13,925 research outputs found
Determining solar abundances using helioseismology
The recent downward revision of solar photospheric abundances of Oxygen and
other heavy elements has resulted in serious discrepancies between solar models
and solar structure as determined through helioseismology. In this work we
investigate the possibility of determining the solar heavy-element abundance
without reference to spectroscopy by using helioseismic data. Using the
dimensionless sound-speed derivative in the solar convection zone, we find that
the heavy element abundance, Z, of 0.0172 +/- 0.002, which is closer to the
older, higher value of the abundances.Comment: To appear in Ap
Modified Bethe-Weizsacker mass formula with isotonic shift and new driplines
Nuclear masses are calculated using the modified Bethe-Weizsacker mass
formula in which the isotonic shifts have been incorporated. The results are
compared with the improved liquid drop model with isotonic shift. Mass excesses
predicted by this method compares well with the microscopic-macroscopic model
while being much more simple. The neutron and proton drip lines have been
predicted using this modified Bethe-Weizsacker mass formula with isotonic
shifts.Comment: 9 pages including 2 figure
Evolution of Topological Defects During Inflation
Topological defects can be formed during inflation by phase transitions as
well as by quantum nucleation. We study the effect of the expansion of the
Universe on the internal structure of the defects. We look for stationary
solutions to the field equations, i.e. solutions that depend only on the proper
distance from the defect core. In the case of very thin defects, whose core
dimensions are much smaller than the de Sitter horizon, we find that the
solutions are well approximated by the flat space solutions. However, as the
flat space thickness parameter increases we notice a deviation from
this, an effect that becomes dramatic as approaches . Beyond this critical value we find no stationary solutions to the field
equations. We conclude that only defects that have flat space thicknesses less
than the critical value survive, while thicker defects are smeared out by the
expansion.Comment: 14 page
Measurement of retinal vessel widths from fundus images based on 2-D modeling
Changes in retinal vessel diameter are an important sign of diseases such as hypertension, arteriosclerosis and diabetes mellitus. Obtaining precise measurements of vascular widths is a critical and demanding process in automated retinal image analysis as the typical vessel is only a few pixels wide. This paper presents an algorithm to measure the vessel diameter to subpixel accuracy. The diameter measurement is based on a two-dimensional difference of Gaussian model, which is optimized to fit a two-dimensional intensity vessel segment. The performance of the method is evaluated against Brinchmann-Hansen's half height, Gregson's rectangular profile and Zhou's Gaussian model. Results from 100 sample profiles show that the presented algorithm is over 30% more precise than the compared techniques and is accurate to a third of a pixel
Folding model analysis of proton radioactivity of spherical proton emitters
Half lives of the decays of spherical nuclei away from proton drip line by
proton emissions are estimated theoretically. The quantum mechanical tunneling
probability is calculated within the WKB approximation. Microscopic
proton-nucleus interaction potentials are obtained by single folding the
densities of the daughter nuclei with M3Y effective interaction supplemented by
a zero-range pseudo-potential for exchange along with the density dependence.
Strengths of the M3Y interaction are extracted by fitting its matrix elements
in an oscillator basis to those elements of the G-matrix obtained with the
Reid-Elliott soft-core nucleon-nucleon interaction. Parameters of the density
dependence are obtained from the nuclear matter calculations. Spherical charge
distributions are used for calculating the Coulomb interaction potentials.
These calculations provide reasonable estimates for the observed proton
radioactivity lifetimes of proton rich nuclei for proton emissions from 26
ground and isomeric states of spherical proton emitters.Comment: 6 page
Multi-frequency observations and spectral analysis of two gigahertz-peaked spectra pulsars
We report the multi-frequency observations of two pulsars: J1740+1000 and
B1800-21, using the Giant Metrewave Radio Telescope and the Green Bank
Telescope. The main aim of these observations was to estimate the flux density
spectrum of these pulsars, as both of them were previously reported to exhibit
gigahertz-peaked spectra. J1740+1000 is a young pulsar far from the Galactic
plane and the interpretation of its spectrum was inconclusive in the light of
the recent flux density measurements. Our result supports the gigahertz-peaked
interpretation of the PSR J1740+1000 spectrum. B1800-21 is a Vela-like pulsar
near the W30 complex, whose spectrum exhibit a significant change between 2012
and 2014 year. Our analysis shows that the current shape of the spectrum is
similar to that observed before 2009 and confirms that the observed spectral
change happen in a time-scale of a few years.Comment: 9 pages, 7 figure
Pairing in High Temperature Superconductors and Berry Phase
The topological approach to the understanding of pairing mechanism in high
superconductors analyses the relevance of the Berry phase factor in this
context. This also gives the evidence for the pairing mechanism to be of
magnetic origin.Comment: 6 page
Novel universality classes of coupled driven diffusive systems
Motivated by the phenomenologies of dynamic roughening of strings in random
media and magnetohydrodynamics, we examine the universal properties of driven
diffusive system with coupled fields. We demonstrate that cross-correlations
between the fields lead to amplitude-ratios and scaling exponents varying
continuosly with the strength of these cross-correlations. The implications of
these results for experimentally relevant systems are discussed.Comment: To appear in Phys. Rev. E (Rapid Comm.) (2003
The relation between and for solar-like oscillations
Establishing relations between global stellar parameters and asteroseismic
quantities can help improve our understanding of stellar astrophysics and
facilitate the interpretation of observations. We present an observed relation
between the large frequency separation, , and the frequency of
maximum power, . We find that is proportional to
, allowing prediction of to about 15 per cent
given . Our result is further supported by established scaling
relations for and and by extended stellar model
calculations, which confirm that can be estimated using this
relation for basically any star showing solar-like oscillations in the
investigated range (0.5<M/Msol<4.0).Comment: 5 pages, 8 figures, Letter accepted by MNRA
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