454 research outputs found

### Internal circulation in tidally locked massive binary stars -- Consequences for double black hole formation

Steady-state circulation currents are predicted in tidally deformed binary
stars, which are believed to be progenitors of double black-hole merger events.
This work aims to quantitatively characterise the steady-state circulation
currents in components of a tidally locked binary system and to explore the
effects of such currents on numerical models. Previous results describing the
circulation in a single rotating star and a binary star are used to deduce a
new prescription for the internal circulation in tidally locked binaries. We
explore the effect of this prescription numerically for binary systems with
primary masses between 25 and 100 solar masses. When comparing circulation
velocities in the radial direction for the single rotating star and binary
star, it is found that the average circulation velocity in the binary star may
be described as an enhancement to the circulation velocity in a single rotating
star. This velocity enhancement is a simple function depending on the masses of
the binary components and amounts to a factor of approximately two when the
components have equal masses. It is found that the ehancement causes the
formation of double helium stars through efficient mixing to occur for systems
with higher initial orbital periods, lower primary masses and lower mass
ratios, compared to the standard circulation scenario. Taking into account
appropriate distributions for primary mass, initial period and mass ratio,
models with enhanced mixing predict 2.4 times more double helium stars being
produced in the parameter space than models without. We conclude that the
effects of companion-induced circulation have strong implications for the
formation of close binary black holes. Not only do the predicted detection
rates increase but double black-hole systems with mass ratios as low as 0.8 may
be formed when companion-induced circulation is taken into account.Comment: 17 pages, 10 figures, accepted for publication in A&

### The Single Star Path to Be Stars

Be stars are rapidly rotating B main sequence stars, which show line emission
due to an outflowing disc. By studying the evolution of rotating single star
models, we can assess their contribution to the observed Be star populations.
We identify the main effects which are responsible for single stars to
approach critical rotation as functions of initial mass and metallicity, and
predict the properties of populations of rotating single stars.
We perform population synthesis with single star models of initial masses
ranging between 3 and 30 solar masses, initial equatorial rotation velocities
between 0 and 600 kms$^{-1}$ at compositions representing the Milky Way, Large
and Small Magellanic Clouds. These models include efficient core-envelope
coupling mediated by internal magnetic fields and correspond to the maximum
efficiency of Be star production. We predict Be star fractions and the
positions of fast rotating stars in the colour-magnitude diagram.
We identify stellar wind mass-loss and the convective core mass fraction as
the key parameters which determine the time dependance of the stellar rotation
rates. Using empirical distributions of initial rotational velocities,our
single star models can reproduce the trends observed in Be star fractions with
mass and metallicity. However,they fail to produce a significant number of
stars rotating very close to critical. We also find that rapidly rotating Be
stars in the Magellanic Clouds should have significant surface nitrogen
enrichments, which may be in conflict with abundance determinations of Be
stars.
Single star evolution may explain the high number of Be stars if 70 to 80% of
critical rotationwould be sufficient to produce the Be phenomenon. However even
in this case, the unexplained presence of many Be stars far below the cluster
turn-off indicates the importance of the binary channel for Be star production.Comment: in pres

### A model of anisotropic winds from rotating stars for evolutionary calculations

Context: The surface properties of rotating stars can vary from pole to
equator, resulting in anisotropic stellar winds which are not included in the
currently available evolutionary models.
Aims: We develop a formalism to describe the mass and angular momentum loss
of rotating stars which takes into account both the varying surface properties
and distortion due to rotation.
Methods: Adopting the mass-loss recipe for non-rotating stars, we assigned to
each point on the surface of a rotating star an equivalent non-rotating star,
for which the surface mass flux is given by the recipe. The global mass-loss
and angular momentum loss rates are then given by integrating over the deformed
stellar surface as appropriate. Evolutionary models were computed and our
prescription is compared to the currently used simple mass-loss enhancement
recipes for rotating stars.
Results: We find that mass-loss rates are largely insensitive to rotation for
models not affected by the bi-stability jump. For those affected by the
bi-stability jump, the increase in mass-loss rates with respect to time is
smoothed. As our prescription considers the variation of physical conditions
over the stellar surface, the region affected by the bi-stability jump is able
to grow gradually instead of the whole star suddenly being affected.
Conclusion: We have provided an easy to implement and flexible, yet
physically meaningful prescription for calculating mass and angular momentum
loss rates of rotating stars in a one-dimensional stellar evolution code which
compares favourably to more physically comprehensive models.
The implementation of our scheme in the stellar evolution code MESA is
available online: https://zenodo.org/record/7437006Comment: Accepted for publication in A&A. 12 page

### A Classification of Infographics

Classifications are useful for describing existing phenomena and guiding further investigation. Several classifications of diagrams have been proposed, typically based on analytical rather than empirical methodologies. A notable exception is the work of Lohse and his colleagues, published in Communications of the ACM in December 1994. The classification of diagrams that Lohse proposed was derived from bottom-up grouping data collected from sixteen participants and based on 60 diagrams. Mean values on ten Likert-scales were used to predict diagram class. We follow a similar methodology to Lohse, using real-world infographics (i.e. embellished data charts) as our stimuli. We propose a structural classification of infographics, and determine whether infographics class can be predicted from values on Likert scales

### Statistics of Partial Minima

Motivated by multi-objective optimization, we study extrema of a set of N
points independently distributed inside the d-dimensional hypercube. A point in
this set is k-dominated by another point when at least k of its coordinates are
larger, and is a k-minimum if it is not k-dominated by any other point. We
obtain statistical properties of these partial minima using exact probabilistic
methods and heuristic scaling techniques. The average number of partial minima,
A, decays algebraically with the total number of points, A ~ N^{-(d-k)/k}, when
1<=k<d. Interestingly, there are k-1 distinct scaling laws characterizing the
largest coordinates as the distribution P(y_j) of the jth largest coordinate,
y_j, decays algebraically, P(y_j) ~ (y_j)^{-alpha_j-1}, with
alpha_j=j(d-k)/(k-j) for 1<=j<=k-1. The average number of partial minima grows
logarithmically, A ~ [1/(d-1)!](ln N)^{d-1}, when k=d. The full distribution of
the number of minima is obtained in closed form in two-dimensions.Comment: 6 pages, 1 figur

### Be aware of drinkaware

In 2006, Drinkaware was established as a charity in the United Kingdom following a memorandum of understanding between the Portman Group and various UK government agencies. This debate piece briefly reviews the international literature on industry social aspects organizations, examines the nature of Drinkaware\u27s activities and considers how the public health community should respond. Although the British addiction field and the wider public health community have distanced themselves from the Portman Group, they have not done so from Drinkaware, even though Drinkaware was devised by the Portman Group to serve industry interests. Both long-standing and more recent developments indicate very high levels of industry influence on British alcohol policy, and Drinkaware provides one mechanism of influence. We suggest that working with, and for, industry bodies such as Drinkaware helps disguise fundamental conflicts of interest and serves only to legitimize corporate efforts to promote partnership as a means of averting evidence-based alcohol policies. We invite vigorous debate on these internationally significant issues and propose that similar industry bodies should be carefully studied in other countries

### Entanglement vs. gap for one-dimensional spin systems

We study the relationship between entanglement and spectral gap for local
Hamiltonians in one dimension. The area law for a one-dimensional system states
that for the ground state, the entanglement of any interval is upper-bounded by
a constant independent of the size of the interval. However, the possible
dependence of the upper bound on the spectral gap Delta is not known, as the
best known general upper bound is asymptotically much larger than the largest
possible entropy of any model system previously constructed for small Delta. To
help resolve this asymptotic behavior, we construct a family of one-dimensional
local systems for which some intervals have entanglement entropy which is
polynomial in 1/Delta, whereas previously studied systems, such as free fermion
systems or systems described by conformal field theory, had the entropy of all
intervals bounded by a constant times log(1/Delta).Comment: 16 pages. v2 is final published version with slight clarification

### Statistical Self-Similarity of One-Dimensional Growth Processes

For one-dimensional growth processes we consider the distribution of the
height above a given point of the substrate and study its scale invariance in
the limit of large times. We argue that for self-similar growth from a single
seed the universal distribution is the Tracy-Widom distribution from the theory
of random matrices and that for growth from a flat substrate it is some other,
only numerically determined distribution. In particular, for the polynuclear
growth model in the droplet geometry the height maps onto the longest
increasing subsequence of a random permutation, from which the height
distribution is identified as the Tracy-Widom distribution.Comment: 11 pages, iopart, epsf, 2 postscript figures, submitted to Physica A,
in an Addendum the distribution for the flat case is identified analyticall

### Massive stars in metal-poor dwarf galaxies are often extreme rotators

We probe how common extremely rapid rotation is among massive stars in the
early universe by measuring the OBe star fraction in nearby metal-poor dwarf
galaxies. We apply a new method that uses broad-band photometry to measure the
galaxy-wide OBe star fractions in the Magellanic Clouds and three more distant,
more metal-poor dwarf galaxies. We find OBe star fractions of ~20% in the Large
Magellanic Cloud (0.5 Z_Solar), and ~30% in the Small Magellanic Cloud (0.2
Z_Solar) as well as in the so-far unexplored metallicity range from 0.1 Z_solar
to 0.2 Z_solar occupied by the other three dwarf galaxies. Our results imply
that extremely rapid rotation is common among massive stars in metal-poor
environments such as the early universe.Comment: Conference proceedings for a talk in IAU Symposium 361: Massive Stars
Near and Far, Ballyconnell, Ireland, 9-13 May 202

### Entropic Tightening of Vibrated Chains

We investigate experimentally the distribution of configurations of a ring
with an elementary topological constraint, a ``figure-8'' twist. Using vibrated
granular chains, which permit controlled preparation and direct observation of
such a constraint, we show that configurations where one of the loops is tight
and the second is large are strongly preferred. This agrees with recent
predictions for equilibrium properties of topologically-constrained polymers.
However, the dynamics of the tightening process weakly violate detailed
balance, a signature of the nonequilibrium nature of this system.Comment: 4 pages, 4 figure

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