323 research outputs found
Approximation faible aux places de bonne réduction sur les surfaces cubiques sur les corps de fonctions
<p>Approximation faible aux places de bonne réduction sur les surfaces cubiques sur les corps de fonctions</p
MACHOs, White Dwarfs, and the Age of the Universe
(Abridged Abstract) A favored interpretation of recent microlensing
measurements towards the Large Magellanic Cloud implies that a large fraction
(i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We
compare model white dwarf luminosity functions to the data from the
observational surveys in order to determine a lower bound on the age of any
substantial white dwarf halo population (and hence possibly on the age of the
Universe). We compare various theoretical white dwarf luminosity functions, in
which we vary hese three parameters, with the abovementioned survey results.
From this comparison, we conclude that if white dwarfs do indeed constitute
more than 10% of the local halo mass density, then the Universe must be at
least 10 Gyr old for our most extreme allowed values of the parameters. When we
use cooling curves that account for chemical fractionation and more likely
values of the IMF and the bolometric correction, we find tighter limits: a
white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5
Gyr). Our analysis also indicates that the halo white dwarfs almost certainly
have helium-dominated atmospheres.Comment: Final version accepted for publication, straight TeX formate, 6 figs,
22 page
ConSum v0: An Experimental Cipher
We present an experimental block cipher, ConSum, based on a hitherto unstudied design element: the Conway transformation. ConSum features an extremely simple design and the ability to operate with arbitrary key lengths, block sizes and round numbers. We study it empirically and statistically so as to illustrate how it might be secure
The Space Density of Extended Ultraviolet (XUV) Disks in the Local Universe and Implications for Gas Accretion on to Galaxies
We present results of the first unbiased search for extended UV (XUV)-disk
galaxies undertaken to determine the space density of such galaxies. Our sample
contains 561 local (0.001 < z < 0.05) galaxies that lie in the intersection of
available GALEX deep imaging (exposure time > 1.5 x 10^4 s) and SDSS DR7
footprints. We explore modifications to the standard classification scheme for
our sample that includes both disk- and bulge-dominated galaxies. Visual
classification of each galaxy in the sample reveals an XUV-disk frequency of up
to 20% for the most nearby portion of our sample. On average over the entire
sample (out to z=0.05) the frequency ranges from a hard limit of 4% to 14%. The
GALEX imaging allows us to detect XUV-disks beyond 100 Mpc. The XUV regions
around XUV-disk galaxies are consistently bluer than the main bodies. We find a
surprisingly high frequency of XUV emission around luminous red (NUV-r > 5) and
green valley (3 < NUV-r < 5) galaxies. The XUV-disk space density in the local
universe is > 1.5-4.2 x 10^-3 Mpc^-3. Using the XUV emission as an indicator of
recent gas accretion, we estimate that the cold gas accretion rate onto these
galaxies is > 1.7-4.6 x 10^-3 Msun Mpc^-3 yr^-1. The number of XUV-disks in the
green valley and the estimated accretion rate onto such galaxies points to the
intriguing possibility that 7%-18% of galaxies in this population are
transitioning away from the red sequence.Comment: 19 pages, 24 figures, ApJ in Pres
Calculabilit\'e de la cohomologie \'etale modulo l
Let be an algebraic scheme over an algebraically closed field and
a prime number invertible on . According to classical results (due
essentially to A. Grothendieck, M. Artin and P. Deligne), the \'etale
cohomology groups are
finite-dimensional. Using an -adic variant of M. Artin's good
neighborhoods and elementary results on the cohomology of pro- groups, we
express the cohomology of as a well controlled colimit of that of toposes
constructed on where the are computable finite -groups. From
this, we deduce that the Betti numbers modulo of are algorithmically
computable (in the sense of Church-Turing). The proof of this fact, along with
certain related results, occupies the first part of this paper. This relies on
the tools collected in the second part, which deals with computational
algebraic geometry. Finally, in the third part, we present a "universal"
formalism for computation on the elements of a field.Comment: In French. v2 has been considerably reworked and expanded. v3
incorporates slight corrections and simplifications and a few additions
(notably: computability of the morphism from hyper\v{c}ech cohomology, graded
algebra structure, and a worked out example); submitted for publicatio
Computing and data processing
The applications of computers and data processing to astronomy are discussed. Among the topics covered are the emerging national information infrastructure, workstations and supercomputers, supertelescopes, digital astronomy, astrophysics in a numerical laboratory, community software, archiving of ground-based observations, dynamical simulations of complex systems, plasma astrophysics, and the remote control of fourth dimension supercomputers
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