32,647 research outputs found
Modeling the Void H I Column Density Spectrum
The equivalent width distribution function (EWDF) of \hone absorbers specific
to the void environment has been recently derived (Manning 2002), revealing a
large line density of clouds (dN/dz ~500 per unit z for Log (N_HI)> 12.4). I
show that the void absorbers cannot be diffuse (or so-called filamentary)
clouds, expanding with the Hubble flow, as suggested by N-body/hydro
simulations. Absorbers are here modeled as the baryonic remnants of
sub-galactic perturbations that have expanded away from their dark halos in
response to reionization at z ~ 6.5. A 1-D Lagrangian hydro/gravity code is
used to follow the dynamic evolution and ionization structure of the baryonic
clouds for a range of halo circular velocities. The simulation products at z=0
can be combined according to various models of the halo velocity distribution
function to form a column density spectrum that can be compared with the
observed. I find that such clouds may explain the observed EWDF if the halo
velocity distribution function is as steep as that advanced by Klypin (1999),
and the halo mass distribution is closer to isothermal than to NFW.Comment: 21 pages, 15 figures. Paper in press; ApJ 591, n
Validation of the communications link analysis and simulation system (CLASS)
CLASS (Communication Link Analysis and Simulation System) is a software package developed for NASA to predict the communication and tracking performance of the Tracking and Data Relay Satellite System (TDRSS) services. The methods used to verify CLASS are described. The usefulness of a software tool such as CLASS depends strongly on the reliability and accuracy of the results it produces. For this reason, considerable attention was paid to validation throughout the CLASS development
The B-L/Electroweak Hierarchy in Smooth Heterotic Compactifications
E8 X E8 heterotic string and M-theory, when appropriately compactified, can
give rise to realistic, N=1 supersymmetric particle physics. In particular, the
exact matter spectrum of the MSSM, including three right-handed neutrino
supermultiplets, one per family, and one pair of Higgs-Higgs conjugate
superfields is obtained by compactifying on Calabi-Yau manifolds admitting
specific SU(4) vector bundles. These "heterotic standard models" have the
SU(3)_{C} X SU(2)_{L} X U(1)_{Y} gauge group of the standard model augmented by
an additional gauged U(1)_{B-L}. Their minimal content requires that the B-L
gauge symmetry be spontaneously broken by a vacuum expectation value of at
least one right-handed sneutrino. In a previous paper, we presented the results
of a renormalization group analysis showing that B-L gauge symmetry is indeed
radiatively broken with a B-L/electroweak hierarchy of O(10) to O(10^{2}). In
this paper, we present the details of that analysis, extending the results to
include higher order terms in tan[beta]^{-1} and the explicit spectrum of all
squarks and sleptons.Comment: 60 pages, 6 figure
Three Generations on the Quintic Quotient
A three-generation SU(5) GUT, that is 3x(10+5bar) and a single 5-5bar pair,
is constructed by compactification of the E_8 heterotic string. The base
manifold is the Z_5 x Z_5-quotient of the quintic, and the vector bundle is the
quotient of a positive monad. The group action on the monad and its
bundle-valued cohomology is discussed in detail, including topological
restrictions on the existence of equivariant structures. This model and a
single Z_5 quotient are the complete list of three generation quotients of
positive monads on the quintic.Comment: 19 pages, LaTeX. v2: section on anomaly cancellation adde
Orbiter global positioning system design and Ku-band problems investigation, exhibit B, revision 1
The LinCom effort in supporting the JSC study of the use of the Global Positioning System (GPS) on the space shuttle and in Ku-band problem investigation is documented. LinCom was tasked to evaluate system implementation, performance, and integration aspects of the shuttle GPS and to provide independent technical assessment of reports submitted to JSC regarding integration studies, system studies and navigation analyses
Periodically driven stochastic un- and refolding transitions of biopolymers
Mechanical single molecule experiments probe the energy profile of
biomolecules. We show that in the case of a profile with two minima (like
folded/unfolded) periodic driving leads to a stochastic resonance-like
phenomenon. We demonstrate that the analysis of such data can be used to
extract four basic parameters of such a transition and discuss the statistical
requirements of the data acquisition. As advantages of the proposed scheme, a
polymeric linker is explicitly included and thermal fluctuations within each
well need not to be resolved.Comment: 7 pages, 5 figures, submitted to EP
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