28,381 research outputs found
Constraining neutrino physics with BBN and CMBR
We perform a likelihood analysis of the recent results on the anisotropy of
Cosmic Microwave Background Radiation from the BOOMERanG and DASI experiments
to show that they single out an effective number of neutrinos in good agreement
with standard Big Bang Nucleosynthesis. We also consider degenerate Big Bang
Nucleosynthesis to provide new bounds on effective relativistic degrees of
freedom and, in particular, on neutrino chemical potential
. When including Supernova Ia data we find, at , and , .Comment: 6 pages, 3 figures, some reference adde
Connectivity and a Problem of Formal Geometry
Let be a product of weighted
projective spaces, and let be the diagonal of . We prove
an algebraization result for formal-rational functions on certain closed
subvarieties of along the intersection .Comment: 9 pages, to appear in the Proceedings volume "Experimental and
Theoretical Methods in Algebra, Geometry and Topology", series Springer
Proceedings in Mathematics & Statistic
Suppression of hole-hole scattering in GaAs/AlGaAs heterostructures under uniaxial compression
Resistance, magnetoresistance and their temperature dependencies have been
investigated in the 2D hole gas at a [001] p-GaAs/AlGaAs
heterointerface under [110] uniaxial compression. Analysis performed in the
frame of hole-hole scattering between carriers in the two spin splitted
subbands of the ground heavy hole state indicates, that h-h scattering is
strongly suppressed by uniaxial compression. The decay time of the
relative momentum reveals 4.5 times increase at a uniaxial compression of 1.3
kbar.Comment: 5 pages, 3 figures. submitted to Phys.Rev.
A reduced coupled-mode description for the electron-ion energy relaxation in dense matter
We present a simplified model for the electron-ion energy relaxation in dense two-temperature systems that includes the effects of coupled collective modes. It also extends the standard Spitzer result to both degenerate and strongly coupled systems. Starting from the general coupled-mode description, we are able to solve analytically for the temperature relaxation time in warm dense matter and strongly coupled plasmas. This was achieved by decoupling the electron-ion dynamics and by representing the ion response in terms of the mode frequencies. The presented reduced model allows for a fast description of temperature equilibration within hydrodynamic simulations and an easy comparison for experimental investigations. For warm dense matter, both fluid and solid, the model gives a slower electron-ion equilibration than predicted by the classical Spitzer result
Message passing for vertex covers
Constructing a minimal vertex cover of a graph can be seen as a prototype for
a combinatorial optimization problem under hard constraints. In this paper, we
develop and analyze message passing techniques, namely warning and survey
propagation, which serve as efficient heuristic algorithms for solving these
computational hard problems. We show also, how previously obtained results on
the typical-case behavior of vertex covers of random graphs can be recovered
starting from the message passing equations, and how they can be extended.Comment: 25 pages, 9 figures - version accepted for publication in PR
A simple analytical model for dark matter halo structure and adiabatic contraction
A simple analytical model for describing inner parts of dark matter halo is
considered. It is assumed that dark matter density is power-law. The model
deals with dark matter distribution function in phase space of adiabatic
invariants (radial action and angular momentum). Two variants are considered
for the angular part of the distribution function: narrow and broad
distribution. The model allows to describe explicitly the process of adiabatic
contraction of halo due to change of gravitational potential caused by
condensation of baryonic matter in the centre. The modification of dark matter
density in the centre is calculated, and is it shown that the standard
algorithm of adiabatic contraction calculation overestimates the compressed
halo density, especially in the case of strong radial anisotropy.Comment: 5 pages, 3 figures. v3 - major improvements, another halo model
introduced, discussion extende
The [Y/Mg] clock works for evolved solar metallicity stars
Previously [Y/Mg] has been proven to be an age indicator for solar twins.
Here, we investigate if this relation also holds for helium-core-burning stars
of solar metallicity. High resolution and high signal-to-noise ratio (S/N)
spectroscopic data of stars in the helium-core-burning phase have been obtained
with the FIES spectrograph on the NOT 2.56m telescope and the HIRES
spectrograph on the Keck I 10 m telescope. They have been analyzed to determine
the chemical abundances of four open clusters with close to solar metallicity;
NGC 6811, NGC 6819, M67 and NGC 188. The abundances are derived from equivalent
widths of spectral lines using ATLAS9 model atmospheres with parameters
determined from the excitation and ionization balance of Fe lines. Results from
asteroseismology and binary studies were used as priors on the atmospheric
parameters, where especially the is determined to much higher
precision than what is possible with spectroscopy. It is confirmed that the
four open clusters are close to solar metallicity and they follow the [Y/Mg]
vs. age trend previously found for solar twins. The [Y/Mg] vs. age clock also
works for giant stars in the helium-core burning phase, which vastly increases
the possibilities to estimate the age of stars not only in the solar
neighborhood, but in large parts of the Galaxy, due to the brighter nature of
evolved stars compared to dwarfs.Comment: 5 pages, 3 figures, accepted for publication as a Letter to A&
Where to find a dark matter sterile neutrino?
We propose a strategy of how to look for dark matter (DM) particles
possessing a radiative decay channel and derive constraints on their parameters
from observations of X-rays from our own Galaxy and its dwarf satellites. When
applied to the sterile neutrinos in keV mass range, it allows a significant
improvement of restrictions to its parameters, as compared with previous works.Comment: 5 pp, revtex; v3: 1-sigma limits have been replaced by more
conservative 3-sigma limits, a picture illustrating the data analysis methods
has been ade
C/O white dwarfs of very low mass: 0.33-0.5 Mo
The standard lower limit for the mass of white dwarfs (WDs) with a C/O core
is roughly 0.5 Mo. In the present work we investigated the possibility to form
C/O WDs with mass as low as 0.33 Mo. Both the pre-WD and the cooling evolution
of such nonstandard models will be described.Comment: Submitted to the "Proceedings of the 16th European White Dwarf
Workshop" (to be published JPCS). 7 pages including 13 figure
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