1,239 research outputs found
Measuring the extent of convective cores in low-mass stars using Kepler data: towards a calibration of core overshooting
Our poor understanding of the boundaries of convective cores generates large
uncertainties on the extent of these cores and thus on stellar ages. Our aim is
to use asteroseismology to consistently measure the extent of convective cores
in a sample of main-sequence stars whose masses lie around the mass-limit for
having a convective core. We first test and validate a seismic diagnostic that
was proposed to probe in a model-dependent way the extent of convective cores
using the so-called ratios, which are built with and
modes. We apply this procedure to 24 low-mass stars chosen among Kepler targets
to optimize the efficiency of this diagnostic. For this purpose, we compute
grids of stellar models with both the CESAM2k and MESA evolution codes, where
the extensions of convective cores are modeled either by an instantaneous
mixing or as a diffusion process. Among the selected targets, we are able to
unambiguously detect convective cores in eight stars and we obtain seismic
measurements of the extent of the mixed core in these targets with a good
agreement between the CESAM2k and MESA codes. By performing optimizations using
the Levenberg-Marquardt algorithm, we then obtain estimates of the amount of
extra-mixing beyond the core that is required in CESAM2k to reproduce seismic
observations for these eight stars and we show that this can be used to propose
a calibration of this quantity. This calibration depends on the prescription
chosen for the extra-mixing, but we find that it should be valid also for the
code MESA, provided the same prescription is used. This study constitutes a
first step towards the calibration of the extension of convective cores in
low-mass stars, which will help reduce the uncertainties on the ages of these
stars.Comment: 27 pages, 15 figures, accepted in A&
Fitomassa aérea e o potencial de adição de carbono de espécies forrageiras.
Em virtude das questões que envolvem as mudanças climáticas globais, quantificar a fitomassa aérea da vegetação é imperativo para determinar a capacidade de aportar carbono para o sistemas agrícolas em análise. O objetivo deste trabalho foi determinar a fitomassa aérea de capim-buffel (Cenchrus ciliaris L.), palma forrageira (Opuntia ficus indica Mill), gliricídia (Gliricidia sepium) e leucena (Leucaena leucocephala L.) para verificar a adição potencial de carbono destas espécies forrageiras cultivadas no Semiárido e fornecer subsídios aos estudos de balanço de carbono no Bioma Caatinga. As espécies forrageiras foram cultivadas em um Argissolo Vermelho-Amarelo distrófico. Foram amostradas a parte área de quatro espécies forrageiras para determinação da fitomassa seca e a quantidade de carbono estocado. A gliricídia, a palma forrageira, o capim-buffel e a leucina produziram 16,25 Mg.ha-1; 15,20 Mg.ha-1; 11,80 Mg.ha-1 e 9,61 Mg.ha-1 de matéria seca e estocaram 6,95 Mg.ha-1; 5,57 Mg.ha-1; 4,50 Mg.ha-1 e 4,15 Mg.ha-1 de carbono, respectivamente
Dissipation instability of Couette-like adiabatic flows in a plane channel
The mixed convection flow in a plane channel with adiabatic boundaries is
examined. The boundaries have an externally prescribed relative velocity
defining a Couette-like setup for the flow. A stationary flow regime is
maintained with a constant velocity difference between the boundaries,
considered as thermally insulated. The effect of viscous dissipation induces a
heat source in the flow domain and, hence, a temperature gradient. The
nonuniform temperature distribution causes, in turn, a buoyancy force and a
combined forced and free flow regime. Dual mixed convection flows occur for a
given velocity difference. Their structure is analysed where, in general, only
one branch of the dual flows is compatible with the Oberbeck-Boussinesq
approximation, for realistic values of the Gebhart number. A linear stability
analysis of the basic stationary flows with viscous dissipation is carried out.
The stability eigenvalue problem is solved numerically, leading to the
determination of the neutral stability curves and the critical values of the
P\'eclet number, for different Gebhart numbers. An analytical asymptotic
solution in the special case of perturbations with infinite wavelength is also
developed.Comment: 24 pages, 11 figure
Green manure contributing for nutrients cycling in irrigated environments of the Brazilian Semi-Arid.
This study aimed to evaluate the dry matter production and accumulation of nutrients of the aerial phytomass of two types of plant mixtures and the spontaneous vegetation kept between the lines of a mango orchard, under two systems of soil managemen
Tutorial sobre normalização bibliográfica.
Esta Circular Técnica traz um tutorial prático e resumido com instruções para normalização bibliográfica no perfil utilizado pela Embrapa. São utilizadas como exemplos situações mais recorrentes quando da normalização de trabalhos científicos e acadêmicos.bitstream/item/101201/1/CT-27.pd
Faithful Squashed Entanglement
Squashed entanglement is a measure for the entanglement of bipartite quantum
states. In this paper we present a lower bound for squashed entanglement in
terms of a distance to the set of separable states. This implies that squashed
entanglement is faithful, that is, strictly positive if and only if the state
is entangled. We derive the bound on squashed entanglement from a bound on
quantum conditional mutual information, which is used to define squashed
entanglement and corresponds to the amount by which strong subadditivity of von
Neumann entropy fails to be saturated. Our result therefore sheds light on the
structure of states that almost satisfy strong subadditivity with equality. The
proof is based on two recent results from quantum information theory: the
operational interpretation of the quantum mutual information as the optimal
rate for state redistribution and the interpretation of the regularised
relative entropy of entanglement as an error exponent in hypothesis testing.
The distance to the set of separable states is measured by the one-way LOCC
norm, an operationally-motivated norm giving the optimal probability of
distinguishing two bipartite quantum states, each shared by two parties, using
any protocol formed by local quantum operations and one-directional classical
communication between the parties. A similar result for the Frobenius or
Euclidean norm follows immediately. The result has two applications in
complexity theory. The first is a quasipolynomial-time algorithm solving the
weak membership problem for the set of separable states in one-way LOCC or
Euclidean norm. The second concerns quantum Merlin-Arthur games. Here we show
that multiple provers are not more powerful than a single prover when the
verifier is restricted to one-way LOCC operations thereby providing a new
characterisation of the complexity class QMA.Comment: 24 pages, 1 figure, 1 table. Due to an error in the published
version, claims have been weakened from the LOCC norm to the one-way LOCC
nor
On Hastings' counterexamples to the minimum output entropy additivity conjecture
Hastings recently reported a randomized construction of channels violating
the minimum output entropy additivity conjecture. Here we revisit his argument,
presenting a simplified proof. In particular, we do not resort to the exact
probability distribution of the Schmidt coefficients of a random bipartite pure
state, as in the original proof, but rather derive the necessary large
deviation bounds by a concentration of measure argument. Furthermore, we prove
non-additivity for the overwhelming majority of channels consisting of a Haar
random isometry followed by partial trace over the environment, for an
environment dimension much bigger than the output dimension. This makes
Hastings' original reasoning clearer and extends the class of channels for
which additivity can be shown to be violated.Comment: 17 pages + 1 lin
A reversible theory of entanglement and its relation to the second law
We consider the manipulation of multipartite entangled states in the limit of
many copies under quantum operations that asymptotically cannot generate
entanglement. As announced in [Brandao and Plenio, Nature Physics 4, 8 (2008)],
and in stark contrast to the manipulation of entanglement under local
operations and classical communication, the entanglement shared by two or more
parties can be reversibly interconverted in this setting. The unique
entanglement measure is identified as the regularized relative entropy of
entanglement, which is shown to be equal to a regularized and smoothed version
of the logarithmic robustness of entanglement.
Here we give a rigorous proof of this result, which is fundamentally based on
a certain recent extension of quantum Stein's Lemma proved in [Brandao and
Plenio, Commun. Math. 295, 791 (2010)], giving the best measurement strategy
for discriminating several copies of an entangled state from an arbitrary
sequence of non-entangled states, with an optimal distinguishability rate equal
to the regularized relative entropy of entanglement. We moreover analyse the
connection of our approach to axiomatic formulations of the second law of
thermodynamics.Comment: 21 pages. revised versio
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