2,816 research outputs found
Some Further Results for the Stationary Points and Dynamics of Supercooled Liquids
We present some new theoretical and computational results for the stationary
points of bulk systems. First we demonstrate how the potential energy surface
can be partitioned into catchment basins associated with every stationary point
using a combination of Newton-Raphson and eigenvector-following techniques.
Numerical results are presented for a 256-atom supercell representation of a
binary Lennard-Jones system. We then derive analytical formulae for the number
of stationary points as a function of both system size and the Hessian index,
using a framework based upon weakly interacting subsystems. This analysis
reveals a simple relation between the total number of stationary points, the
number of local minima, and the number of transition states connected on
average to each minimum. Finally we calculate two measures of localisation for
the displacements corresponding to Hessian eigenvectors in samples of
stationary points obtained from the Newton-Raphson-based geometry optimisation
scheme. Systematic differences are found between the properties of eigenvectors
corresponding to positive and negative Hessian eigenvalues, and localised
character is most pronounced for stationary points with low values of the
Hessian index.Comment: 16 pages, 2 figure
Space-efficient detection of unusual words
Detecting all the strings that occur in a text more frequently or less
frequently than expected according to an IID or a Markov model is a basic
problem in string mining, yet current algorithms are based on data structures
that are either space-inefficient or incur large slowdowns, and current
implementations cannot scale to genomes or metagenomes in practice. In this
paper we engineer an algorithm based on the suffix tree of a string to use just
a small data structure built on the Burrows-Wheeler transform, and a stack of
bits, where is the length of the string and
is the size of the alphabet. The size of the stack is except for very
large values of . We further improve the algorithm by removing its time
dependency on , by reporting only a subset of the maximal repeats and
of the minimal rare words of the string, and by detecting and scoring candidate
under-represented strings that in the string. Our
algorithms are practical and work directly on the BWT, thus they can be
immediately applied to a number of existing datasets that are available in this
form, returning this string mining problem to a manageable scale.Comment: arXiv admin note: text overlap with arXiv:1502.0637
VLTI/MIDI 10 micron interferometry of the forming massive star W33A
We report on resolved interferometric observations with VLTI/MIDI of the
massive young stellar object (MYSO) W33A. The MIDI observations deliver
spectrally dispersed visibilities with values between 0.03 and 0.06, for a
baseline of 45m over the wavelength range 8-13 micron. The visibilities
indicate that W33A has a FWHM size of approximately 120AU (0.030'') at 8 micron
which increases to 240AU at 13 micron, scales previously unexplored among
MYSOs. This observed trend is consistent with the temperature falling off with
distance. 1D dust radiative transfer models are simultaneously fit to the
visibility spectrum, the strong silicate feature and the shape of the mid
infrared spectral energy distribution (SED). For any powerlaw density
distribution, we find that the sizes (as implied by the visibilities) and the
stellar luminosity are incompatible. A reduction to a third of W33A's
previously adopted luminosity is required to match the visibilities; such a
reduction is consistent with new high resolution 70 micron data from Spitzer's
MIPSGAL survey. We obtain best fits for models with shallow dust density
distributions of r^(-0.5) and r^(-1.0) and for increased optical depth in the
silicate feature produced by decreasing the ISM ratio of graphite to silicates
and using optical grain properties by Ossenkopf et al. (1992).Comment: 4 pages, 4 figures. Accepted for ApJ letter
Even faster sorting of (not only) integers
In this paper we introduce RADULS2, the fastest parallel sorter based on
radix algorithm. It is optimized to process huge amounts of data making use of
modern multicore CPUs. The main novelties include: extremely optimized
algorithm for handling tiny arrays (up to about a hundred of records) that
could appear even billions times as subproblems to handle and improved
processing of larger subarrays with better use of non-temporal memory stores
Bound States of the q-Deformed AdS5 x S5 Superstring S-matrix
The investigation of the q deformation of the S-matrix for excitations on the
string world sheet in AdS5 x S5 is continued. We argue that due to the lack of
Lorentz invariance the situation is more subtle than in a relativistic theory
in that the nature of bound states depends on their momentum. At low enough
momentum |p|<E the bound states transform in the anti-symmetric representation
of the super-algebra symmetry and become the solitons of the Pohlmeyer reduced
theory in the relativistic limit. At a critical momentum |p|=E they become
marginally unstable, and at higher momenta the stable bound states are in the
symmetric representation and become the familiar magnons in the string limit as
q->1. This subtlety fixes a problem involving the consistency of crossing
symmetry with the relativistic limit found in earlier work. With mirror
kinematics, obtained after a double Wick rotation, the bound state structure is
simpler and there are no marginally unstable bound states.Comment: 25 page
Near-infrared integral field spectroscopy of Massive Young Stellar Objects
We present medium resolution () -band integral field
spectroscopy of six MYSOs. The targets are selected from the RMS survey, and we
used the NIFS on the Gemini North telescope. The data show various spectral
line features including Br, CO, H, and \mbox{He\,{\sc i}}. The
Br line is detected in emission in all objects with
-- 200 kms. V645 Cyg shows a high-velocity
P-Cygni profile between -800 kms and -300 kms. We performed
three-dimensional spectroastrometry to diagnose the circumstellar environment
in the vicinity of the central stars using the Br line. We measured the
centroids of the velocity components with sub-mas precision. The centroids
allow us to discriminate the blueshifted and redshifted components in a roughly
east--west direction in both IRAS 18151--1208 and S106 in Br. This lies
almost perpendicular to observed larger scale outflows. We conclude, given the
widths of the lines and the orientation of the spectroastrometric signature,
that our results trace a disc wind in both IRAS 18151--1208 and S106. The CO
absorption lines at low transitions are detected in IRAS
18151--1208 and AFGL 2136. We analysed the velocity structure of the neutral
gas discs. In IRAS 18151--1208, the absorption centroids of the blueshifted and
redshifted components are separated in a direction of north-east to south-west,
nearly perpendicular to that of the larger scale jet. The
position-velocity relations of these objects can be reproduced with central
masses of 30 M_{\sun} for IRAS 18151--1208 and 20 M_{\sun} for AFGL 2136.
We also detect CO bandhead emission in IRAS 18151--1208, S106 and
V645 Cyg. The results can be fitted reasonably with a Keplerian rotation model,
with masses of 15, 20 and 20 M_{\sun} respectively.Comment: 17 pages, 10 figures, accepted by MNRA
Global Optimization by Basin-Hopping and the Lowest Energy Structures of Lennard-Jones Clusters Containing up to 110 Atoms
We describe a global optimization technique using `basin-hopping' in which
the potential energy surface is transformed into a collection of
interpenetrating staircases. This method has been designed to exploit the
features which recent work suggests must be present in an energy landscape for
efficient relaxation to the global minimum. The transformation associates any
point in configuration space with the local minimum obtained by a geometry
optimization started from that point, effectively removing transition state
regions from the problem. However, unlike other methods based upon hypersurface
deformation, this transformation does not change the global minimum. The lowest
known structures are located for all Lennard-Jones clusters up to 110 atoms,
including a number that have never been found before in unbiased searches.Comment: 8 pages, 3 figures, revte
Probing discs around massive young stellar objects with CO first overtone emission
We present high resolution (R~50,000) spectroastrometry over the CO 1st
overtone bandhead of a sample of seven intermediate/massive young stellar
objects. These are primarily drawn from the red MSX source (RMS) survey, a
systematic search for young massive stars which has returned a large, well
selected sample of such objects. The mean luminosity of the sample is
approximately 5 times 10^4 L_\odot, indicating the objects typically have a
mass of ~15 solar masses. We fit the observed bandhead profiles with a model of
a circumstellar disc, and find good agreement between the models and
observations for all but one object. We compare the high angular precision
(0.2-0.8 mas) spectroastrometric data to the spatial distribution of the
emitting material in the best-fitting models. No spatial signatures of discs
are detected, which is entirely consistent with the properties of the
best-fitting models. Therefore, the observations suggest that the CO bandhead
emission of massive young stellar objects originates in small-scale disks, in
agreement with previous work. This provides further evidence that massive stars
form via disc accretion, as suggested by recent simulations.Comment: Accepted for publication in MNRA
Ionization of clusters in intense laser pulses through collective electron dynamics
The motion of electrons and ions in medium-sized rare gas clusters (1000
atoms) exposed to intense laser pulses is studied microscopically by means of
classical molecular dynamics using a hierarchical tree code. Pulse parameters
for optimum ionization are found to be wavelength dependent. This resonant
behavior is traced back to a collective electron oscillation inside the charged
cluster. It is shown that this dynamics can be well described by a driven and
damped harmonic oscillator allowing for a clear discrimination against other
energy absorption mechanisms.Comment: 4 pages (4 figures
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