3,791 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
Generalized Miura Transformations, Two-Boson KP Hierarchies and their Reduction to KDV Hierarchies
Bracket preserving gauge equivalence is established between several two-boson
generated KP type of hierarchies. These KP hierarchies reduce under symplectic
reduction (via Dirac constraints) to KdV, mKdV and Schwarzian KdV hierarchies.
Under this reduction the gauge equivalence is taking form of the conventional
Miura maps between the above KdV type of hierarchies.Comment: 12 pgs., LaTeX, IFT-P/011/93, UICHEP-TH/93-
Gravitational Topological Quantum Field Theory Versus N = 2 D = 8 Supergravity and its lift to N = 1 D = 11 Supergravity
In a previous work, it was shown that the 8-dimensional topological quantum
field theory for a metric and a Kalb-Ramond 2-form gauge field determines N = 1
D = 8 supergravity. It is shown here that, the combination of this TQFT with
that of a 3-form determines N = 2 D = 8 supergravity, that is, an untruncated
dimensional reduction of N = 1 D = 11 supergravity. Our construction holds for
8-dimensional manifolds with Spin(7) \subset SO(8) holonomy. We suggest that
the origin of local Poincare supersymmetry is the gravitational topological
symmetry. We indicate a mechanism for the lift of the TQFT in higher
dimensions, which generates Chern-Simons couplings.Comment: one section has been adde
On the link between rotation, chromospheric activity and Li abundance in subgiant stars
The connection rotation-CaII emission flux-lithium abundance is analyzed for
a sample of bona fide subgiant stars, with evolutionary status determined from
HIPPARCOS trigonometric parallax measurements and from the Toulouse-Geneva
code.Comment: 9 pages, 8 figure
Condutividade hidráulica e Índice S em solo sob Integração-Lavoura-Pecuária-Floresta.
O objetivo deste estudo foi avaliar o efeito da conversão de uma pastagem convencional para iLPF.Pôster - pós-graduação
Flora apícola arbórea nativa na região serrana de Pelotas para a apicultura sustentável na Metade Sul do Rio Grande do Sul.
bitstream/CPACT-2009-09/12057/1/documento_242.pd
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
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