2,689 research outputs found
The NASA Space Station program plans
The design of a permanently manned space station is discussed. The role of the space shuttle, planning guidelines, international cooperation, and commercial possibilities are among the topics discussed
The influence of initial mass segregation on the runaway merging of stars
We have investigated the effect of initial mass segregation on the runaway
merging of stars. The evolution of multi-mass, dense star clusters was followed
by means of direct N-body simulations of up to 131.072 stars. All clusters
started from King models with dimensionless central potentials of 3.0 <= W_0 <=
9.0. Initial mass segregation was realized by varying the minimum mass of a
certain fraction of stars whose either (1) distances were closest to the
cluster center or (2) total energies were lowest. The second case is more
favorable to promote the runaway merging of stars by creating a high-mass core
of massive, low-energy stars. Initial mass segregation could decrease the
central relaxation time and thus help the formation of a high-mass core.
However, we found that initial mass segregation does not help the runaway
stellar merger to happen if the overall mass density profile is kept constant.
This is due to the fact that the collision rate of stars is not increased due
to initial mass segregation. Our simulations show that initial mass segregation
is not sufficient to allow runaway merging of stars to occur in clusters with
central densities typical for star clusters in the Milky Way.Comment: 25 pages, 9 figures, 3 tables, accepted for publication in Ap
The partnership: Space shuttle, space science, and space station
An overview of the NASA Space Station Program functions, design, and planned implementation is presented. The discussed functions for the permanently manned space facility include: (1) development of new technologies and related commercial products; (2) observations of the Earth and the universe; (3) provision of service facilities for resupply, maintenance, upgrade and repair of payloads and spacecraft; (4) provision of a transportation node for stationing, processing and dispatching payloads and vehicles; (5) provision of manufacturing and assembly facilities; (6) provision of a storage depot for parts and payloads; and (7) provision of a staging base for future space endeavors. The fundamental concept for the Space Station, as given, is that it be designed, operated, and evolved in response to a broad variety of scientific, technological, and commercial user interests. The Space Shuttle's role as the principal transportation system for the construction and maintenance of the Space Station and the servicing and support of the station crew is also discussed
Permalloy-based carbon nanotube spin-valve
In this Letter we demonstrate that Permalloy (Py), a widely used Ni/Fe alloy,
forms contacts to carbon nanotubes (CNTs) that meet the requirements for the
injection and detection of spin-polarized currents in carbon-based spintronic
devices. We establish the material quality and magnetization properties of Py
strips in the shape of suitable electrical contacts and find a sharp
magnetization switching tunable by geometry in the anisotropic
magnetoresistance (AMR) of a single strip at cryogenic temperatures. In
addition, we show that Py contacts couple strongly to CNTs, comparable to Pd
contacts, thereby forming CNT quantum dots at low temperatures. These results
form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance
switchings in the tunneling magnetoresistance, which directly correspond to the
magnetization reversals in the individual contacts observed in AMR experiments.Comment: 3 page
On Convergence of the Inexact Rayleigh Quotient Iteration with the Lanczos Method Used for Solving Linear Systems
For the Hermitian inexact Rayleigh quotient iteration (RQI), the author has
established new local general convergence results, independent of iterative
solvers for inner linear systems. The theory shows that the method locally
converges quadratically under a new condition, called the uniform positiveness
condition. In this paper we first consider the local convergence of the inexact
RQI with the unpreconditioned Lanczos method for the linear systems. Some
attractive properties are derived for the residuals, whose norms are
's, of the linear systems obtained by the Lanczos method. Based on
them and the new general convergence results, we make a refined analysis and
establish new local convergence results. It is proved that the inexact RQI with
Lanczos converges quadratically provided that with a
constant . The method is guaranteed to converge linearly provided
that is bounded by a small multiple of the reciprocal of the
residual norm of the current approximate eigenpair. The results are
fundamentally different from the existing convergence results that always
require , and they have a strong impact on effective
implementations of the method. We extend the new theory to the inexact RQI with
a tuned preconditioned Lanczos for the linear systems. Based on the new theory,
we can design practical criteria to control to achieve quadratic
convergence and implement the method more effectively than ever before.
Numerical experiments confirm our theory.Comment: 20 pages, 8 figures. arXiv admin note: text overlap with
arXiv:0906.223
In vivo analysis of staphylococcus aureus-infected mice reveals differential temporal and spatial expression patterns of fhuD2
Staphylococcus aureus is an opportunistic human pathogen and a major cause of invasive infections such as bacteremia, endocarditis, pneumonia and wound infections. FhuD2 is a staphylococcal lipoprotein involved in the uptake of iron-hydroxymate and is under the control of the iron uptake regulator Fur. The protein is part of an investigational multi-component vaccine formulation that has shown protective efficacy in several murine models of infection. Even though fhuD2 expression was shown to be upregulated in murine kidneys infected with S. aureus, it is unknown whether the bacterium undergoes increased iron deprivation during prolonged infection. Furthermore, different infection niches of S. aureus might provide different environments and iron availability resulting in different fhuD2 expression pattern within different host organs. To address these questions, we characterized the in vitro expression of the fhuD2 gene and confirmed Fur-dependent iron-regulation of its expression. We further investigated its expression in mice infected with a bioluminescent reporter strain of S. aureus expressing the luciferase operon under the control of the fhuD2 promoter. The emission of bioluminescence in different organs was followed over a seven-day time course, as well as quantitative real-time PCR analysis of the RNA transcribed from the endogenous fhuD2 gene. Using this approach, we could show that fhuD2 expression was induced during infection in all organs analyzed and that differences in expression were observed in the temporal expression profiles, and between infected organs. Our data suggest that S. aureus undergoes increased iron deprivation during progression of infection in diverse host organs and accordingly induces dedicated iron acquisition mechanisms. Since FhuD2 plays a central role in providing the pathogen with the required iron, further knowledge of the patterns of fhuD2 expression in vivo during infection is instrumental in better defining the role of this antigen in S. aureus pathogenesis and as a vaccine antigen
Dynamical Interactions and the Black Hole Merger Rate of the Universe
Binary black holes can form efficiently in dense young stellar clusters, such
as the progenitors of globular clusters, via a combination of gravitational
segregation and cluster evaporation. We use simple analytic arguments supported
by detailed N-body simulations to determine how frequently black holes born in
a single stellar cluster should form binaries, be ejected from the cluster, and
merge through the emission of gravitational radiation. We then convolve this
``transfer function'' relating cluster formation to black hole mergers with (i)
the distribution of observed cluster masses and (ii) the star formation history
of the universe, assuming that a significant fraction gcl of star formation
occurs in clusters and that a significant fraction gcand of clusters undergo
this segregation and evaporation process. We predict future ground--based
gravitational wave (GW) detectors could observe ~500 (gcl/0.5) (gcand/0.1)
double black hole mergers per year, and the presently operating LIGO
interferometer would have a chance (50%) at detecting a merger during its first
full year of science data. More realistically, advanced LIGO and similar
next-generation gravitational wave observatories provide unique opportunities
to constrain otherwise inaccessible properties of clusters formed in the early
universe.Comment: 4 pages, 2 figures. To appear in PRD Rapid Communication
Merger of Black Holes in the Galactic Center
We present the results of three body simulations focused on understanding the
fates of intermediate mass black holes (IBH) that drift within the central 0.5
pc of the Galaxy. In particular, we modeled the interactions between pairs of
black holes as they orbit a central blac k hole of mass
. The simulations performed assume a
Schwarzschild geometry and account for Chandrasekhar dynamical friction as well
as acceleration resulting from energy lost due to gravitational radiation.
We found the branching ratio for one of the orbiting IBHs to merge with the
CBH was 0.95 and is independent of the inner IBH's initial eccentricity as well
as the rate of sinking. This, coupled with an infall rate of yrs
for an IBH to drift into the Galactic center, results in an IBH-CBH merger
every Myrs. Lastly we found that the IBH-IBH-CBH triple body
system ``resets'' itself, in the sense that a system with an inner I BH with an
initially circular orbit generally left behind an IBH with a large
eccentricity, whereas a system in which the inner IBH had a high eccentricity
() usually left a remnant with low eccentricity. Branching ratios
for different outcomes are also similar in the two cases.Comment: Official paper to appear in November 2008 issue of Ap
Rapid formation of exponential disks and bulges at high redshift from the dynamical evolution of clump cluster and chain galaxies
Many galaxies at high redshift have peculiar morphologies dominated by
10^8-10^9 Mo kpc-sized clumps. Using numerical simulations, we show that these
"clump clusters" can result from fragmentation in gravitationally unstable
primordial disks. They appear as "chain galaxies" when observed edge-on. In
less than 1 Gyr, clump formation, migration, disruption, and interaction with
the disk cause these systems to evolve from initially uniform disks into
regular spiral galaxies with an exponential or double-exponential disk profile
and a central bulge. The inner exponential is the initial disk size and the
outer exponential is from material flung out by spiral arms and clump torques.
A nuclear black hole may form at the same time as the bulge from smaller black
holes that grow inside the dense cores of each clump. The properties and
lifetimes of the clumps in our models are consistent with observations of the
clumps in high redshift galaxies, and the stellar motions in our models are
consistent with the observed velocity dispersions and lack of organized
rotation in chain galaxies. We suggest that violently unstable disks are the
first step in spiral galaxy formation. The associated starburst activity gives
a short timescale for the initial stellar disk to form.Comment: ApJ Accepted, 13 pages, 9 figure
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