57,765 research outputs found
On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars
The Fourier Transform method is a popular tool to derive the rotational
velocities of stars from their spectral line profiles. However, its domain of
validity does not include line-profile variables with time-dependent profiles.
We investigate the performance of the method for such cases, by interpreting
the line-profile variations of spotted B stars, and of pulsating B tars, as if
their spectral lines were caused by uniform surface rotation along with
macroturbulence. We perform time-series analysis and harmonic least-squares
fitting of various line diagnostics and of the outcome of several
implementations of the Fourier Transform method. We find that the projected
rotational velocities derived from the Fourier Transform vary appreciably
during the pulsation cycle whenever the pulsational and rotational velocity
fields are of similar magnitude. The macroturbulent velocities derived while
ignoring the pulsations can vary with tens of km/s during the pulsation cycle.
The temporal behaviour of the deduced rotational and macroturbulent velocities
are in antiphase with each other. The rotational velocity is in phase with the
second moment of the line profiles. The application of the Fourier method to
stars with considerable pulsational line broadening may lead to an appreciable
spread in the values of the rotation velocity, and, by implication, of the
deduced value of the macroturbulence. These two quantities should therefore not
be derived from single snapshot spectra if the aim is to use them as a solid
diagnostic for the evaluation of stellar evolution models of slow to moderate
rotators.Comment: 13 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Algebraic theory of affine curvature tensors
We use curvature decompositions to construct generating sets for the space of
algebraic curvature tensors and for the space of tensors with the same
symmetries as those of a torsion free, Ricci symmetric connection; the latter
naturally appear in relative hypersurface theory.Comment: The paper is dedicated to the memory of the first author (N. Blazic)
who passed away Monday 10 October 200
Critical Behavior of Coupled q-state Potts Models under Weak Disorder
We investigate the effect of weak disorder on different coupled -state
Potts models with using two loops renormalisation group. This study
presents new examples of first order transitions driven by randomness. We found
that weak disorder makes the models decouple. Therefore, it appears that no
relations emerge, at a perturbation level, between the disordered -state Potts model and the two disordered , -state Potts models
(), despite their central charges are similar according to recent
numerical investigations. Nevertheless, when two -state Potts models are
considered (), the system remains always driven in a strong coupling
regime, violating apparently the Imry-Wortis argument.Comment: 7 pages + 1 PS figure (Latex
Peres-Horodecki separability criterion for continuous variable systems
The Peres-Horodecki criterion of positivity under partial transpose is
studied in the context of separability of bipartite continuous variable states.
The partial transpose operation admits, in the continuous case, a geometric
interpretation as mirror reflection in phase space. This recognition leads to
uncertainty principles, stronger than the traditional ones, to be obeyed by all
separable states. For all bipartite Gaussian states, the Peres-Horodecki
criterion turns out to be necessary and sufficient condition for separability.Comment: 6 pages, no figure
A landscape of non-supersymmetric AdS vacua on coset manifolds
We construct new families of non-supersymmetric sourceless type IIA AdS4
vacua on those coset manifolds that also admit supersymmetric solutions. We
investigate the spectrum of left-invariant modes and find that most, but not
all, of the vacua are stable under these fluctuations. Generically, there are
also no massless moduli.Comment: 20 pages, 11 figures, v2: added some clarifications, references, v3:
corrections addressing comments refere
Optimal and Robust Quantum Metrology Using Interaction-Based Readouts
Useful quantum metrology requires nonclassical states with a high particle
number and (close to) the optimal exploitation of the state's quantum
correlations. Unfortunately, the single-particle detection resolution demanded
by conventional protocols, such as spin squeezing via one-axis twisting, places
severe limits on the particle number. Additionally, the challenge of finding
optimal measurements (that saturate the quantum Cram{\'e}r-Rao bound) for an
arbitrary nonclassical state limits most metrological protocols to only
moderate levels of quantum enhancement. "Interaction-based readout" protocols
have been shown to allow optimal interferometry \emph{or} to provide robustness
against detection noise at the expense of optimality. In this Letter, we prove
that one has great flexibility in constructing an optimal protocol, thereby
allowing it to also be robust to detection noise. This requires the full
probability distribution of outcomes in an optimal measurement basis, which is
typically easily accessible and can be determined from specific criteria we
provide. Additionally, we quantify the robustness of several classes of
interaction-based readouts under realistic experimental constraints. We
determine that optimal \emph{and} robust quantum metrology is achievable in
current spin-squeezing experiments.Comment: 7 pages, 3 figure
Recommended from our members
Estimating sustainability gaps: methods and preliminary applications for the UK and the Netherlands
This paper sets out and applies a new methodology for determining whether or not economic activity is environmentally sustainable. By comparing current environmental impacts with standards of environmental sustainability, it calculates the 'sustainability gaps' (SGAPs) with respect to different environmental impacts. For the UK, SGAPs are computed for CO2, SO2, and other air pollutants. Across none of these environmental themes can current UK use of the environment be said to be sustainable. The SGAP indicators can be combined with current trends to show how long it would take, on continuation of the trends, for the sustainability standard to be attained. The paper calls this indicator the 'Years to Sustainability' (YS) measure. For the policy objective of sustainable development to be made fully operational, it is necessary for the concept of environmental sustainability to be clearly defined by quantitative indicators. On the basis of the indicators set out in this paper it can be judged whether economic activity is moving towards or away from environmental sustainability, and at what speed
- …