35,132 research outputs found
Is the Redshift Clustering of Long-Duration Gamma-Ray Bursts Significant?
The 26 long-duration gamma-ray bursts (GRBs) with known redshifts form a
distinct cosmological set, selected differently than other cosmological probes
such as quasars and galaxies. Since the progenitors are now believed to be
connected with active star-formation and since burst emission penetrates dust,
one hope is that with a uniformly-selected sample, the large-scale redshift
distribution of GRBs can help constrain the star-formation history of the
Universe. However, we show that strong observational biases in ground-based
redshift discovery hamper a clean determination of the large-scale GRB rate and
hence the connection of GRBs to the star formation history. We then focus on
the properties of the small-scale (clustering) distribution of GRB redshifts.
When corrected for heliocentric motion relative to the local Hubble flow, the
observed redshifts appear to show a propensity for clustering: 8 of 26 GRBs
occurred within a recession velocity difference of 1000 km/s of another GRB.
That is, 4 pairs of GRBs occurred within 30 h_65^-1 Myr in cosmic time, despite
being causally separated on the sky. We investigate the significance of this
clustering. Comparison of the numbers of close redshift pairs expected from the
simulation with that observed shows no significant small-scale clustering
excess in the present sample; however, the four close pairs occur only in about
twenty percent of the simulated datasets (the precise significance of the
clustering is dependent upon the modeled biases). We conclude with some
impetuses and suggestions for future precise GRB redshift measurements.Comment: Published in the Astronomical Journal, June 2003: see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003AJ....125.2865
Coreshine in L1506C - Evidence for a primitive big-grain component or indication for a turbulent core history?
The recently discovered coreshine effect can aid in exploring the core
properties and in probing the large grain population of the ISM. We discuss the
implications of the coreshine detected from the molecular cloud core L1506C in
the Taurus filament for the history of the core and the existence of a
primitive ISM component of large grains becoming visible in cores. The
coreshine surface brightness of L1506C is determined from IRAC Spitzer images
at 3.6 micron. We perform grain growth calculations to estimate the grain size
distribution in model cores similar in gas density, radius, and turbulent
velocity to L1506C. Scattered light intensities at 3.6 micron are calculated
for a variety of MRN and grain growth distributions to compare with the
observed coreshine. For a core with the overall physical properties of L1506C,
no detectable coreshine is predicted for an MRN size distribution. Extending
the distribution to grain radii of about 0.65 m allows to reproduce the
observed surface brightness level in scattered light. Assuming the properties
of L1506C to be preserved, models for the growth of grains in cores do not
yield sufficient scattered light to account for the coreshine within the
lifetime of the Taurus complex. Only increasing the core density and the
turbulence amplifies the scattered light intensity to a level consistent with
the observed coreshine brightness. The grains could be part of primitive
omni-present large grain population becoming visible in the densest part of the
ISM, could grow under the turbulent dense conditions of former cores, or in
L1506C itself. In the later case, L1506C must have passed through a period of
larger density and stronger turbulence. This would be consistent with the
surprisingly strong depletion usually attributed to high column densities, and
with the large-scale outward motion of the core envelope observed today.Comment: 6 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Robustness of the nodal d-wave spectrum to strongly fluctuating competing order
We resolve an existing controversy between, on the one hand, convincing
evidence for the existence of competing order in underdoped cuprates, and, on
the other hand, spectroscopic data consistent with a seemingly homogeneous
d-wave superconductor in the very same compounds. Specifically, we show how
short-range fluctuations of the competing order essentially restore the nodal
d-wave spectrum from the qualitatively distinct folded dispersion resulting
from homogeneous coexisting phases. The signatures of the fluctuating competing
order can be found mainly in a splitting of the antinodal quasi-particles and,
depending of the strength of the competing order, also in small induced nodal
gaps as found in recent experiments on underdoped La{2-x}SrxCuO4.Comment: 5 pages, 4 figure
Mixed Models and Reduction Techniques for Large-Rotation, Nonlinear Analysis of Shells of Revolution with Application to Tires
An effective computational strategy is presented for the large-rotation, nonlinear axisymmetric analysis of shells of revolution. The three key elements of the computational strategy are: (1) use of mixed finite-element models with discontinuous stress resultants at the element interfaces; (2) substantial reduction in the total number of degrees of freedom through the use of a multiple-parameter reduction technique; and (3) reduction in the size of the analysis model through the decomposition of asymmetric loads into symmetric and antisymmetric components coupled with the use of the multiple-parameter reduction technique. The potential of the proposed computational strategy is discussed. Numerical results are presented to demonstrate the high accuracy of the mixed models developed and to show the potential of using the proposed computational strategy for the analysis of tires
Model of Electronic Structure and Superconductivity in Orbitally Ordered FeSe
We provide a band structure with low-energy properties consistent with recent
photoemission and quantum oscillations measurements on FeSe, assuming
mean-field like s and/or d-wave orbital ordering at the structural transition.
We show how the resulting model provides a consistent explanation of the
temperature dependence of the measured Knight shift and the spin-relaxation
rate. Furthermore, the superconducting gap structure obtained from spin
fluctuation theory exhibits nodes on the electron pockets, consistent with the
'V'-shaped density of states obtained by tunneling spectroscopy on this
material, and the temperature dependence of the London penetration depth. Our
studies prove that the recent experimental observations of the electronic
properties of FeSe are consistent with orbital order, but leave open the
microscopic origin of the unusual band structure of this material.Comment: 12 pages, 15 figures, T.B hopping error corrected, d-wave orbital
order added, real space hoppings included in tex fil
Pinning of stripes by local structural distortions in cuprate high-Tc superconductors
We study the spin-density wave (stripe) instability in lattices with mixed
low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) crystal
symmetry. Within an explicit mean-field model it is shown how local LTT regions
act as pinning centers for static stripe formation. We calculate the
modulations in the local density of states near these local stripe regions and
find that mainly the coherence peaks and the van Hove singularity (VHS) are
spatially modulated. Lastly, we use the real-space approach to simulate recent
tunneling data in the overdoped regime where the VHS has been detected by
utilizing local normal state regions.Comment: Conference proceedings for Stripes1
Superconducting phase diagram of itinerant antiferromagnets
We study the phase diagram of the Hubbard model in the weak-coupling limit
for coexisting spin-density-wave order and spin-fluctuation-mediated
superconductivity. Both longitudinal and transverse spin fluctuations
contribute significantly to the effective interaction potential, which creates
Cooper pairs of the quasi-particles of the antiferromagnetic metallic state. We
find a dominant -wave solution in both electron- and hole-doped
cases. In the quasi-spin triplet channel, the longitudinal fluctuations give
rise to an effective attraction supporting a -wave gap, but are overcome by
repulsive contributions from the transverse fluctuations which disfavor
-wave pairing compared to . The sub-leading pair instability is
found to be in the -wave channel, but complex admixtures of and are
not energetically favored since their nodal structures coincide. Inclusion of
interband pairing, in which each fermion in the Cooper pair belongs to a
different spin-density-wave band, is considered for a range of electron dopings
in the regime of well-developed magnetic order. We demonstrate that these
interband pairing gaps, which are non-zero in the magnetic state, must have the
same parity under inversion as the normal intraband gaps. The self-consistent
solution to the full system of five coupled gap equations give intraband and
interband pairing gaps of structure and similar gap magnitude. In
conclusion, the gap dominates for both hole and electron doping
inside the spin-density-wave phase.Comment: 14 pages, 9 figure
The mass content of the Sculptor dwarf spheroidal galaxy
We present a new determination of the mass content of the Sculptor dwarf
spheroidal galaxy, based on a novel approach which takes into account the two
distinct stellar populations present in this galaxy. This method helps to
partially break the well-known mass-anisotropy degeneracy present in the
modelling of pressure-supported stellar systems.Comment: 6 pages, 3 figures. To appear in the proceedings of IAU Symposium 254
"The Galaxy disk in a cosmological context", Copenhagen, June 200
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