500 research outputs found
Statistical properties of antisymmetrized molecular dynamics for non-nucleon-emission and nucleon-emission processes
Statistical properties of the antisymmetrized molecular dynamics (AMD) are
classical in the case of nucleon emission processes, while they are quantum
mechanical for the processes without nucleon emission. We first clarify that
there coexist mutually opposite two statistics in the AMD framework: One is the
classical statistics of the motion of wave packet centroids and the other is
the quantum statistics of the motion of wave packets which is described by the
AMD wave function. We prove the classical statistics of wave packet centroids
by using the framework of the microcanonical ensemble of the nuclear system. We
show that the quantum statistics of wave packets emerges from the classical
statistics of wave packet centroids. It is emphasized that the temperature of
the classical statistics of wave packet centroids is different from the
temperature of the quantum statistics of wave packets. We then explain that the
statistical properties of AMD for nucleon emission processes are classical
because nucleon emission processes in AMD are described by the motion of wave
packet centroids. When we improve the description of the nucleon emission
process so as to take into account the momentum fluctuation due to the wave
packet spread, the AMD statistical properties for nucleon emission processes
change drastically into quantum statistics. Our study of nucleon emission
processes can be conversely regarded as giving another kind of proof of the
fact that the statistics of wave packets is quantum mechanical while that of
wave packet centroids is classical.Comment: 20 pages, LaTeX with revtex and epsf, uuenocded postscript figures,
postscript version available at http://pearl.scphys.kyoto-u.ac.jp/~ono
X-ray spectral diagnostics of activity in massive stars
X-rays give direct evidence of instabilities, time-variable structure, and
shock heating in the winds of O stars. The observed broad X-ray emission lines
provide information about the kinematics of shock-heated wind plasma, enabling
us to test wind-shock models. And their shapes provide information about wind
absorption, and thus about the wind mass-loss rates. Mass-loss rates determined
from X-ray line profiles are not sensitive to density-squared clumping effects,
and indicate mass-loss rate reductions of factors of 3 to 6 over traditional
diagnostics that suffer from density-squared effects. Broad-band X-ray spectral
energy distributions also provide mass-loss rate information via soft X-ray
absorption signatures. In some cases, the degree of wind absorption is so high
that the hardening of the X-ray SED can be quite significant. We discuss these
results as applied to the early O stars zeta Pup (O4 If), 9 Sgr (O4 V((f))),
and HD 93129A (O2 If*).Comment: To appear in the proceedings of IAU 272: Active OB Star
Identifying High Metallicity M Giants at Intragroup Distances with SDSS
Tidal stripping and three-body interactions with the central supermassive
black hole may eject stars from the Milky Way. These stars would comprise a set
of `intragroup' stars that trace the past history of interactions in our
galactic neighborhood. Using the Sloan Digital Sky Survey DR7, we identify
candidate solar metallicity red giant intragroup stars using color cuts that
are designed to exclude nearby M and L dwarfs. We present 677 intragroup
candidates that are selected between 300 kpc and 2 Mpc, and are either the
reddest intragroup candidates (M7-M10) or are L dwarfs at larger distances than
previously detected.Comment: 8 pages, 6 figures, 1 table (for full version, see
http://astro.phy.vanderbilt.edu/~palladl2), Accepted for publication in A
Effective s- and p-Wave Contact Interactions in Trapped Degenerate Fermi Gases
The structure and stability of dilute degenerate Fermi gases trapped in an
external potential is discussed with special emphasis on the influence of s-
and p-wave interactions. In a first step an Effective Contact Interaction for
all partial waves is derived, which reproduces the energy spectrum of the full
potential within a mean-field model space. Using the s- and p-wave part the
energy density of the multi-component Fermi gas is calculated in Thomas-Fermi
approximation. On this basis the stability of the one- and two-component Fermi
gas against mean-field induced collapse is investigated. Explicit stability
conditions in terms of density and total particle number are given. For the
single-component system attractive p-wave interactions limit the density of the
gas. In the two-component case a subtle competition of s- and p-wave
interactions occurs and gives rise to a rich variety of phenomena. A repulsive
p-wave part, for example, can stabilize a two-component system that would
otherwise collapse due to an attractive s-wave interaction. It is concluded
that the p-wave interaction may have important influence on the structure of
degenerate Fermi gases and should not be discarded from the outset.Comment: 18 pages, 11 figures (using RevTEX4
A Monte Carlo approach to study neutron and fragment emission in heavy-ion reactions
Quantum Molecular Dynamics models (QMD) are Monte Carlo approaches targeted
at the description of nucleon-ion and ion-ion collisions. We have developed a
QMD code, which has been used for the simulation of the fast stage of ion-ion
collisions, considering a wide range of system masses and system mass
asymmetries. The slow stage of the collisions has been described by statistical
methods. The combination of both stages leads to final distributions of
particles and fragments, which have been compared to experimental data
available in literature. A few results of these comparisons, concerning neutron
double-differential production cross-sections for C, Ne and Ar ions impinging
on C, Cu and Pb targets at 290 - 400 MeV/A bombarding energies and fragment
isotopic distributions from Xe + Al at 790 MeV/A, are shown in this paper.Comment: 12 pages, 3 figures, submitted for publication in Adv. Space Re
Connection between the Largest Lyapunov Exponent, Density Fluctuation and Multifragmentation in Excited Nuclear Systems
Within a quantum molecular dynamics model we calculate the largest Lyapunov
exponent (LLE), density fluctuation and mass distribution of fragments for a
series of nuclear systems at different initial temperatures. It is found that
the peaks at the temperature ("critical temperature") where the density
fluctuation reaches a maximal value and the mass distribution of fragments is
best fitted by the Fisher's power law from which the critical exponents for
mass and charge distribution are obtained. The time-dependent behavior of the
LLE and density fluctuation is studied. We find that the time scale of the
density fluctuation is much longer than the inverse LLE, which indicates that
the chaotic motion can be well developed during the process of fragment
formation. The finite-size effect on "critical temperature" for nuclear systems
ranging from Calcium to superheavy nuclei is also studied.Comment: 18 pages, 8 figures Submited to Phys. Rev.
Chandra HETGS Multi-Phase Spectroscopy of the Young Magnetic O Star theta^1 Orionis C
We report on four Chandra grating observations of the oblique magnetic
rotator theta^1 Ori C (O5.5 V) covering a wide range of viewing angles with
respect to the star's 1060 G dipole magnetic field. We employ line-width and
centroid analyses to study the dynamics of the X-ray emitting plasma in the
circumstellar environment, as well as line-ratio diagnostics to constrain the
spatial location, and global spectral modeling to constrain the temperature
distribution and abundances of the very hot plasma. We investigate these
diagnostics as a function of viewing angle and analyze them in conjunction with
new MHD simulations of the magnetically channeled wind shock mechanism on
theta^1 Ori C. This model fits all the data surprisingly well, predicting the
temperature, luminosity, and occultation of the X-ray emitting plasma with
rotation phase.Comment: 52 pages, 14 figures (1 color), 6 tables. To appear in the
Astrophysical Journal, 1 August 2005, v628, issue 2. New version corrects
e-mail address, figure and table formatting problem
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