2,922 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
Bopp-Podolsky black holes and the no-hair theorem
Bopp-Podolsky electrodynamics is generalized to curved space-times. The
equations of motion are written for the case of static spherically symmetric
black holes and their exterior solutions are analyzed using Bekenstein's
method. It is shown the solutions split-up into two parts, namely a
non-homogeneous (asymptotically massless) regime and a homogeneous
(asymptotically massive) sector which is null outside the event horizon. In
addition, in the simplest approach to Bopp-Podolsky black holes, the
non-homogeneous solutions are found to be Maxwell's solutions leading to a
Reissner-Nordstr\"om black hole. It is also demonstrated that the only exterior
solution consistent with the weak and null energy conditions is the Maxwell's
one. Thus, in light of energy conditions, it is concluded that only Maxwell
modes propagate outside the horizon and, therefore, the no-hair theorem is
satisfied in the case of Bopp-Podolsky fields in spherically symmetric
space-times.Comment: 9 pages, updated to match published versio
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
Expressão de citocinas no intestino grosso de ovinos somalis naturalmente infectados por nematoides gastrintestinais.
O presente trabalho teve como objetivo quantificar a expressão de citocinas (IL4, IL13, TNF? e IFN?) em ovinos da raça Somalis resistentes e susceptíveis às infecções por Oesophagostomum sp. Para isso, um rebanho de 75 ovinos jovens, com idade entre 4 e 5 meses foram mantidos durante 98 dias em caatinga nativa naturalmente contaminada por nematoides gastrintestinais e sem administração antihelmíntic
Adaptação de genótipos de arroz irrigado com tolerância à herbicida em Roraima.
O objetivo deste trabalho foi o de testar genótipos com tolerância à herbicidas, e principalmente, selecionar aqueles mais adaptados às condições locais, com produtividade de grãos compatíveis com as cultivares convencionais atualmente utilizadas, visando subsidiar os sistemas de produção do Estado
Predição da maturidade de fibras de algodão colorido empregando espectroscopia no infravermelho próximo (NIR) e regressão por mínimos quadrados parciais.
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