180 research outputs found
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.
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
Close encounters in young stellar clusters: implications for planetary systems in the solar neighbourhood
The stars that populate the solar neighbourhood were formed in stellar
clusters. Through N-body simulations of these clusters, we measure the rate of
close encounters between stars. By monitoring the interaction histories of each
star, we investigate the singleton fraction in the solar neighbourhood. A
singleton is a star which formed as a single star, has never experienced any
close encounters with other stars or binaries, or undergone an exchange
encounter with a binary. We find that, of the stars which formed as single
stars, a significant fraction are not singletons once the clusters have
dispersed. If some of these stars had planetary systems, with properties
similar to those of the solar system, the planets orbits may have been
perturbed by the effects of close encounters with other stars or the effects of
a companion star within a binary. Such perturbations can lead to strong
planet-planet interactions which eject several planets, leaving the remaining
planets on eccentric orbits. Some of the single stars exchange into binaries.
Most of these binaries are broken up via subsequent interactions within the
cluster, but some remain intact beyond the lifetime of the cluster. The
properties of these binaries are similar to those of the observed binary
systems containing extra-solar planets. Thus, dynamical processes in young
stellar clusters will alter significantly any population of solar-system-like
planetary systems. In addition, beginning with a population of planetary
systems exactly resembling the solar system around single stars, dynamical
encounters in young stellar clusters may produce at least some of the
extra-solar planetary systems observed in the solar neighbourhood.Comment: 11 pages, 9 figures, 1 table. Accepted for publication in MNRA
The Casimir Problem of Spherical Dielectrics: Quantum Statistical and Field Theoretical Approaches
The Casimir free energy for a system of two dielectric concentric nonmagnetic
spherical bodies is calculated with use of a quantum statistical mechanical
method, at arbitrary temperature. By means of this rather novel method, which
turns out to be quite powerful (we have shown this to be true in other
situations also), we consider first an explicit evaluation of the free energy
for the static case, corresponding to zero Matsubara frequency ().
Thereafter, the time-dependent case is examined. For comparison we consider the
calculation of the free energy with use of the more commonly known field
theoretical method, assuming for simplicity metallic boundary surfaces.Comment: 31 pages, LaTeX, one new reference; version to appear in Phys. Rev.
- …