309,413 research outputs found
The dynamical fate of planetary systems in young star clusters
We carry out N-body simulations to examine the effects of dynamical
interactions on planetary systems in young open star clusters. We explore how
the planetary populations in these star clusters evolve, and how this evolution
depends on the initial amount of substructure, the virial ratio, the cluster
mass and density, and the initial semi-major axis of the planetary systems. The
fraction of planetary systems that remains intact as a cluster member, fbps, is
generally well-described by the functional form fbps=f0(1+[a/a0]^c)^-1, where
(1-f0) is the fraction of stars that escapes from the cluster, a0 the critical
semi-major axis for survival, and c a measure for the width of the transition
region. The effect of the initial amount of substructure over time can be
quantified as fbps=A(t)+B(D), where A(t) decreases nearly linearly with time,
and B(D) decreases when the clusters are initially more substructured. Provided
that the orbital separation of planetary systems is smaller than the critical
value a0, those in clusters with a higher initial stellar density (but
identical mass) have a larger probability of escaping the cluster intact. These
results help us to obtain a better understanding of the difference between the
observed fractions of exoplanets-hosting stars in star clusters and in the
Galactic field. It also allows us to make predictions about the free-floating
planet population over time in different stellar environments.Comment: 14 pages, 9 figures, accepted for publication in MNRA
Optical spectroscopy study of Nd(O,F)BiS2 single crystals
We present an optical spectroscopy study on F-substituted NdOBiS
superconducting single crystals grown using KCl/LiCl flux method. The
measurement reveals a simple metallic response with a relatively low screened
plasma edge near 5000 \cm. The plasma frequency is estimated to be 2.1 eV,
which is much smaller than the value expected from the first-principles
calculations for an electron doping level of x=0.5, but very close to the value
based on a doping level of 7 of itinerant electrons per Bi site as
determined by ARPES experiment. The energy scales of the interband transitions
are also well reproduced by the first-principles calculations. The results
suggest an absence of correlation effect in the compound, which essentially
rules out the exotic pairing mechanism for superconductivity or scenario based
on the strong electronic correlation effect. The study also reveals that the
system is far from a CDW instability as being widely discussed for a doping
level of x=0.5.Comment: 5 pages, 5 figure
Where is the jet quenching in Pb+Pb collisions at 158 AGeV?
Because of the rapidly falling particle spectrum at large from jet
fragmentation at the CERN SPS energy, the high- hadron distribution should
be highly sensitive to parton energy loss inside a dense medium as predicted by
recent perturbative QCD (pQCD) studies. A careful analysis of recent data from
CERN SPS experiments via pQCD calculation shows little evidence of energy loss.
This implies that either the life-time of the dense partonic matter is very
short or one has to re-think about the problem of parton energy loss in dense
matter. The hadronic matter does not seem to cause jet quenching in
collisions at the CERN SPS. High- two particle correlation in the
azimuthal angle is proposed to further clarify this issue.Comment: 4 pages with 2 ps figures. Minors changes are made in the text with
updated references. Revised version to appear in Phys. Rev. Letter
Azimuthal asymmetry in transverse energy flow in nuclear collisions at high energies
The azimuthal pattern of transverse energy flow in nuclear collisions at RHIC
and LHC energies is considered. We show that the probability distribution of
the event-by-event azimuthal disbalance in transverse energy flow is
essentially sensitive to the presence of the semihard minijet component.Comment: 6 pages, 2 figure
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