569 research outputs found
Regional price targets appropriate for advanced coal extraction
A methodology is presented for predicting coal prices in regional markets for the target time frames 1985 and 2000 that could subsequently be used to guide the development of an advanced coal extraction system. The model constructed is a supply and demand model that focuses on underground mining since the advanced technology is expected to be developed for these reserves by the target years. Coal reserve data and the cost of operating a mine are used to obtain the minimum acceptable selling price that would induce the producer to bring the mine into production. Based on this information, market supply curves can be generated. Demand by region is calculated based on an EEA methodology that emphasizes demand by electric utilities and demand by industry. The demand and supply curves are then used to obtain the price targets. The results show a growth in the size of the markets for compliance and low sulphur coal regions. A significant rise in the real price of coal is not expected even by the year 2000. The model predicts heavy reliance on mines with thick seams, larger block size and deep overburden
Charged-Particle and Neutron-Capture Processes in the High-Entropy Wind of Core-Collapse Supernovae
The astrophysical site of the r-process is still uncertain, and a full
exploration of the systematics of this process in terms of its dependence on
nuclear properties from stability to the neutron drip-line within realistic
stellar environments has still to be undertaken. Sufficiently high neutron to
seed ratios can only be obtained either in very neutron-rich low-entropy
environments or moderately neutron-rich high-entropy environments, related to
neutron star mergers (or jets of neutron star matter) and the high-entropy wind
of core-collapse supernova explosions. As chemical evolution models seem to
disfavor neutron star mergers, we focus here on high-entropy environments
characterized by entropy , electron abundance and expansion velocity
. We investigate the termination point of charged-particle reactions,
and we define a maximum entropy for a given and ,
beyond which the seed production of heavy elements fails due to the very small
matter density. We then investigate whether an r-process subsequent to the
charged-particle freeze-out can in principle be understood on the basis of the
classical approach, which assumes a chemical equilibrium between neutron
captures and photodisintegrations, possibly followed by a -flow
equilibrium. In particular, we illustrate how long such a chemical equilibrium
approximation holds, how the freeze-out from such conditions affects the
abundance pattern, and which role the late capture of neutrons originating from
-delayed neutron emission can play.Comment: 52 pages, 31 figure
SU(4) Skyrmions and Activation Energy Anomaly in Bilayer Quantum Hall Systems
The bilayer QH system has four energy levels in the lowest Landau level,
corresponding to the layer and spin degrees of freedom. We investigate the
system in the regime where all four levels are nearly degenerate and equally
active. The underlying group structure is SU(4). At the QH state is a
charge-transferable state between the two layers and the SU(4) isospin
coherence develops spontaneously. Quasiparticles are isospin textures to be
identified with SU(4) skyrmions. The skyrmion energy consists of the Coulomb
energy, the Zeeman energy and the pseudo-Zeeman energy. The Coulomb energy
consists of the self-energy, the capacitance energy and the exchange energy. At
the balanced point only pseudospins are excited unless the tunneling gap is too
large. Then, the SU(4) skyrmion evolves continuously from the
pseudospin-skyrmion limit into the spin-skyrmion limit as the system is
transformed from the balanced point to the monolayer point by controlling the
bias voltage. Our theoretical result explains quite well the experimental data
due to Murphy et al. and Sawada et al. on the activation energy anomaly induced
by applying parallel magnetic field.Comment: 22 pagets, 6 figures, the final version to be published in PR
Finite temperature effects on cosmological baryon diffusion and inhomogeneous Big-Bang nucleosynthesis
We have studied finite temperature corrections to the baryon transport cross
sections and diffusion coefficients. These corrections are based upon the
recently computed renormalized electron mass and the modified state density due
to the background thermal bath in the early universe. It is found that the
optimum nucleosynthesis yields computed using our diffusion coefficients shift
to longer distance scales by a factor of about 3. We also find that the minimum
value of abundance decreases by while and
increase. Effects of these results on constraints from primordial
nucleosynthesis are discussed. In particular, we find that a large baryonic
contribution to the closure density (\Omega_b h_{50}^{2} \lsim 0.4) may be
allowed in inhomogeneous models corrected for finite temperature.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Rayleigh-Taylor Instabilities in Young Supernova Remnants Undergoing Efficient Particle Acceleration
We employ hydrodynamic simulations to study the effects of high shock
compression ratios, as expected for fast shocks with efficient particle
acceleration, on the convective instability of driven waves in supernova
remnants. We find that the instability itself does not depend significantly on
the compression ratio, but because the width of the interaction region between
the forward and reverse shocks can shrink significantly with increasing shock
compression, we find that convective instabilities can reach all the way to the
forward shock front if compression ratios are high enough.Comment: Submitted to The Astrophysical Journa
Effects of Beta-Decays of Excited-State Nuclei on the Astrophysical r-Process
A rudimentary calculation is employed to evaluate the possible effects of
beta- decays of excited-state nuclei on the astrophysical r-process.
Single-particle levels calculated with the FRDM are adapted to the calculation
of beta-decay rates of these excited-state nuclei. Quantum numbers are
determined based on proximity to Nilson model levels. The resulting rates are
used in an r-process network calculation in which a supernova hot-bubble model
is coupled to an extensive network calculation including all nuclei between the
valley of stability and the neutron drip line and with masses A<284. Beta-decay
rates are included as functional forms of the environmental temperature. While
the decay rate model used is simple and phenomenological, it is consistent
across all 3700 nuclei involved in the r-process network calculation. This
represents an approximate first estimate to gauge the possible effects of
excited-state beta-decays on r-process freeze-out abundances
The influence of collective neutrino oscillations on a supernova r-process
Recently, it has been demonstrated that neutrinos in a supernova oscillate
collectively. This process occurs much deeper than the conventional
matter-induced MSW effect and hence may have an impact on nucleosynthesis. In
this paper we explore the effects of collective neutrino oscillations on the
r-process, using representative late-time neutrino spectra and outflow models.
We find that accurate modeling of the collective oscillations is essential for
this analysis. As an illustration, the often-used "single-angle" approximation
makes grossly inaccurate predictions for the yields in our setup. With the
proper multiangle treatment, the effect of the oscillations is found to be less
dramatic, but still significant. Since the oscillation patterns are sensitive
to the details of the emitted fluxes and the sign of the neutrino mass
hierarchy, so are the r-process yields. The magnitude of the effect also
depends sensitively on the astrophysical conditions - in particular on the
interplay between the time when nuclei begin to exist in significant numbers
and the time when the collective oscillation begins. A more definitive
understanding of the astrophysical conditions, and accurate modeling of the
collective oscillations for those conditions, is necessary.Comment: 27 pages, 10 figure
Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line
By performing MHD simulations, we investigate the mass loss of intermediate-
and low-mass stars from main sequence (MS) to red giant branch (RGB) phases.
Alfven waves, which are excited by the surface convections travel outwardly and
dissipate by nonlinear processes to accelerate and heat the stellar winds. We
dynamically treat these processes in open magnetic field regions from the
photospheres to 25 stellar radii. When the stars evolve to slightly blueward
positions of the dividing line (Linsky & Haisch), the steady hot corona with
temperature, ~ 1MK, suddenly disappears. Instead, many hot (~1MK) and warm
(~10^5K) bubbles are formed in cool (T<~2x10^4K) chromospheric winds because of
thermal instability; the red giant wind is not a steady stream but structured
outflow. As a result, the mass loss rates, \dot{M}, largely vary in time by 3-4
orders or magnitude in the RGB stars. Supported by magnetic pressure, the
density of hot bubbles can be kept low to reduce the radiative cooling and to
maintain the high temperature long time. Even in the stars redward of the
dividing line, hot bubbles intermittently exist, and they can be sources of
UV/soft X-ray emissions from hybrid stars. Nearly static regions are formed
above the photospheres of the RGB stars, and the stellar winds are effectively
accelerated from several stellar radii. Then, the wind velocity is much smaller
than the surface escape speed, because it is regulated by the slower escape
speed at that location. We finally derive an equation that determines \dot{M}
from the energetics of the simulated wave-driven winds in a forward manner. The
relation explains \dot{M} from MS to RGB, and it can play a complementary role
to the Reimers' formula, which is mainly for more luminous stars.Comment: 19 pages, 15 figures embedded (emulate ApJ style), submitted to ApJ,
mpeg movie is available at
http://www.esa.c.u-tokyo.ac.jp/~stakeru/research/st633.mp
Fractal Reconnection in Solar and Stellar Environments
Recent space based observations of the Sun revealed that magnetic
reconnection is ubiquitous in the solar atmosphere, ranging from small scale
reconnection (observed as nanoflares) to large scale one (observed as long
duration flares or giant arcades). Often the magnetic reconnection events are
associated with mass ejections or jets, which seem to be closely related to
multiple plasmoid ejections from fractal current sheet. The bursty radio and
hard X-ray emissions from flares also suggest the fractal reconnection and
associated particle acceleration. We shall discuss recent observations and
theories related to the plasmoid-induced-reconnection and the fractal
reconnection in solar flares, and their implication to reconnection physics and
particle acceleration. Recent findings of many superflares on solar type stars
that has extended the applicability of the fractal reconnection model of solar
flares to much a wider parameter space suitable for stellar flares are also
discussed.Comment: Invited chapter to appear in "Magnetic Reconnection: Concepts and
Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016),
33 pages, 18 figure
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