219 research outputs found
Escaping stars from young low-N clusters
With the use of N-body calculations the amount and properties of escaping
stars from low-N (N = 100 and 1000) young embedded star clusters prior to gas
expulsion are studied over the first 5 Myr of their existence. Besides the
number of stars also different initial radii and binary populations are
examined as well as virialised and collapsing clusters. It is found that these
clusters can loose substantial amounts (up to 20%) of stars within 5 Myr with
considerable velocities up to more than 100 km/s. Even with their mean
velocities between 2 and 8 km/s these stars will still be travelling between 2
and 30 pc during the 5 Myr. Therefore can large amounts of distributed stars in
star-forming regions not necessarily be counted as evidence for the isolated
formation of stars.Comment: 10 pages, 10 figures, accepted for publication by MNRA
What is the Temperature Dependence of the Casimir Effect?
There has been recent criticism of our approach to the Casimir force between
real metallic surfaces at finite temperature, saying it is in conflict with the
third law of thermodynamics and in contradiction with experiment. We show that
these claims are unwarranted, and that our approach has strong theoretical
support, while the experimental situation is still unclear.Comment: 6 pages, REVTeX, final revision includes two new references and
related discussio
On the Temperature Dependence of the Casimir Effect
The temperature dependence of the Casimir force between a real metallic plate
and a metallic sphere is analyzed on the basis of optical data concerning the
dispersion relation of metals such as gold and copper. Realistic permittivities
imply, together with basic thermodynamic considerations, that the transverse
electric zero mode does not contribute. This results in observable differences
with the conventional prediction, which does not take this physical requirement
into account. The results are shown to be consistent with the third law of
thermodynamics, as well as being consistent with current experiments. However,
the predicted temperature dependence should be detectable in future
experiments. The inadequacies of approaches based on {\it ad hoc} assumptions,
such as the plasma dispersion relation and the use of surface impedance without
transverse momentum dependence, are discussed.Comment: 14 pages, 3 eps figures, revtex4. New version includes clarifications
and new reference. Accepted for publication in Phys. Rev.
Analytical and Numerical Verification of the Nernst Theorem for Metals
In view of the current discussion on the subject, an effort is made to show
very accurately both analytically and numerically how the Drude dispersion
model gives consistent results for the Casimir free energy at low temperatures.
Specifically, for the free energy near T=0 we find the leading term to be
proportional to T^2 and the next-to-leading term proportional to T^{5/2}. These
terms give rise to zero Casimir entropy as T approaches zero, and is thus in
accordance with Nernst's theorem.Comment: 19 pages latex, 3 figures. v4: Figures updated. This is the final
version, accepted for publication in Physical Review
The Tidal Tails of NGC 5466
The study of substructure in the stellar halo of the Milky Way has made a lot
of progress in recent years, especially with the advent of surveys like the
Sloan Digital Sky Survey. Here, we study the newly discovered tidal tails of
the Galactic globular cluster NGC 5466. By means of numerical simulations, we
reproduce the shape, direction and surface density of the tidal tails, as well
as the structural and kinematical properties of the present-day NGC 5466.
Although its tails are very extended in SDSS data (> 45 degrees), NGC 5466 is
only losing mass slowly at the present epoch and so can survive for probably a
further Hubble time. The effects of tides at perigalacticon and disc crossing
are the dominant causes of the slow dissolution of NGC 5466, accounting for
about 60 % of the mass loss over the course of its evolution. The morphology of
the tails provides a constraint on the proper motion -- the observationally
determined proper motion has to be refined (within the stated error margins) to
match the location of the tidal tails.Comment: MNRAS, in pres
Calibration of radii and masses of open clusters with a simulation
Context: A recent new approach to apply a simple dynamical mass estimate of
tidally limited star clusters is based on the identification of the tidal
radius in a King profile with the dynamical Jacobi radius. The application to
an unbiased open cluster catalogue yields significantly higher cluster masses
compared to the classical methods. Aims: We quantify the bias in the mass
determination as function of projection direction and cluster age by analysing
a simulated star cluster. Methods: We use direct -body simulations of a star
cluster including stellar evolution in an analytic Milky Way potential and
apply a best fit to the projected number density of cluster stars. Results: We
obtain significantly overestimated star cluster masses which depend strongly on
the viewing direction. The overestimation is typically in the range of 10-50
percent and reaches a factor of 3.5 for young clusters. Mass segregation
reduces the derived limiting radii systematically.Comment: 9 pages, 10+1 figures, accepted by Astronomy and Astrophysic
Phase transitions in simplified models with long-range interactions
We study the origin of phase transitions in some simplified models with long
range interactions. For the ring model, we show that a possible new phase
transition predicted in a recent paper by Nardini and Casetti from an energy
landscape analysis does not occur. Instead of such phase transitions we observe
a sharp, although without any non-analiticity, change from a core-halo to an
only core configuration in the spatial distribution functions for low energies.
By introducing a new class of solvable simplified models without any critical
points in the potential energy, we show that a similar behaviour to the ring
model is obtained, with a first order phase transition from an almost
homogeneous high energy phase to a clustered phase, and the same core-halo to
core configuration transition at lower energies. We discuss the origin of these
features of the simplified models, and show that the first order phase
transition comes from the maximization of the entropy of the system as a
function of energy an an order parameter, as previously discussed by Kastner,
which seems to be the main mechanism causing phase transitions in long-range
interacting systems
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