1,910 research outputs found
Granular flow down a rough inclined plane: transition between thin and thick piles
The rheology of granular particles in an inclined plane geometry is studied
using molecular dynamics simulations. The flow--no-flow boundary is determined
for piles of varying heights over a range of inclination angles . Three
angles determine the phase diagram: , the angle of repose, is the
angle at which a flowing system comes to rest; , the maximum angle
of stability, is the inclination required to induce flow in a static system;
and is the maximum angle for which stable, steady state flow is
observed. In the stable flow region , three
flow regimes can be distinguished that depend on how close is to
: i) : Bagnold rheology, characterized by a
mean particle velocity in the direction of flow that scales as
, for a pile of height , ii)
: the slow flow regime, characterized by a linear
velocity profile with depth, and iii) : avalanche flow
characterized by a slow underlying creep motion combined with occasional free
surface events and large energy fluctuations. We also probe the physics of the
initiation and cessation of flow. The results are compared to several recent
experimental studies on chute flows and suggest that differences between
measured velocity profiles in these experiments may simply be a consequence of
how far the system is from jamming.Comment: 19 pages, 14 figs, submitted to Physics of Fluid
Co-infections and multiple stressors in fish
Fish are typically exposed to multiple physical, chemical and biological stressors. The cumulative impact of co-infections between parasites, bacteria, viruses and (a)biotic environmental pressures may trigger complex interactions, eliciting different pathological and immunological outcomes than those classically assessed. New cross-disciplinary studies attempt to measure the impact of environmental stressors in modulating the host response to pathogens. Scientific advances are needed to reduce pressure on natural populations, improve fish stock management, and to design more efficient diagnostic tools or vaccination strategies. An EAFP-promoted workshop, held on 10th September 2019 in Porto, Portugal, was dedicated to sharing research experiences on the interaction between heterogenous pathogens and multiple stressors in fish. The workshop involved around 200 attendants, opened by a keynote talk (Fast), and followed by a further twelve oral presentations, including three in the format of lash poster presentations. Contributions illustrated cross-disciplinary approaches to study complex host-pathogen and stressors interactions
Astrophysical Constraints on Modifying Gravity at Large Distances
Recently, several interesting proposals were made modifying the law of
gravity on large scales, within a sensible relativistic formulation. This
allows a precise formulation of the idea that such a modification might account
for galaxy rotation curves, instead of the usual interpretation of these curves
as evidence for dark matter. We here summarize several observational
constraints which any such modification must satisfy, and which we believe make
more challenging any interpretation of galaxy rotation curves in terms of new
gravitational physics.Comment: References added, submitted to Classical & Quantum Gravit
Post-Einsteinian tests of gravitation
Einstein gravitation theory can be extended by preserving its geometrical
nature but changing the relation between curvature and energy-momentum tensors.
This change accounts for radiative corrections, replacing the Newton
gravitation constant by two running couplings which depend on scale and differ
in the two sectors of traceless and traced tensors. The metric and curvature
tensors in the field of the Sun, which were obtained in previous papers within
a linearized approximation, are then calculated without this restriction.
Modifications of gravitational effects on geodesics are then studied, allowing
one to explore phenomenological consequences of extensions lying in the
vicinity of general relativity. Some of these extended theories are able to
account for the Pioneer anomaly while remaining compatible with tests involving
the motion of planets. The PPN Ansatz corresponds to peculiar extensions of
general relativity which do not have the ability to meet this compatibility
challenge.Comment: 19 pages Corrected typo
Virus Replication as a Phenotypic Version of Polynucleotide Evolution
In this paper we revisit and adapt to viral evolution an approach based on
the theory of branching process advanced by Demetrius, Schuster and Sigmund
("Polynucleotide evolution and branching processes", Bull. Math. Biol. 46
(1985) 239-262), in their study of polynucleotide evolution. By taking into
account beneficial effects we obtain a non-trivial multivariate generalization
of their single-type branching process model. Perturbative techniques allows us
to obtain analytical asymptotic expressions for the main global parameters of
the model which lead to the following rigorous results: (i) a new criterion for
"no sure extinction", (ii) a generalization and proof, for this particular
class of models, of the lethal mutagenesis criterion proposed by Bull,
Sanju\'an and Wilke ("Theory of lethal mutagenesis for viruses", J. Virology 18
(2007) 2930-2939), (iii) a new proposal for the notion of relaxation time with
a quantitative prescription for its evaluation, (iv) the quantitative
description of the evolution of the expected values in in four distinct
"stages": extinction threshold, lethal mutagenesis, stationary "equilibrium"
and transient. Finally, based on these quantitative results we are able to draw
some qualitative conclusions.Comment: 23 pages, 1 figure, 2 tables. arXiv admin note: substantial text
overlap with arXiv:1110.336
Post-Einsteinian tests of linearized gravitation
The general relativistic treatment of gravitation can be extended by
preserving the geometrical nature of the theory but modifying the form of the
coupling between curvature and stress tensors. The gravitation constant is thus
replaced by two running coupling constants which depend on scale and differ in
the sectors of traceless and traced tensors. When calculated in the solar
system in a linearized approximation, the metric is described by two
gravitation potentials. This extends the parametrized post-Newtonian (PPN)
phenomenological framework while allowing one to preserve compatibility with
gravity tests performed in the solar system. Consequences of this extension are
drawn here for phenomena correctly treated in the linear approximation. We
obtain a Pioneer-like anomaly for probes with an eccentric motion as well as a
range dependence of Eddington parameter to be seen in light deflection
experiments.Comment: 15 pages. Accepted version, to appear in Classical and Quantum
Gravit
Lamost observations in the kepler field. I. Database of low-resolution spectra*
The nearly continuous light curves with micromagnitude precision provided by the space mission Kepler are revolutionizing our view of pulsating stars. They have revealed a vast sea of low-amplitude pulsation modes that were undetectable from Earth. The long time base of Kepler light curves allows for the accurate determination of the frequencies and amplitudes of pulsation modes needed for in-depth asteroseismic modeling. However, for an asteroseismic study to be successful, the first estimates of stellar parameters need to be known and they cannot be derived from the Kepler photometry itself. The Kepler Input Catalog provides values for the effective temperature, surface gravity, and metallicity, but not always with sufficient accuracy. Moreover, information on the chemical composition and rotation rate is lacking. We are collecting low-resolution spectra for objects in the Kepler field of view with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Xinglong observatory, China). All of the requested fields have now been observed at least once. In this paper, we describe those observations and provide a useful database for the whole astronomical communit
Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology
We present a study of 33 {\it Kepler} planet-candidate host stars for which
asteroseismic observations have sufficiently high signal-to-noise ratio to
allow extraction of individual pulsation frequencies. We implement a new
Bayesian scheme that is flexible in its input to process individual oscillation
frequencies, combinations of them, and average asteroseismic parameters, and
derive robust fundamental properties for these targets. Applying this scheme to
grids of evolutionary models yields stellar properties with median statistical
uncertainties of 1.2\% (radius), 1.7\% (density), 3.3\% (mass), 4.4\%
(distance), and 14\% (age), making this the exoplanet host-star sample with the
most precise and uniformly determined fundamental parameters to date. We assess
the systematics from changes in the solar abundances and mixing-length
parameter, showing that they are smaller than the statistical errors. We also
determine the stellar properties with three other fitting algorithms and
explore the systematics arising from using different evolution and pulsation
codes, resulting in 1\% in density and radius, and 2\% and 7\% in mass and age,
respectively. We confirm previous findings of the initial helium abundance
being a source of systematics comparable to our statistical uncertainties, and
discuss future prospects for constraining this parameter by combining
asteroseismology and data from space missions. Finally we compare our derived
properties with those obtained using the global average asteroseismic
observables along with effective temperature and metallicity, finding an
excellent level of agreement. Owing to selection effects, our results show that
the majority of the high signal-to-noise ratio asteroseismic {\it Kepler} host
stars are older than the Sun.Comment: 25 pages, 17 figures, MNRAS accepte
A precise asteroseismic age and radius for the evolved Sun-like star KIC 11026764
The primary science goal of the Kepler Mission is to provide a census of
exoplanets in the solar neighborhood, including the identification and
characterization of habitable Earth-like planets. The asteroseismic
capabilities of the mission are being used to determine precise radii and ages
for the target stars from their solar-like oscillations. Chaplin et al. (2010)
published observations of three bright G-type stars, which were monitored
during the first 33.5 days of science operations. One of these stars, the
subgiant KIC 11026764, exhibits a characteristic pattern of oscillation
frequencies suggesting that it has evolved significantly. We have derived
asteroseismic estimates of the properties of KIC 11026764 from Kepler
photometry combined with ground-based spectroscopic data. We present the
results of detailed modeling for this star, employing a variety of independent
codes and analyses that attempt to match the asteroseismic and spectroscopic
constraints simultaneously. We determine both the radius and the age of KIC
11026764 with a precision near 1%, and an accuracy near 2% for the radius and
15% for the age. Continued observations of this star promise to reveal
additional oscillation frequencies that will further improve the determination
of its fundamental properties.Comment: 16 pages, 6 figures, 4 tables, ApJ in pres
Biological parameters of Amblyomma coelebs Neumann, 1906 (Acari: Ixodidae) under experimental conditions.
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