6,320 research outputs found
Modeling the secular evolution of migrating planet pairs
The subject of this paper is the secular behaviour of a pair of planets
evolving under dissipative forces. In particular, we investigate the case when
dissipative forces affect the planetary semi-major axes and the planets move
inward/outward the central star, in a process known as planet migration. To
perform this investigation, we introduce fundamental concepts of conservative
and dissipative dynamics of the three-body problem. Based on these concepts, we
develop a qualitative model of the secular evolution of the migrating planetary
pair. Our approach is based on analysis of the energy and the orbital angular
momentum exchange between the two-planet system and an external medium; thus no
specific kind of dissipative forces is invoked. We show that, under assumption
that dissipation is weak and slow, the evolutionary routes of the migrating
planets are traced by the Mode I and Mode II stationary solutions of the
conservative secular problem. The ultimate convergence and the evolution of the
system along one of these secular modes of motion is determined uniquely by the
condition that the dissipation rate is sufficiently smaller than the proper
secular frequency of the system. We show that it is possible to reassemble the
starting configurations and migration history of the systems on the basis of
their final states and consequently to constrain the parameters of the physical
processes involved.Comment: 20 pages, 17 figures. Accepted for publication in MNRA
An advance Towards the Synthesis of Ag Nanorod Arrays with Controlled Surface Roughness for SERS Substrates
An innovative approach to produce silver nanorod (NRs) arrays with controlled morphological parameters and surface roughness is presented. The Ag NRs were obtained using a three-stage fabrication process based on the electron beam exposure of a metal-polymer nanocomposite resist on a transparent substrate and development, a post bake and then a series of non-electrochemical metallization steps. After each step the evolution of the Ag NRs was characterized by scanning electron microscopy (SEM) for morphology and optical transmittance (T) measurements for Localized Surface Plasmon Resonance (LSPR). The transmittance measurements were interpreted using models based on the Finite Element Method (FEM)
Origin and Sustainability of The Population of Asteroids Captured in the Exterior Resonance 1:2 with Mars
At present, approximately 1500 asteroids are known to evolve inside or
sticked to the exterior 1:2 resonance with Mars at a = 2.418 AU, being (142)
Polana the largest member of this group. The effect of the forced secular modes
superposed to the resonance gives rise to a complex dynamical evolution.
Chaotic diffusion, collisions, close encounters with massive asteroids and
mainly orbital migration due to the Yarkovsky effect generate continuous
captures to and losses from the resonance, with a fraction of asteroids
remaining captured over long time scales and generating a concentration in the
semimajor axis distribution that exceeds by 20% the population of background
asteroids. The Yarkovsky effect induces different dynamics according to the
asteroid size, producing an excess of small asteroids inside the resonance. The
evolution in the resonance generates a signature on the orbits, mainly in
eccentricity, that depends on the time the asteroid remains captured inside the
resonance and on the magnitude of the Yarkovsky effect. The greater the
asteroids, the larger the time they remain captured in the resonance, allowing
greater diffusion in eccentricity and inclination. The resonance generates a
discontinuity and mixing in the space of proper elements producing
misidentification of dynamical family members, mainly for Vesta and Nysa-Polana
families. The half-life of resonant asteroids large enough for not being
affected by the Yarkovsky effect is about 1 Gyr. From the point of view of
taxonomic classes, the resonant population does not differ from the background
population and the excess of small asteroids is confirmed.Comment: Accepted for publication in Icaru
Signal sequence analysis of expressed sequence tags from the nematode Nippostrongylus brasiliensis and the evolution of secreted proteins in parasites
BACKGROUND: Parasitism is a highly successful mode of life and one that requires suites of gene adaptations to permit survival within a potentially hostile host. Among such adaptations is the secretion of proteins capable of modifying or manipulating the host environment. Nippostrongylus brasiliensis is a well-studied model nematode parasite of rodents, which secretes products known to modulate host immunity. RESULTS: Taking a genomic approach to characterize potential secreted products, we analyzed expressed sequence tag (EST) sequences for putative amino-terminal secretory signals. We sequenced ESTs from a cDNA library constructed by oligo-capping to select full-length cDNAs, as well as from conventional cDNA libraries. SignalP analysis was applied to predicted open reading frames, to identify potential signal peptides and anchors. Among 1,234 ESTs, 197 (~16%) contain predicted 5' signal sequences, with 176 classified as conventional signal peptides and 21 as signal anchors. ESTs cluster into 742 distinct genes, of which 135 (18%) bear predicted signal-sequence coding regions. Comparisons of clusters with homologs from Caenorhabditis elegans and more distantly related organisms reveal that the majority (65% at P < e(-10)) of signal peptide-bearing sequences from N. brasiliensis show no similarity to previously reported genes, and less than 10% align to conserved genes recorded outside the phylum Nematoda. Of all novel sequences identified, 32% contained predicted signal peptides, whereas this was the case for only 3.4% of conserved genes with sequence homologies beyond the Nematoda. CONCLUSIONS: These results indicate that secreted proteins may be undergoing accelerated evolution, either because of relaxed functional constraints, or in response to stronger selective pressure from host immunity
Growth and reductive transformation of a gold shell around pyramidal cadmium selenide nanocrystals
We report the growth of an unstable shell-like gold structure around
dihexagonal pyramidal CdSe nanocrystals in organic solution and the structural
transformation to spherical domains by two means: i) electron beam irradiation
(in situ) and (ii) addition of a strong reducing agent during synthesis. By
varying the conditions of gold deposition, such as ligands present or the
geometry of the CdSe nanocrystals, we were able to tune the gold domain size
between 1.4 nm to 3.9 nm and gain important information on the role of surface
chemistry in hetero nanoparticle synthesis and seed reactivity, both of which
are crucial points regarding the chemical design of new materials for
photocatalysis and optoelectronic applications.Comment: 5 pages, 4 figure
Bonsais in a Wild Forest? A Historical Interpretation of the Longevity of Large Spanish Family Firms
En este artículo se presentan los primeros resultados de una investigación en curso sobre la longevidad de las grandes empresas familiares españolas. Su principal objetivo es proporcionar datos sólidos sobre un tema en el que abundan más las opiniones que las estadísticas científicas, así como identificar las claves de la supervivencia y competitividad de las cerca de 250 empresas en que se basa el estudio. Se concluye que esta realidad no es sólo el resultado de dos décadas de integración económica en Europa o de una buena dotación de recursos naturales, sino también de un largo período de aprendizaje y de la combinación de dos factores: la especialización en nichos de mercado que no fueron objeto de interés estratégico por parte del Estado, y la habilidad para consolidar redes personales de cooperación e influencia dentro y fuera del país.This study presents the first results of an on-going study of longevity of large Spanish family firms. Its main objective is to provide solid data for a topic in which opinions are more abundant than scientific statistics, as well as to identify the keys to the survival and competitiveness of the almost 250 firms included in the study. It concludes that this reality is not just the result of two decades of integration in Europe or a fortunate endowment with some natural resources, but the outcome of a long learning process and the combination of two factors: specialization in market niches in which the state had no strategic interests and the creation of personal networks of cooperation and influence within and outside Spain
On the anomalous secular increase of the eccentricity of the orbit of the Moon
A recent analysis of a Lunar Laser Ranging (LLR) data record spanning 38.7 yr
revealed an anomalous increase of the eccentricity of the lunar orbit amounting
to de/dt_meas = (9 +/- 3) 10^-12 yr^-1. The present-day models of the
dissipative phenomena occurring in the interiors of both the Earth and the Moon
are not able to explain it. We examine several dynamical effects, not modeled
in the data analysis, in the framework of long-range modified models of gravity
and of the standard Newtonian/Einsteinian paradigm. It turns out that none of
them can accommodate de/dt_meas. Many of them do not even induce long-term
changes in e; other models do, instead, yield such an effect, but the resulting
magnitudes are in disagreement with de/dt_meas. In particular, the general
relativistic gravitomagnetic acceleration of the Moon due to the Earth's
angular momentum has the right order of magnitude, but the resulting
Lense-Thirring secular effect for the eccentricity vanishes. A potentially
viable Newtonian candidate would be a trans-Plutonian massive object (Planet
X/Nemesis/Tyche) since it, actually, would affect e with a non-vanishing
long-term variation. On the other hand, the values for the physical and orbital
parameters of such a hypothetical body required to obtain the right order of
magnitude for de/dt are completely unrealistic. Moreover, they are in neat
disagreement with both the most recent theoretical scenarios envisaging the
existence of a distant, planetary-sized body and with the model-independent
constraints on them dynamically inferred from planetary motions. Thus, the
issue of finding a satisfactorily explanation for the anomalous behavior of the
Moon's eccentricity remains open.Comment: LaTex2e, 8 pages, 1 table, 1 figure. Matching the version at press in
Monthly Notices of the Royal Astronomical Society (MNRAS
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