3,139 research outputs found
Why haven't loose globular clusters collapsed yet?
We report on the discovery of a surprising observed correlation between the
slope of the low-mass stellar global mass function (GMF) of globular clusters
(GCs) and their central concentration parameter c=log(r_t/r_c), i.e. the
logarithmic ratio of tidal and core radii. This result is based on the analysis
of a sample of twenty Galactic GCs with solid GMF measurements from deep HST or
VLT data. All the high-concentration clusters in the sample have a steep GMF,
most likely reflecting their initial mass function. Conversely,
low-concentration clusters tend to have a flatter GMF implying that they have
lost many stars via evaporation or tidal stripping. No GCs are found with a
flat GMF and high central concentration. This finding appears
counter-intuitive, since the same two-body relaxation mechanism that causes
stars to evaporate and the cluster to eventually dissolve should also lead to
higher central density and possibly core-collapse. Therefore, more concentrated
clusters should have lost proportionately more stars and have a shallower GMF
than low concentration clusters, contrary to what is observed. It is possible
that severely depleted GCs have also undergone core collapse and have already
recovered a normal radial density profile. It is, however, more likely that GCs
with a flat GMF have a much denser and smaller core than suggested by their
surface brightness profile and may well be undergoing collapse at present. In
either case, we may have so far seriously underestimated the number of post
core-collapse clusters and many may be lurking in the Milky Way.Comment: Four pages, one figure, accepted for publication in ApJ Letter
Greedy kernel methods for accelerating implicit integrators for parametric ODEs
We present a novel acceleration method for the solution of parametric ODEs by
single-step implicit solvers by means of greedy kernel-based surrogate models.
In an offline phase, a set of trajectories is precomputed with a high-accuracy
ODE solver for a selected set of parameter samples, and used to train a kernel
model which predicts the next point in the trajectory as a function of the last
one. This model is cheap to evaluate, and it is used in an online phase for new
parameter samples to provide a good initialization point for the nonlinear
solver of the implicit integrator. The accuracy of the surrogate reflects into
a reduction of the number of iterations until convergence of the solver, thus
providing an overall speedup of the full simulation. Interestingly, in addition
to providing an acceleration, the accuracy of the solution is maintained, since
the ODE solver is still used to guarantee the required precision. Although the
method can be applied to a large variety of solvers and different ODEs, we will
present in details its use with the Implicit Euler method for the solution of
the Burgers equation, which results to be a meaningful test case to demonstrate
the method's features
The EU’s comprehensive approach as the dominant discourse: A corpus-linguistics analysis of the EU’s counter-piracy narrative
Using corpus linguistics techniques, this article discusses the EU’s comprehensive approach narrative in the context of counter-piracy at the Horn of Africa. It shows that (1) the comprehensive approach is systematically put forward by the EU in its counter-piracy discourse, (2) this approach is presented as the best (if not the unique) and normal way to deal with the problem, and (3) the EU claims the paternity of this approach and attributes successes to its own activities and approach. The article demonstrates that the EU uses the comprehensive approach narrative to showcase its positive and unique contribution as a global security actor and to normalize its power projection practice. This case study also contributes to demonstrate that the comprehensive approach tends to achieve discursive dominance at the EU level. By integrating interpretative framework from the field of International Relations (IR) with the empirical, data driven descriptions that corpus linguistics analysis provides this article makes an original contribution to European foreign policy studies and contributes to the methodological enrichment of the discipline
Novel Scintillating Materials Based on Phenyl-Polysiloxane for Neutron Detection and Monitoring
Neutron detectors are extensively used at many nuclear research facilities
across Europe. Their application range covers many topics in basic and applied
nuclear research: in nuclear structure and reaction dynamics (reaction
reconstruction and decay studies); in nuclear astrophysics (neutron emission
probabilities); in nuclear technology (nuclear data measurements and
in-core/off-core monitors); in nuclear medicine (radiation monitors,
dosimeters); in materials science (neutron imaging techniques); in homeland
security applications (fissile materials investigation and cargo inspection).
Liquid scintillators, widely used at present, have however some drawbacks given
by toxicity, flammability, volatility and sensitivity to oxygen that limit
their duration and quality. Even plastic scintillators are not satisfactory
because they have low radiation hardness and low thermal stability. Moreover
organic solvents may affect their optical properties due to crazing. In order
to overcome these problems, phenyl-polysiloxane based scintillators have been
recently developed at Legnaro National Laboratory. This new solution showed
very good chemical and thermal stability and high radiation hardness. The
results on the different samples performance will be presented, paying special
attention to a characterization comparison between synthesized phenyl
containing polysiloxane resins where a Pt catalyst has been used and a
scintillating material obtained by condensation reaction, where tin based
compounds are used as catalysts. Different structural arrangements as a result
of different substituents on the main chain have been investigated by High
Resolution X-Ray Diffraction, while the effect of improved optical
transmittance on the scintillation yield has been elucidated by a combination
of excitation/fluorescence measurements and scintillation yield under exposure
to alpha and {\gamma}-rays.Comment: InterM 2013 - International Multidisciplinary Microscopy Congres
Star Formation in the Starburst Cluster in NGC 3603
We have used new, deep, visible and near infrared observations of the compact
starburst cluster in the giant HII region NGC 3603 and its surroundings with
the WFC3 on HST and HAWK-I on the VLT to study in detail the physical
properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show
that after correction for differential extinction and actively accreting stars,
and the study of field star contamination, strong evidence remains for a
continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~
30 Myr within the temporal resolution available. Existing differences among
presently available theoretical models account for the largest possible
variation in shape of the measured age histograms within these limits. We also
find that this isochronal age spread in the near infrared and visible
Colour-Magnitude Diagrams cannot be reproduced by any other presently known
source of astrophysical or instrumental scatter that could mimic the luminosity
spread seen in our observations except, possibly, episodic accretion. The
measured age spread and the stellar spatial distribution in the cluster are
consistent with the hypothesis that star formation started at least 20-30 Myrs
ago progressing slowly but continuously up to at least a few million years ago.
All the stars in the considered mass range are distributed in a flattened
oblate spheroidal pattern with the major axis oriented in an approximate
South-East - North-West direction, and with the length of the equatorial axis
decreasing with increasing age. This asymmetry is most likely due to the fact
that star formation occurred along a filament of gas and dust in the natal
molecular cloud oriented locally in this direction.Comment: 21 pages, 19 figures, accepted for publication in Astrophysics &
Space Scienc
WIYN Open Cluster Study 1: Deep Photometry of NGC 188
We have employed precise V and I photometry of NGC 188 at WIYN to explore the
cluster luminosity function (LF) and study the cluster white dwarfs (WDs). Our
photometry is offset by V = 0.052 (fainter) from Sandage (1962) and Eggen &
Sandage (1969). All published photometry for the past three decades have been
based on these two calibrations, which are in error by 0.05 +- 0.01. We employ
the Pinsonneault etal (1998) fiducial main sequence to derive a cluster
distance modulus of 11.43 +- 0.08. We report observations that are >= 50%
complete to V = 24.6 and find that the cluster central-field LF peaks at M_I ~
3 to 4. This is unlike the solar neighborhood LF and unlike the LFs of
dynamically unevolved portions of open and globular clusters, which rise
continuously until M_I ~ 9.5. Although we find that >= 50% of the unresolved
cluster objects are multiple systems, their presence cannot account for the
shape of the NGC 188 LF. For theoretical reasons (Terlevich 1987; Vesperini &
Heggie 1997) having to do with the survivability of NGC 188 we believe the
cluster is highly dynamically evolved and that the missing low luminosity stars
are either in the cluster outskirts or have left the cluster altogether. We
identify nine candidate WDs, of which we expect three to six are bona fide
cluster WDs. The luminosities of the faintest likely WD indicates an age
(Bergeron, Wesemael, & Beauchamp 1995) of 1.14 +- 0.09 Gyrs. This is a lower
limit to the cluster age and observations probing to V = 27 or 28 will be
necessary to find the faintest cluster WDs and independently determine the
cluster age. While our age limit is not surprising for this ~6 Gyr old cluster,
our result demonstrates the value of the WD age technique with its very low
internal errors. (abridged)Comment: 26 pages, uuencoded gunzip'ed latex + 16 postscrip figures, to be
published in A
Apollo asteroids (1566) Icarus and 2007 MK6: Icarus family members?
Although it is more complicated to search for near-Earth object (NEO)
families than main belt asteroid (MBA) families, since differential orbital
evolution within a NEO family can cause current orbital elements to drastically
differ from each other, we have found that Apollo asteroids (1566) Icarus and
the newly discovered 2007 MK6 are almost certainly related. Specifically, their
orbital evolutions show a similar profile, time shifted by only ~1000 yr, based
on our time-lag theory. The dynamical relationship between Icarus and 2007 MK6
along with a possible dust band, the Taurid-Perseid meteor swarm, implies the
first detection of an asteroidal NEO family, namely the "Icarus asteroid
family".Comment: 11 pages, 1 figure, to appear on Astrophysical Journal Letters
(journal info added
Position and singularity analysis of a class of planar parallel manipulators with a reconfigurable end-effector
Parallel robots with configurable platforms are a class of robots in which the end-effector has an inner mobility, so that its overall shape can be reconfigured: in most cases, the end-effector is thus a closed-loop kinematic chain composed of rigid links. These robots have a greater flexibility in their motion and control with respect to rigid-platform parallel architectures, but their kinematics is more challenging to analyze. In our work, we consider n-RRR planar configurable robots, in which the end-effector is a chain composed of n links and revolute joints, and is controlled by n rotary actuators located on the base of the mechanism. In particular, we study the geometrical design of such robots and their direct and inverse kinematics for n = 4, n = 5 and n = 6; we employ the bilateration method, which can simplify the kinematic analysis and allows us to generalize the approach and the results obtained for the 3-RRR mechanism to n-RRR robots (with n > 3). Then, we study the singularity configurations of these robot architectures. Finally, we present the results from experimental tests that have been performed on a 5–RRR robot prototype
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