363 research outputs found
Indicators of weed competition on Organic Winter Wheat
Organic winter wheat production is limited by climatic and agronomic factors, including weed competition. The incidence of weeds on yield limitation can be characterized through various early indicators to predict weed occurrence and competition. A network of 10 fields of organic winter wheat was implemented in the South East of France in 2005-2006. Results showed that weed density, dynamics and diversity are good indicators for weed occurrence and competition
The Mira-based distance to the Galactic centre
Mira variables are useful distance indicators, due to their high luminosities
and well-defined period-luminosity relation. We select 1863 Miras from SAAO and
MACHO observations to examine their use as distance estimators in the Milky
Way. We measure a distance to the Galactic centre of kpc,
which is in good agreement with other literature values. The uncertainty has
two components of 0.2 kpc each: the first is from our analysis and
predominantly due to interstellar extinction, the second is due to zero-point
uncertainties extrinsic to our investigation, such as the distance to the Large
Magellanic Cloud (LMC). In an attempt to improve existing period-luminosity
calibrations, we use theoretical models of Miras to determine the dependence of
the period-luminosity relation on age, metallicity, and helium abundance, under
the assumption that Miras trace the bulk stellar population. We find that at a
fixed period of , changes in the predicted magnitudes can
be approximated by , and these coefficients are
nearly independent of period. The expected overestimate in the Galactic centre
distance from using an LMC-calibrated relation is 0.3 kpc. This
prediction is not validated by our analysis; a few possible reasons are
discussed. We separately show that while the predicted color-color diagrams of
solar-neighbourhood Miras work well in the near-infrared, though there are
offsets from the model predictions in the optical and mid-infrared.Comment: Accepted for publication in The Astrophysical Journal. 16 pages, 8
figures, 6 table
Timing as a sexually selected trait: the right mate at the right moment
Sexual selection favours the expression of traits in one sex that attract members of the opposite sex for mating. The nature of sexually selected traits such as vocalization, colour and ornamentation, their fitness benefits as well as their costs have received ample attention in field and laboratory studies. However, sexually selected traits may not always be expressed: coloration and ornaments often follow a seasonal pattern and behaviours may be displayed only at specific times of the day. Despite the widely recognized differences in the daily and seasonal timing of traits and their consequences for reproductive success, the actions of sexual selection on the temporal organization of traits has received only scant attention. Drawing on selected examples from bird and mammal studies, here we summarize the current evidence for the daily and seasonal timing of traits. We highlight that molecular advances in chronobiology have opened exciting new opportunities for identifying the genetic targets that sexual selection may act on to shape the timing of trait expression. Furthermore, known genetic links between daily and seasonal timing mechanisms lead to the hypothesis that selection on one timescale may simultaneously also affect the other. We emphasize that studies on the timing of sexual displays of both males and females from wild populations will be invaluable for understanding the nature of sexual selection and its potential to act on differences within and between the sexes in timing. Molecular approaches will be important for pinpointing genetic components of biological rhythms that are targeted by sexual selection, and to clarify whether these represent core or peripheral components of endogenous clocks. Finally, we call for a renewed integration of the fields of evolution, behavioural ecology and chronobiology to tackle the exciting question of how sexual selection contributes to the evolution of biological clocks.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'
A POLĂTICA DE ASSISTĂNCIA ESTUDANTIL NO BRASIL: O CASO DA UNIVERSIDADE FEDERAL DE JUIZ DE FORA
O objetivo deste artigo Ă© apresentar a trajetĂłria das polĂticas de AssistĂȘncia ao Estudante no Brasil, bem como analisar a PolĂtica de AssistĂȘncia Estudantil de uma Universidade Federal, a fim de observar as açÔes de assistĂȘncia desenvolvidas nas universidades, identificando os seus instrumentos de monitoramento e de avaliação. Para tanto, este trabalho baseia-se em um estudo de caso, que procura abordar a PolĂtica de Apoio Estudantil da Universidade Federal de Juiz de Fora. O Decreto nÂș 7.234/10, que dispĂ”e sobre o Programa Nacional de AssistĂȘncia Estudantil, estabelece que as instituiçÔes federais de ensino superior devem fixar mecanismos de acompanhamento e avaliação e, neste sentido, em anĂĄlise ao normativo interno da UFJF, que dispĂ”e sobre o Apoio Estudantil, nĂŁo foram identificados dispositivos que estabeleçam tais mecanismos. Portanto, com o intuito de melhorar a gestĂŁo do Programa de Apoio Estudantil e de cumprir o Decreto nÂș 7.234/10, torna-se necessĂĄrio que a UFJF estabeleça e formalize os mecanismos de acompanhamento e avaliação de sua PolĂtica de Apoio Estudantil
On the Color-Metallicity Relation of the Red Clump and the Reddening Toward the Magellanic Clouds
The zero point of the reddening toward the Large Magellanic Cloud (LMC) has
been the subject of some dispute. Its uncertainty propagates as a systematic
error for methods which measure the extragalactic distance scale through
knowledge of the absolute extinction of LMC stars. In an effort to resolve this
issue, we used three different methods to calibrate the most widely-used metric
to predict LMC extinction, the intrinsic color of the red clump,
, for the inner 3 degrees of that galaxy. The first
approach was to empirically calibrate the color zeropoints of the BaSTI
isochrones over a wide metallicity range of
using measurements of red clump stars in 47 Tuc, the Solar Neighborhood, and
NGC 6791.
From these efforts we also measure these properties of the Solar Neighborhood
red clump, (, , , , ,
, , ) (1.02, 2.75, 2.18, 1.52, 0.64, 0.15,
0.23, 1.63). The second and third methods were to compare the observed
colors of the red clump to those of Cepheids and RR Lyrae in the LMC. With
these three methods, we estimated the intrinsic color of the red clump of the
LMC to be respectively, and similarly using the first and third method we
estimated
respectively for the Small Magellanic Cloud. We estimate the luminosities to be
and . We show that this has
important implications for recent calibrations of the tip of the red giant
branch in the Magellanic Clouds used to measure .Comment: 20 pages, 5 figures, 4 tables. Accepted for Publication in The
Astrophysical Journal as of February 9th, 202
A threshold mechanism ensures minimum-path flow in lightning discharge
A well-known property of linear resistive electrical networks is that the current distribution minimizes the total dissipated power. When the circuit includes resistors with nonlinear monotonic characteristic, the current distribution minimizes in general a diferent functional. We show that, if the nonlinear characteristic is a threshold-like function and the current generator is concentrated in a single point, as in the case of lightning or dielectric discharge, then the current flow is concentrated along a single path, which is a minimum path to the ground with respect to the threshold. We also propose a dynamic model that explains and qualitatively reproduces the lightning transient behavior: initial generation of several plasma branches and subsequent dismissal of all branches but the one reaching the ground frst, which is the optimal one
Asteroseismology of the Hyades with K2: first detection of main-sequence solar-like oscillations in an open cluster
The Hyades open cluster was targeted during Campaign 4 (C4) of the NASA K2
mission, and short-cadence data were collected on a number of cool
main-sequence stars. Here, we report results on two F-type stars that show
detectable oscillations of a quality that allows asteroseismic analyses to be
performed. These are the first ever detections of solar-like oscillations in
main-sequence stars in an open cluster.Comment: 12 pages, 8 figures, 2 tables; accepted for publication in MNRA
Asteroseismic properties of solar-type stars observed with the NASA K2 mission: results from Campaigns 1-3 and prospects for future observations
We present an asteroseismic analysis of 33 solar-type stars observed in short
cadence during Campaigns (C) 1-3 of the NASA K2 mission. We were able to
extract both average seismic parameters and individual mode frequencies for
stars with dominant frequencies up to ~3300{\mu}Hz, and we find that data for
some targets are good enough to allow for a measurement of the rotational
splitting. Modelling of the extracted parameters is performed by using
grid-based methods using average parameters and individual frequencies together
with spectroscopic parameters. For the target selection in C3, stars were
chosen as in C1 and C2 to cover a wide range in parameter space to better
understand the performance and noise characteristics. For C3 we still detected
oscillations in 73% of the observed stars that we proposed. Future K2 campaigns
hold great promise for the study of nearby clusters and the chemical evolution
and age-metallicity relation of nearby field stars in the solar neighbourhood.
We expect oscillations to be detected in ~388 short-cadence targets if the K2
mission continues until C18, which will greatly complement the ~500 detections
of solar-like oscillations made for short-cadence targets during the nominal
Kepler mission. For ~30-40 of these, including several members of the Hyades
open cluster, we furthermore expect that inference from interferometry should
be possible.Comment: 17 pages, 15 figures, 4 tables; accepted for publication in PAS
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