3,881 research outputs found
Formulating the Net Gain of MISO-SFN in the Presence of Self-Interferences
In this study, an analytical formula for multiple-input single-output single frequency network gain (MISO-SFNG) is investigated. To formulate the net MISO-SFNG, we derived the average signal to interference plus noise ratio (SINR) where the gain achieved by the distributed MISO diversity as a function of power imbalance is curve-fitted. Further, we analyzed the losses owing to self-interferences resulting from the delay spread and imperfect channel estimation. We verified the accuracy and effectiveness of the derived formula by comparing the measurement results with the analytical results. The derived formula helps to understand how various system factors affect the gain under a given condition. The formula can be used to evaluate the MISO-SFNG and to predict the MISO-SFN coverage in various system configurations
The Stream-Stream Collision after the Tidal Disruption of a Star Around a Massive Black Hole
A star can be tidally disrupted around a massive black hole. It has been
known that the debris forms a precessing stream, which may collide with itself.
The stream collision is a key process determining the subsequent evolution of
the stellar debris: if the orbital energy is efficiently dissipated, the debris
will eventually form a circular disk (or torus). In this paper, we have
numerically studied such stream collision resulting from the encounter between
a 10^6 Msun black hole and a 1 Msun normal star with a pericenter radius of 100
Rsun. A simple treatment for radiative cooling has been adopted for both
optically thick and thin regions. We have found that approximately 10 to 15% of
the initial kinetic energy of the streams is converted into thermal energy
during the collision. The angular momentum of the incoming stream is increased
by a factor of 2 to 3, and such increase, together with the decrease in kinetic
energy, significantly helps the circularization process. Initial luminosity
burst due to the collision may reach as high as 10^41 erg/sec in 10^4 sec,
after which the luminosity increases again (but slowly this time) to a steady
value of a few 10^40 erg/sec in a few times of 10^5 sec. The radiation from the
system is expected to be close to Planckian with effective temperature of
\~10^5K.Comment: 19 pages including 12 figures; Accepted for publication in Ap
Lyman alpha line formation in starbursting galaxies II. Extremely Thick, Dustless, and Static HI Media
The Lya line transfer in an extremely thick medium of neutral hydrogen is
investigated by adopting an accelerating scheme in our Monte Carlo code to skip
a large number of core or resonant scatterings. This scheme reduces computing
time significantly with no sacrifice in the accuracy of the results. We applied
this numerical method to the Lya transfer in a static, uniform, dustless, and
plane-parallel medium. Two types of photon sources have been considered, the
midplane source and the uniformly distributed sources. The emergent profiles
show double peaks and absorption trough at the line-center. We compared our
results with the analytic solutions derived by previous researchers, and
confirmed that both solutions are in good agreement with each other. We
investigated the directionality of the emergent Lya photons and found that limb
brightening is observed in slightly thick media while limb darkening appears in
extremely thick media. The behavior of the directionality is noted to follow
that of the Thomson scattered radiation in electron clouds, because both Lya
wing scattering and Thomson scattering share the same Rayleigh scattering phase
function. The mean number of wing scatterings just before escape is in exact
agreement with the prediction of the diffusion approximation. The Lya photons
constituting the inner part of the emergent profiles follow the relationship
derived from the diffusion approximation. We present a brief discussion on the
application of our results to the formation of Lya broad absorption troughs and
P-Cygni type Lya profiles seen in the UV spectra of starburst galaxies.Comment: 24 papges, 12 figures, The revised version submitted to Ap
N-Body Simulations of Compact Young Clusters near the Galactic Center
We investigate the dynamical evolution of compact young star clusters (CYCs)
near the Galactic center (GC) using Aarseth's Nbody6 codes. The relatively
small number of stars in the cluster (5,000-20,000) makes real-number N-body
simulations for these clusters feasible on current workstations. Using
Fokker-Planck (F-P) models, Kim, Morris, & Lee (1999) have made a survey of
cluster lifetimes for various initial conditions, and have found that clusters
with a mass <~ 2x10^4 Msun evaporate in ~10 Myr. These results were, however,
to be confirmed by N-body simulations because some extreme cluster conditions,
such as strong tidal forces and a large stellar mass range participating in the
dynamical evolution, might violate assumptions made in F-P models. Here we find
that, in most cases, the CYC lifetimes of previous F-P calculations are 5-30%
shorter than those from the present N-body simulations. The comparison of
projected number density profiles and stellar mass functions between N-body
simulations and HST/NICMOS observations by Figer et al. (1999) suggests that
the current tidal radius of the Arches cluster is ~1.0 pc, and the following
parameters for the initial conditions of that cluster: total mass of 2x10^4
Msun and mass function slope for intermediate-to-massive stars of 1.75 (the
Salpeter function has 2.35). We also find that the lower stellar mass limit,
the presence of primordial binaries, the amount of initial mass segregation,
and the choice of initial density profile (King or Plummer models) do not
significantly affect the dynamical evolution of CYCs.Comment: 20 pages including 6 figures, To appear in ApJ, Dec 20 issu
Self Heating of Corona by Electrostatic Fields Driven by Sheared Flows
A mechanism of self-heating of solar corona is pointed out. It is shown that
the free energy available in the form of sheared flows gives rise to unstable
electrostatic waves which accelerate the particles and heat them. The
electrostatic perturbations take place through two processes (a) by purely
growing sheared flow-driven instability and (b) by sheared flow-driven drift
waves. These processes occur throughout the corona and hence the self-heating
is very important in this plasma. These instabilities can give rise to local
electrostatic potentials of the order of about 100 volts or less
within to a few seconds time if the initial perturbation is
assumed to be about one percent that is .
The components of wave lengths in the direction perpendicular to external
magnetic field vary from about 10m to 1m. The purely growing
instability creates electrostatic fields by sheared flows even if the density
gradient does not exist whereas the density gradient is crucial for the
concurrence of drift wave instability. Subject headings: Sun: self-heating of
corona, sheared flow-driven instability, drift waves
Constant cross section of loops in the solar corona
The corona of the Sun is dominated by emission from loop-like structures.
When observed in X-ray or extreme ultraviolet emission, these million K hot
coronal loops show a more or less constant cross section. In this study we show
how the interplay of heating, radiative cooling, and heat conduction in an
expanding magnetic structure can explain the observed constant cross section.
We employ a three-dimensional magnetohydrodynamics (3D MHD) model of the
corona. The heating of the coronal plasma is the result of braiding of the
magnetic field lines through footpoint motions and subsequent dissipation of
the induced currents. From the model we synthesize the coronal emission, which
is directly comparable to observations from, e.g., the Atmospheric Imaging
Assembly on the Solar Dynamics Observatory (AIA/SDO). We find that the
synthesized observation of a coronal loop seen in the 3D data cube does match
actually observed loops in count rate and that the cross section is roughly
constant, as observed. The magnetic field in the loop is expanding and the
plasma density is concentrated in this expanding loop; however, the temperature
is not constant perpendicular to the plasma loop. The higher temperature in the
upper outer parts of the loop is so high that this part of the loop is outside
the contribution function of the respective emission line(s). In effect, the
upper part of the plasma loop is not bright and thus the loop actually seen in
coronal emission appears to have a constant width. From this we can conclude
that the underlying field-line-braiding heating mechanism provides the proper
spatial and temporal distribution of the energy input into the corona --- at
least on the observable scales.Comment: 8 pages, 9 figures, accepted for publication in A&
Patient-Reported Symptoms and Impact of Treatment With Osimertinib Versus Chemotherapy in Advanced Non-Small-Cell Lung Cancer: The AURA3 Trial
Non-ancient solution of the Ricci flow
For any complete noncompact Khler manifold with nonnegative and
bounded holomorphic bisectional curvature,we provide the necessary and
sufficient condition for non-ancient solution to the Ricci flow in this paper.Comment: seven pages, latex fil
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