugates, since in the former the Lac activity becomes residual at the end of 3 days. By enabling many ionic interactions, chitosan restricted the mass transfer between Lac and substrate and, thus, inhibited the enzymatic activity.
These hybrid nanocomposites made up of inorganic NPs, organic polymers and immobilized antimicrobial oxidoreductive enzymes represent a new class of materials with improved synergistic performance. Moreover, the Lac and the AgNPs different antimicrobial action, both in time and mechanism, may also constitute a new alternative to reduce the probability of developing resistance-associated mutations.This work was funded by Portuguese Foundation for Science and TechnologyFCT/MCTES (PIDDAC) and co-financed by European funds (FEDER) through the PT2020 program, research projectM-ERA-NET/0006/2014. A. Zille and H. P. Felgueiras also acknowledge funding from FCT within the scope of the project POCI-01-0145-FEDER-007136 and UID/CTM/00264
Noether's Symmetry Theorem for Variational and Optimal Control Problems with Time Delay
We extend the DuBois-Reymond necessary optimality condition and Noether's
symmetry theorem to the time delay variational setting. Both Lagrangian and
Hamiltonian versions of Noether's theorem are proved, covering problems of the
calculus of variations and optimal control with delays.Comment: This is a preprint of a paper whose final and definite form will
appear in the international journal Numerical Algebra, Control and
Optimization (NACO). Paper accepted for publication 15-March-201
Amino acids in the development of Prodrugs
Although drugs currently used for the various types of diseases (e.g., antiparasitic, antiviral, antibacterial, etc.) are effective, they present several undesirable pharmacological and pharmaceutical properties. Most of the drugs have low bioavailability, lack of sensitivity, and do not target only the damaged cells, thus also affecting normal cells. Moreover, there is the risk of developing resistance against drugs upon chronic treatment. Consequently, their potential clinical applications might be limited and therefore, it is mandatory to find strategies that improve those properties of therapeutic agents. The development of prodrugs using amino acids as moieties has resulted in improvements in several properties, namely increased bioavailability, decreased toxicity of the parent drug, accurate delivery to target tissues or organs, and prevention of fast metabolism. Herein, we provide an overview of models currently in use of prodrug design with amino acids. Furthermore, we review the challenges related to the permeability of poorly absorbed drugs and transport and deliver on target organs.NV acknowledges support from Fundação para a Ciência e Tecnologia (FCT, Lisbon, Portugal) and FEDER (European Union), award number IF/00092/2014/CP1255/CT0004. NV also thanks FCT for the IF position and Fundação Manuel António da Mota (FMAM, Porto, Portugal) and Pfizer (Portugal) for support for the Nuno Vale Research Group. The contents of this report are solely the responsibility of the authors and do not necessarily represent the official views of the FCT, FMAM and Pfizer
Evidence for a Very Large-Scale Fractal Structure in the Universe from Cobe Measurements
In this work, we analyse the temperature fluctuations of the cosmic microwave
background radiation observed by COBE and show that the distribution can be
fitted by a fractal distribution with a fractal dimension .
This value is in close agreement with the fractal dimension obtained by Coleman
and Pietronero (1992) and Luo and Schramm (1992) from galaxy-galaxy and
cluster-cluster correlations up to . The fact that the
observed temperature fluctuations correspond to scales much larger than and are signatures of the primordial density fluctuations at the
recombination layer suggests that the structure of the matter at the early
universe was already fractal and thus non-homogeneous on those scales. This
result may have important consequences for the theoretical framework that
describes the universe.Comment: 11 pages, postscript file, 2 figures available upon request. To
appear in ApJ Letter
Deflection of ultra high energy cosmic rays by the galactic magnetic field: from the sources to the detector
We report the results of 3D simulations of the trajectories of ultra-high
energy protons and Fe nuclei (with energies and ) propagating through the galactic magnetic field from the
sources to the detector. A uniform distribution of anti-particles is
backtracked from the detector, at the Earth, to the halo of the Galaxy. We
assume an axisymmetric, large scale spiral magnetic field permeating both the
disc and the halo. A normal field component to the galactic plane () is
also included in part of the simulations. We find that the presence of a large
scale galactic magnetic field does not generally affect the arrival directions
of the protons, although the inclusion of a component may cause
significant deflection of the lower energy protons (
eV). Error boxes larger than or equal to are most expected in
this case. On the other hand, in the case of heavy nuclei, the arrival
direction of the particles is strongly dependent on the coordinates of the
particle source. The deflection may be high enough () as to make
extremely difficult any identification of the sources unless the real magnetic
field configuration is accurately determined. Moreover, not every incoming
particle direction is allowed between a given source and the detector. This
generates sky patches which are virtually unobservable from the Earth. In the
particular case of the UHE events of Yakutsk, Fly's Eye, and Akeno, they come
from locations for which the deflection caused by the assumed magnetic field is
not significant.Comment: LaTeX + 2 postscript figures - Color versions of both figures (highly
recommended) available via anonymous ftp at
ftp://capc07.ast.cam.ac.uk/pub/uhecr_gmf as fig*.g
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