2,205 research outputs found
Self-Assembly of Monatomic Complex Crystals and Quasicrystals with a Double-Well Interaction Potential
For the study of crystal formation and dynamics we introduce a simple
two-dimensional monatomic model system with a parametrized interaction
potential. We find in molecular dynamics simulations that a surprising variety
of crystals, a decagonal and a dodecagonal quasicrystal are self-assembled. In
the case of the quasicrystals the particles reorder by phason flips at elevated
temperatures. During annealing the entropically stabilized decagonal
quasicrystal undergoes a reversible phase transition at 65% of the melting
temperature into an approximant, which is monitored by the rotation of the de
Bruijn surface in hyperspace.Comment: 4 pages, 6 figures. Physical Review Letters, in Press (April 2007
Quasicrystalline Order in Binary Dipolar Systems
Motivated by recent experimental findings, we investigate the possible
occurrence and characteristics of quasicrystalline order in two-dimensional
mixtures of point dipoles with two sorts of dipole moments. Despite the fact
that the dipolar interaction potential does not exhibit an intrinsic length
scale and cannot be tuned a priori to support the formation of quasicrystalline
order, we find that configurations with long--range quasicrystallinity yield
minima in the potential energy surface of the many particle system. These
configurations emanate from an ideal or perturbed ideal decoration of a binary
tiling by steepest descent relaxation. Ground state energy calculations of
alternative ordered states and parallel tempering Monte-Carlo simulations
reveal that the quasicrystalline configurations do not correspond to a
thermodynamically stable state. On the other hand, steepest descent relaxations
and conventional Monte-Carlo simulations suggest that they are rather robust
against fluctuations. Local quasicrystalline order in the disordered
equilibrium states can be strong.Comment: 10 pages, 7 figure
Near-IR spectroscopic ages of massive star clusters in M82
Like other starburst galaxies, M82 hosts compact, massive young star clusters
that are interesting both in their own right and as benchmarks for population
synthesis models. Can spectral synthesis models at resolutions around 1000
adequately reproduce the near-IR spectral features and the energy distribution
of these clusters between 0.8 and 2.4 microns? How do the derived cluster
properties compare with previous results from optical studies?
We analyse the spectra of 5 massive clusters in M82, using data acquired with
the spectrograph SpeX on the InfraRed Telescope Facility (NASA/IRTF) and a new
population synthesis tool with a highly improved near-IR extension, based on a
recent collection of empirical and theoretical spectra of red supergiant stars.
We obtain excellent fits across the near-IR with models at quasi-solar
metallicity and a solar neighbourhood extinction law. Spectroscopy breaks a
strong degeneracy between age and extinction in the near-IR colours in the red
supergiant-dominated phase of evolution. The estimated near-IR ages cluster
between 9 and 30 Myr, i.e. the ages at which the molecular bands due to
luminous red supergiants are strongest in the current models. They do not
always agree with optical spectroscopic ages. Adding optical data sometimes
leads to the rejection of the solar neighbourhood extinction law. This is not
surprising considering small-scale structure around the clusters, but it has no
significant effect on the near-IR based spectroscopic ages. [abridged]Comment: 14 pages, 20 figures, uses aa.cl
Scaling properties of step bunches induced by sublimation and related mechanisms: A unified perspective
This work provides a ground for a quantitative interpretation of experiments
on step bunching during sublimation of crystals with a pronounced
Ehrlich-Schwoebel (ES) barrier in the regime of weak desorption. A strong step
bunching instability takes place when the kinetic length is larger than the
average distance between the steps on the vicinal surface. In the opposite
limit the instability is weak and step bunching can occur only when the
magnitude of step-step repulsion is small. The central result are power law
relations of the between the width, the height, and the minimum interstep
distance of a bunch. These relations are obtained from a continuum evolution
equation for the surface profile, which is derived from the discrete step
dynamical equations for. The analysis of the continuum equation reveals the
existence of two types of stationary bunch profiles with different scaling
properties. Through a mathematical equivalence on the level of the discrete
step equations as well as on the continuum level, our results carry over to the
problems of step bunching induced by growth with a strong inverse ES effect,
and by electromigration in the attachment/detachment limited regime. Thus our
work provides support for the existence of universality classes of step
bunching instabilities [A. Pimpinelli et al., Phys. Rev. Lett. 88, 206103
(2002)], but some aspects of the universality scenario need to be revised.Comment: 21 pages, 8 figure
Fundamental properties and atmospheric structure of the red supergiant VY CMa based on VLTI/AMBER spectro-interferometry
We investigate the atmospheric structure and fundamental properties of the
red supergiant VY CMa. We obtained near-infrared spectro-interferometric
observations of VY CMa with spectral resolutions of 35 and 1500 using the AMBER
instrument at the VLTI. The visibility data indicate the presence of molecular
layers of water vapor and CO in the extended atmosphere with an asymmetric
morphology. The uniform disk diameter in the water band around 2.0 mu is
increased by \sim20% compared to the near-continuum bandpass at 2.20-2.25 mu
and in the CO band at 2.3-2.5 mu it is increased by up to \sim50%. The closure
phases indicate relatively small deviations from point symmetry close to the
photospheric layer, and stronger deviations in the extended H2O and CO layers.
Making use of the high spatial and spectral resolution, a near-continuum
bandpass can be isolated from contamination by molecular and dusty layers, and
the Rosseland-mean photospheric angular diameter is estimated to 11.3 +/- 0.3
mas based on a PHOENIX atmosphere model. Together with recent high-precision
estimates of the distance and spectro-photometry, this estimate corresponds to
a radius of 1420 +/- 120 Rsun and an effective temperature of 3490 +/- 90 K. VY
CMa exhibits asymmetric, possibly clumpy, atmospheric layers of H2O and CO,
which are not co-spatial, within a larger elongated dusty envelope. Our revised
fundamental parameters put VY CMa close to the Hayashi limit of recent
evolutionary tracks of initial mass 25 Msun with rotation or 32 Msun without
rotation, shortly before evolving blueward in the HR-diagram.Comment: 5 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics (A&A) as a Lette
Organisation en réseau et durabilité systémique de deux filières alimentaires
La durabilité des filières alimentaires est questionnée par les pouvoirs publics et les consommateurs, qui tendent à défendre des formes alternatives de production, respectueuses de l'environnement, économiquement et socialement acceptables. Ces formes sont toutefois confrontées à des freins techniques, économiques et institutionnels d'où un questionnement sur leur capacité à se développer et à se maintenir, et donc sur leur durabilité. A partir d'une étude systémique de deux filières alternatives, le petit épeautre de Haute Provence et le riz biologique de Camargue, nous nous sommes attachés à comprendre comment l'organisation des acteurs peut influer sur les trois piliers du développement durable. Nous exposons une manière de considérer le développement durable qui met en avant les relations entre les acteurs, à l'aide de l'outil de l'analyse de réseau, et des entretiens menés avec un échantillon de différents acteurs des filières. Nous montrons que la volonté de pérennisation de la filière influe sur le choix du type d'organisation au sein de la filière, qui peut être orienté pour plus ou moins favoriser la prise en compte des piliers du développement durable. Nos observations montrent l'importance de considérer les réseaux dans une vision systémique de la durabilité. Il reste toutefois difficile d'établir des relations de causalité entre d'une part le type d'organisation et d'autre part la durabilité
First starlight spectrum captured using an integrated photonic micro-spectrograph
Photonic technologies have received growing consideration for incorporation
into next-generation astronomical instrumentation, owing to their miniature
footprint and inherent robustness. In this paper we present results from the
first on-telescope demonstration of a miniature photonic spectrograph for
astronomy, by obtaining spectra spanning the entire H-band from several stellar
targets. The prototype was tested on the 3.9 m Anglo-Australian telescope. In
particular, we present a spectrum of the variable star Pi 01 Gru, with observed
CO molecular absorption bands, at a resolving power R = 2500 at 1600 nm.
Furthermore, we successfully demonstrate the simultaneous acquisition of
multiple spectra with a single spectrograph chip by using multiple fibre
inputs.Comment: 5 Pages, 4 Figures; A&A, Volume 544 (2012
Dynamics of Viscoplastic Deformation in Amorphous Solids
We propose a dynamical theory of low-temperature shear deformation in
amorphous solids. Our analysis is based on molecular-dynamics simulations of a
two-dimensional, two-component noncrystalline system. These numerical
simulations reveal behavior typical of metallic glasses and other viscoplastic
materials, specifically, reversible elastic deformation at small applied
stresses, irreversible plastic deformation at larger stresses, a stress
threshold above which unbounded plastic flow occurs, and a strong dependence of
the state of the system on the history of past deformations. Microscopic
observations suggest that a dynamically complete description of the macroscopic
state of this deforming body requires specifying, in addition to stress and
strain, certain average features of a population of two-state shear
transformation zones. Our introduction of these new state variables into the
constitutive equations for this system is an extension of earlier models of
creep in metallic glasses. In the treatment presented here, we specialize to
temperatures far below the glass transition, and postulate that irreversible
motions are governed by local entropic fluctuations in the volumes of the
transformation zones. In most respects, our theory is in good quantitative
agreement with the rich variety of phenomena seen in the simulations.Comment: 16 pages, 9 figure
The M/L ratio of massive young clusters
We point out a strong time-evolution of the mass-to-light conversion factor
\eta commonly used to estimate masses of dense star clusters from observed
cluster radii and stellar velocity dispersions. We use a gas-dynamical model
coupled with the Cambridge stellar evolution tracks to compute line-of-sight
velocity dispersions and half-light radii weighted by the luminosity. Stars at
birth are assumed to follow the Salpeter mass function in the range [0.15--17
M_\sun]. We find that , and hence the estimated cluster mass, increases
by factors as large as 3 over time-scales of 20 million years. Increasing the
upper mass limit to 50 M_\sun leads to a sharp rise of similar amplitude but
in as little as 10 million years.
Fitting truncated isothermal (Michie-King) models to the projected light
profile leads to over-estimates of the concentration par ameter c of compared to the same functional fit applied to the proj ected
mass density.Comment: Draft version of an ApJ lette
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