4,251 research outputs found
Wave attenuation and dispersion due to floating ice covers
Experiments investigating the attenuation and dispersion of surface waves in
a variety of ice covers are performed using a refrigerated wave flume. The ice
conditions tested in the experiments cover naturally occurring combinations of
continuous, fragmented, pancake and grease ice. Attenuation rates are shown to
be a function of ice thickness, wave frequency, and the general rigidity of the
ice cover. Dispersion changes were minor except for large wavelength increases
when continuous covers were tested. Results are verified and compared with
existing literature to show the extended range of investigation in terms of
incident wave frequency and ice conditions
Reduction of a metapopulation genetic model to an effective one island model
We explore a model of metapopulation genetics which is based on a more
ecologically motivated approach than is frequently used in population genetics.
The size of the population is regulated by competition between individuals,
rather than by artificially imposing a fixed population size. The increased
complexity of the model is managed by employing techniques often used in the
physical sciences, namely exploiting time-scale separation to eliminate fast
variables and then constructing an effective model from the slow modes.
Remarkably, an initial model with 2 variables, where
is the number of islands in the metapopulation, can be reduced to a model with
a single variable. We analyze this effective model and show that the
predictions for the probability of fixation of the alleles and the mean time to
fixation agree well with those found from numerical simulations of the original
model.Comment: 16 pages, 4 figures. Supplementary material: 22 pages, 3 figure
A VLT/FLAMES survey for massive binaries in Westerlund 1 VIII. Binary systems and orbital parameters
Context. The galactic cluster Westerlund 1 contains a rich population of evolved, massive stars. A high binary fraction has been inferred from previous multi-wavelength observations. Aims. We use multi-epoch spectroscopy of a large sample of early-type stars in Westerlund 1 to identify new binaries and binary candidates in the cluster. Methods. VLT/FLAMES was used with the GIRAFFE spectrograph in HR21 mode to obtain spectra of ∼100 OB stars over a 14-month baseline in 2008 and 2009, supplemented with follow-up observations in 2011 and 2013. Radial velocities were obtained from strong Paschen series absorption lines in the I-band. Results. We identify 20 new OB I–III binaries, a WN9h binary, and a WC9d binary, greatly increasing the number of directly confirmed binary systems in Westerlund 1, while 12 O9−9.5 Iab−III stars are identified as candidate binaries based on radial velocity changes that are inconsistent with photospheric variability. The 173.9 day SB1 W1030 represents the first longer-period system identified in the cluster, while the determination of a 53.95 day period for W44 (WR L) makes it the first Wolf-Rayet binary in Westerlund 1 with a confirmed orbital period greater than ten days. Our results suggest the binary fraction in the OB population is at least ∼40%, and may be significantly higher. Conclusions. These results demonstrate that binary systems can be effectively identified in the population of OB I−III stars evolving off the main sequence in Westerlund 1. Future multi-epoch surveys will be able to fully characterise this population.This research is partially supported by the Spanish Government Ministerio de Ciencia, Innovación y Universidades under grant PGC2018-093741-B-C21 (MICIU/AEI/FEDER, UE). I.N. is also supported by the Generalitat Valenciana through grant PROMETEO/2019/041. F.N. acknowledges financial support through Spanish grant PID2019-105552RB-C41 (MINECO/MCIU/AEI/FEDER) and from the Spanish State Research Agency (AEI) through the Unidad de Excelencia “María de Maeztu”-Centro de Astrobiología (CSIC-INTA) project No. MDM-2017-0737
Positivity and conservation of superenergy tensors
Two essential properties of energy-momentum tensors T_{\mu\nu} are their
positivity and conservation. This is mathematically formalized by,
respectively, an energy condition, as the dominant energy condition, and the
vanishing of their divergence \nabla^\mu T_{\mu\nu}=0. The classical Bel and
Bel-Robinson superenergy tensors, generated from the Riemann and Weyl tensors,
respectively, are rank-4 tensors. But they share these two properties with
energy momentum tensors: the Dominant Property (DP) and the divergence-free
property in the absence of sources (vacuum). Senovilla defined a universal
algebraic construction which generates a basic superenergy tensor T{A} from any
arbitrary tensor A. In this construction the seed tensor A is structured as an
r-fold multivector, which can always be done. The most important feature of the
basic superenergy tensors is that they satisfy automatically the DP,
independently of the generating tensor A. In a previous paper we presented a
more compact definition of T{A} using the r-fold Clifford algebra. This form
for the superenergy tensors allowed to obtain an easy proof of the DP valid for
any dimension. In this paper we include this proof. We explain which new
elements appear when we consider the tensor T{A} generated by a
non-degree-defined r-fold multivector A and how orthogonal Lorentz
transformations and bilinear observables of spinor fields are included as
particular cases of superenergy tensors. We find some sufficient conditions for
the seed tensor A, which guarantee that the generated tensor T{A} is
divergence-free. These sufficient conditions are satisfied by some physical
fields, which are presented as examples.Comment: 19 pages, no figures. Language and minor changes. Published versio
Considering Fluctuation Energy as a Measure of Gyrokinetic Turbulence
In gyrokinetic theory there are two quadratic measures of fluctuation energy,
left invariant under nonlinear interactions, that constrain the turbulence. The
recent work of Plunk and Tatsuno [Phys. Rev. Lett. 106, 165003 (2011)] reported
on the novel consequences that this constraint has on the direction and
locality of spectral energy transfer. This paper builds on that work. We
provide detailed analysis in support of the results of Plunk and Tatsuno but
also significantly broaden the scope and use additional methods to address the
problem of energy transfer. The perspective taken here is that the fluctuation
energies are not merely formal invariants of an idealized model
(two-dimensional gyrokinetics) but are general measures of gyrokinetic
turbulence, i.e. quantities that can be used to predict the behavior of the
turbulence. Though many open questions remain, this paper collects evidence in
favor of this perspective by demonstrating in several contexts that constrained
spectral energy transfer governs the dynamics.Comment: Final version as published. Some cosmetic changes and update of
reference
Perpendicular momentum injection by lower hybrid wave in a tokamak
The injection of lower hybrid waves for current drive into a tokamak affects
the profile of intrinsic rotation. In this article, the momentum deposition by
the lower hybrid wave on the electrons is studied. Due to the increase in the
poloidal momentum of the wave as it propagates into the tokamak, the parallel
momentum of the wave increases considerably. The change of the perpendicular
momentum of the wave is such that the toroidal angular momentum of the wave is
conserved. If the perpendicular momentum transfer via electron Landau damping
is ignored, the transfer of the toroidal angular momentum to the plasma will be
larger than the injected toroidal angular momentum. A proper quasilinear
treatment proves that both perpendicular and parallel momentum are transferred
to the electrons. The toroidal angular momentum of the electrons is then
transferred to the ions via different mechanisms for the parallel and
perpendicular momentum. The perpendicular momentum is transferred to ions
through an outward radial electron pinch, while the parallel momentum is
transferred through collisions.Comment: 22 pages, 4 figure
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