2,766 research outputs found
Measuring stellar differential rotation with high-precision space-borne photometry
We introduce a method of measuring a lower limit to the amplitude of surface
differential rotation from high-precision, evenly sampled photometric time
series. It is applied to main-sequence late-type stars whose optical flux
modulation is dominated by starspots. An autocorrelation of the time series was
used to select stars that allow an accurate determination of starspot rotation
periods. A simple two-spot model was applied together with a Bayesian
information criterion to preliminarily select intervals of the time series
showing evidence of differential rotation with starspots of almost constant
area. Finally, the significance of the differential rotation detection and a
measurement of its amplitude and uncertainty were obtained by an a posteriori
Bayesian analysis based on a Monte Carlo Markov Chain approach. We applied our
method to the Sun and eight other stars for which previous spot modelling had
been performed to compare our results with previous ones. We find that
autocorrelation is a simple method for selecting stars with a coherent
rotational signal that is a prerequisite for successfully measuring
differential rotation through spot modelling. For a proper Monte Carlo Markov
Chain analysis, it is necessary to take the strong correlations among different
parameters that exist in spot modelling into account. For the planet-hosting
star Kepler-30, we derive a lower limit to the relative amplitude of the
differential rotation of \Delta P / P = 0.0523 \pm 0.0016. We confirm that the
Sun as a star in the optical passband is not suitable for measuring
differential rotation owing to the rapid evolution of its photospheric active
regions. In general, our method performs well in comparison to more
sophisticated and time-consuming approaches.Comment: Accepted to Astronomy and Astrophysics, 15 pages, 13 figures, 4
tables and an Appendi
Soil faunal activity in natural and improved secondary vegetation (capoeira) and in cropped fields under different land preparation systems.
Testing the recovery of stellar rotation signals from Kepler light curves using a blind hare-and-hounds exercise
We present the results of a blind exercise to test the recoverability of
stellar rotation and differential rotation in Kepler light curves. The
simulated light curves lasted 1000 days and included activity cycles, Sun-like
butterfly patterns, differential rotation and spot evolution. The range of
rotation periods, activity levels and spot lifetime were chosen to be
representative of the Kepler data of solar like stars. Of the 1000 simulated
light curves, 770 were injected into actual quiescent Kepler light curves to
simulate Kepler noise. The test also included five 1000-day segments of the
Sun's total irradiance variations at different points in the Sun's activity
cycle.
Five teams took part in the blind exercise, plus two teams who participated
after the content of the light curves had been released. The methods used
included Lomb-Scargle periodograms and variants thereof, auto-correlation
function, and wavelet-based analyses, plus spot modelling to search for
differential rotation. The results show that the `overall' period is well
recovered for stars exhibiting low and moderate activity levels. Most teams
reported values within 10% of the true value in 70% of the cases. There was,
however, little correlation between the reported and simulated values of the
differential rotation shear, suggesting that differential rotation studies
based on full-disk light curves alone need to be treated with caution, at least
for solar-type stars.
The simulated light curves and associated parameters are available online for
the community to test their own methods.Comment: Accepted for publication in MNRAS. Accepted, 13 April 2015. Received,
26 March 2015; in original form, 9 November 201
On the chromospheric activity of stars with planets
Context. Signatures of chromospheric activity enhancement have been found for
a dozen stars, pointing to a possible star-planet interaction. Nevertheless in
the coronal activity regime, there is no conclusive observational evidence for
such an interaction. Does star-planet interaction manifest itself only for a
few particular cases, without having a major effect on stars with planets in
general? Aims. We aim to add additional observational constraints to support or
reject the major effects of star-planet interactions in stellar activity, based
on CaII chromospheric emission flux. Methods. We performed a statistical
analysis of CaII emission flux of stars with planets, as well as a comparison
between CaII and X-ray emission fluxes, searching for dependencies on planetary
parameters. Results. In the present sample of stars with planets, there are no
significant correlations between chromospheric activity indicator log(R'HK) and
planetary parameters. Further, the distribution of the chromospheric activity
indicator for stars without planets is not distinguishable from the one with
planets.Comment: 6 pages, 5 figures, submitted to A&
The variability behavior of CoRoT M-giant Stars
For 6 years the Convection, Rotation, and Planetary Transits (CoRoT) space
mission has acquired photometric data from more than one hundred thousand point
sources towards and directly opposite from the inner and outer regions of the
Galaxy. The high temporal resolution of the CoRoT data combined with the wide
time span of the observations has enabled the study of short and long time
variations in unprecedented detail. From the initial sample of 2534 stars
classified as M-giants in the CoRoT databasis, we selected 1428 targets that
exhibit well defined variability, using visual inspection. The variability
period and amplitude of C1 stars (stars having Teff < 4200 K) were computed
using Lomb-Scargle and harmonic fit methods. The trends found in the V-I vs J-K
color-color diagram are in agreement with standard empirical calibrations for
M-giants. The sources located towards the inner regions of the Galaxy are
distributed throughout the diagram while the majority of the stars towards the
outer regions of the Galaxy are spread between the calibrations of M-giants and
the predicted position for Carbon stars. The stars classified as supergiants
follow a different sequence from the one found for giant stars. We also
performed a KS test of the period and amplitude of stars towards the inner and
outer regions of the Galaxy. We obtained a low probability that the two samples
come from the same parent distribution. The observed behavior of the
period-amplitude and period-Teff diagrams are, in general, in agreement with
those found for Kepler sources and ground based photometry, with pulsation
being the dominant cause responsible for the observed modulation. We also
conclude that short-time variations on M-Giant stars do not exist orare very
rare and the few cases we found are possibly related to biases or background
stars.Comment: 11 pages, 6 figure
Efeito da irrigação e fertirrigação sobre o estado nutricional de macieiras.
A análise foliar permite o diagnóstico nutricional das plantas, podendo indicar desequilíbrios nutricionais durante o ciclo de desenvolvimento da cultura. Situações de déficits hídricos no solo podem afetar a absorção e o estado nutricional das plantas, influenciando a qualidade e produtividade de macieiras. O objetivo do trabalho foi avaliar o estado nutricional de macieiras em função da aplicação de irrigação e fertirrigação, comparadas ao cultivo convencional.(Embrapa Uva e Vinho. Documentos, 99
Indicadores de maturação para definição de ponto de colheita do maracujá selvagem (passiflora tenuifila) cultivado na região de cerrado.
O Maracujá-selvagem é uma das espécies depositadas no BAG de Passifloras da Embrapa Cerrados que estão sendo estudadas para fins de melhoramento genético. Neste trabalho buscou-se avaliar uma metodologia de campo, para definição do ponto de colheita do fruto tomando-se como referência indicadores físicos, físico-químicos e de aparência dos frutos em 7 (sete) estádios. Fez-se comparações entre índices de aparência do fruto com as variáveis massa total,da casca, da polpa, das sementes e seus respectivos percentuais em relação ao fruto total, o diâmetro transversal e longitudinal e teor de açucares solúveis totais na polpa. Os resultados indicaram que o ponto ótimo de colheita dos frutos dá-se no estádio 3, quando apresentam aparência com coloração amarela em até 20% da casca e valores médios de 18,45g de massa total. Na polpa 26,5o Brix de açucares solúveis totais, 0,82% de acidez total titulável e pH 4,43
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