5,805 research outputs found
Evolution and Explosion of Very Massive Primordial Stars
While the modern stellar IMF shows a rapid decline with increasing mass,
theoretical investigations suggest that very massive stars (>100 solar masses)
may have been abundant in the early universe. Other calculations also indicate
that, lacking metals, these same stars reach their late evolutionary stages
without appreciable mass loss. After central helium burning, they encounter the
electron-positron pair instability, collapse, and burn oxygen and silicon
explosively. If sufficient energy is released by the burning, these stars
explode as brilliant supernovae with energies up to 100 times that of an
ordinary core collapse supernova. They also eject up to 50 solar masses of
radioactive Ni56. Stars less massive than 140 solar masses or more massive than
260 solar masses should collapse into black holes instead of exploding, thus
bounding the pair-creation supernovae with regions of stellar mass that are
nucleosynthetically sterile. Pair-instability supernovae might be detectable in
the near infrared out to redshifts of 20 or more and their ashes should leave a
distinctive nucleosynthetic pattern.Comment: 7 pages, including 4 figures; in. proc. MPA/ESO/MPE/USM Joint
Astronomy Conference "Lighthouses of the Universe: The Most Luminous
Celestial Objects and their use for Cosmology
X-ray Scattering Study of the spin-Peierls transition and soft phonon behavior in TiOCl
We have studied the S=1/2 quasi-one-dimensional antiferromagnet TiOCl using
single crystal x-ray diffraction and inelastic x-ray scattering techniques. The
Ti ions form staggered spin chains which dimerize below Tc1 = 66 K and have an
incommensurate lattice distortion between Tc1 and Tc2 = 92 K. Based on our
measurements of the intensities, wave vectors, and harmonics of the
incommensurate superlattice peaks, we construct a model for the incommensurate
modulation. The results are in good agreement with a soliton lattice model,
though some quantitative discrepancies exist near Tc2. The behavior of the
phonons has been studied using inelastic x-ray scattering with ~2 meV energy
resolution. For the first time, a zone boundary phonon which softens at the
spin-Peierls temperature Tsp has been observed. Our results show reasonably
good quantitative agreement with the Cross-Fisher theory for the phonon
dynamics at wave vectors near the zone boundary and temperatures near Tsp.
However, not all aspects of the data can be described, such as the strong
overdamping of the soft mode above Tsp. Overall, our results show that TiOCl is
a good realization of a spin-Peierls system, where the phonon softening allows
us to identify the transition temperature as Tsp=Tc2=92 KComment: 14 pages, 14 figure
Fragmentation and the formation of primordial protostars: the possible role of Collision Induced Emission
The mechanisms which could lead to chemo-thermal instabilities and
fragmentation during the formation of primordial protostars are investigated
analytically. We introduce approximations for H2 cooling rates bridging the
optically thin and thick regimes. These allow us to discuss instabilities up to
densities when protostars become optically thick to continuum radiation
(n~10^16 cm^-3). During the collapse, instability arises at two different
stages: at low density (n~10^8-10^11 cm^-3), it is due to fast 3-body reactions
converting H into H2; at high density (n>10^13 cm^-3), it is due to Collisional
Induced Emission (CIE). In agreement with the 3D simulations, we find that the
instability at low densities cannot lead to fragmentation, because fluctuations
do not survive turbulent mixing, and because their growth is slow. The
situation at high density is similar. The CIE-induced instability is as weak as
the low density one, with similar ratios of growth and dynamical time scales.
Fluctuation growth time is longer than free fall time, and fragmentation seems
unlikely. One then expects the first stars to be massive, not to form binaries
nor harbour planets. Nevertheless, full 3D simulations are required. They could
become possible using simplified estimates of radiative transfer effects, which
we show to work very well in the 1D case. This indicates that the effects of
radiative transfer during the initial stages of formation of primordial
protostars can be treated as local corrections to cooling. (Abridged)Comment: 17 pages, 9 figures; accepted for publication in MNRA
Liver Resection for Primary Hepatic Neoplasms.
Subtotal hepatic resection was performed in 356 patients; 87 had primary hepatic malignancies, 108 had metastatic tumors, and 161 had benign lesions including 8 traumatic injuries. The global mortality was 4.2%. The experience has elucidated the role of subtotal hepatic resection both for benign and malignant neoplasms
Cosmic Reionisation by Stellar Sources: Population II Stars
We study the reionisation of the Universe by stellar sources using a
numerical approach that combines fast 3D radiative transfer calculations with
high resolution hydrodynamical simulations. Ionising fluxes for the sources are
derived from intrinsic star formation rates computed in the underlying
hydrodynamical simulations. Our mass resolution limit for sources is M~ 4.0 x
10^7 h^-1 M_sol, which is roughly an order of magnitude smaller than in
previous studies of this kind. Our calculations reveal that the reionisation
process is sensitive to the inclusion of dim sources with masses below ~10^9
h^-1 M_sol. We present the results of our reionisation simulation assuming a
range of escape fractions for ionising photons and make statistical comparisons
with observational constraints on the neutral fraction of hydrogen at z~6
derived from the z=6.28 SDSS quasar of Becker and coworkers. Our best fitting
model has an escape fraction of ~20% and causes reionisation to occur by z~8,
although the IGM remains fairly opaque until z~6. In order to simultaneously
match the observations from the z=6.28 SDSS quasar and the optical depth
measurement from WMAP with the sources modeled here, we require an evolving
escape fraction that rises from f_esc=0.20 near z~6 to f_esc>~10 at z~18.Comment: 42 pages, 13 figure
Design and simulation of an effective backup power supply for academic institutions in Nigeria: A case study of NDA postgraduate school
This research work is aimed to mitigate the adverse effect of numerous portable generators used in academic environments due to the unstable power supply experienced in Nigeria. Data for the study on the existing backup, availability hours from the national grid, and load demand for the area of study were obtained from the residents of the campus, facility managers, and Kaduna Distribution Company as the grid supplier from August 2017 to December 2020. The average load of the campus was obtained to be 80kW. These were used as a baseline to obtain the required size and quantity of material to generate the backup power needed. A total ampere-hour requirement of the battery to be used was obtained to be 4,278.07Ah considering the average battery depth of discharge of 80%. This resulted in a total number of cells required to be 134 considering a battery with a 200Ah rating and a nominal voltage rating of 48V. A solar photovoltaic (PV) system rating of 166.4kW is required to sufficiently charge the battery bank and also serve the load. This amounts to a minimum of 5 panels per string connected in series and 34 number panels per string connected in parallel based on which the total number of panels required summed up to 666. The inverter rating for the load was obtained to be 150 kVA with a total load of 100 kVA, an efficiency of 80%, and an average future expansion of 20 %. A diesel generator rating of 100kVA with a starting kVA rating of 113.64kVA is required to efficiently serve the load considering future expansion of 1.1 and operating efficiency of 80 %. These obtained parameters were simulated using MATLAB/Simulink to test the feasibility of the backup systems. The generation cost of each backup was calculated based on which solar PV with battery bank has an initial energy generation cost of 81.9 ₦/kWh and a future energy generation cost of 0.27 ₦/kWh while diesel generator has an initial energy generation cost of 1602.04 ₦/kWh and a future energy generation cost of 8.07 ₦/kWh as such, PV has the least energy cost and more economical for the academic environment
Impact of faults on bus stability on an island 330kV mesh network on the Nigerian grid
This study carried out an assessment on the impact of faults on bus stability along the Benin-IkejaWest-Aiyede-Oshogbo-Benin (BIAOB) 330kV island network. The sensitivity of BIAOB as a ring network on the Nigerian grid aroused the interest behind its choice for this study. The network parameters were collated from the National Control Centre, Oshogbo and the network was modeled on the MATLAB 2015 environment using the obtained data. A high reactive power flow was observed in all the buses while the lowest voltage profile was observed on the Line-Line-Line-Ground (L-L-L-G) simulated in bus 1. This is an indication that symmetrical faults have the greatest impact on the network. Further results showed that the BIAOB network has a better voltage profile when compared with other radial network from existing literature. The paper concluded by recommending the closure of more radial network on the grid in order to improve its performance
On the EDM Cancellations in D-brane models
We analyze the possibility of simultaneous electron, neutron, and mercury
electric dipole moment (EDM) cancellations in the mSUGRA and D--brane models.
We find that the mercury EDM constraint practically rules out the cancellation
scenario in D-brane models whereas in the context of mSUGRA it is still allowed
with some fine-tuning.Comment: 10 pages, to appear in Phys. Rev. Let
Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl
We report on the magnetic, thermodynamic and optical properties of the
quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been
discussed as spin-Peierls compounds. The observed deviations from canonical
spin-Peierls behavior, e.g. the existence of two distinct phase transitions,
have been attributed previously to strong orbital fluctuations. This can be
ruled out by our optical data of the orbital excitations. We show that the
frustration of the interchain interactions in the bilayer structure gives rise
to incommensurate order with a subsequent lock-in transition to a commensurate
dimerized state. In this way, a single driving force, the spin-Peierls
mechanism, induces two separate transitions.Comment: 4 pages, 4 figure
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