7,055 research outputs found
Orbiter/launch system
The system includes reusable turbojet propelled booster vehicles releasably connected to a reusable rocket powered orbit vehicle. The coupled orbiter-booster combination takes off horizontally and ascends to staging altitude and speed under booster power with both orbiter and booster wings providing lift. After staging, the booster vehicles fly back to Earth for horizontal landing and the orbiter vehicle continues ascending to orbit
Paper Session II-A - The Testability of Software for the Space Station Freedom Program
The Space Station Freedom Data Management System consists of state-of-the-art hardware and software technology that exceeds the capabilities of earlier test tools and methods used to verify and certify man-rated space systems. New technologies and techniques are being developed to meet these challenges
Fecundity of Wild Northern Bobwhite Hens Under Hatchery Conditions
We describe egg production by 88 pairs of randomly selected, mature, wild-caught northern bobwhite (Colinus virginianus) hens housed under optimal conditions of food, water, climate, and a 17-hr photoperiod in a hatchery. We collected eggs daily using an 18-day period to differentiate between clutches. Hens continuously laid eggs until ceasing production. We evaluated number of eggs laid by each hen individually and hens collectively including total number, number/clutch, number/day, hatching success, and egg mass. Eighty-six hens produced 5,888 eggs. Number of eggs produced by individual hens ranged from 0 to 172 over ~ 200 days. Mean number of eggs laid/hen/day was 0.86. Clutch size ranged from 0 (n 1⁄4 2) to 12 (n 1⁄4 1). Mean number of eggs/clutch was 8.57. There was a strong correlation between clutch size and number of clutches. Some hens demonstrated continuous production of several large clutches. Hatching success of 5,793 eggs included for analysis was 61.6% (3,571 hatched, 2,222 failed to hatch). Hatched eggs had a greater mean mass compared to those that did not hatch
Quantum Structure of Space Near a Black Hole Horizon
We describe a midi-superspace quantization scheme for generic single horizon
black holes in which only the spatial diffeomorphisms are fixed. The remaining
Hamiltonian constraint yields an infinite set of decoupled eigenvalue
equations: one at each spatial point. The corresponding operator at each point
is the product of the outgoing and ingoing null convergences, and describes the
scale invariant quantum mechanics of a particle moving in an attractive
potential. The variable that is analoguous to particle position is the
square root of the conformal mode of the metric. We quantize the theory via
Bohr quantization, which by construction turns the Hamiltonian constraint
eigenvalue equation into a finite difference equation. The resulting spectrum
gives rise to a discrete spatial topology exterior to the horizon. The spectrum
approaches the continuum in the asymptotic region.Comment: References added and typos corrected. 21 pages, 1 figur
Multimodal Imaging of Alzheimer Pathophysiology in the Brain's Default Mode Network
The spatial correlations between the brain's default mode network (DMN) and the brain regions known to develop pathophysiology in Alzheimer's disease (AD) have recently attracted much attention. In this paper, we compare results of different functional and structural imaging modalities, including MRI and PET, and highlight different patterns of anomalies observed within the DMN. Multitracer PET imaging in subjects with and without dementia has demonstrated that [C-11]PIB- and [F-18]FDDNP-binding patterns in patients with AD overlap within nodes of the brain's default network including the prefrontal, lateral parietal, lateral temporal, and posterior cingulate cortices, with the exception of the medial temporal cortex (especially, the hippocampus) where significant discrepancy between increased [F-18]FDDNP binding and negligible [C-11]PIB-binding was observed. [F-18]FDDNP binding in the medial temporal cortex—a key constituent of the DMN—coincides with both the presence of amyloid and tau pathology, and also with cortical areas with maximal atrophy as demonstrated by T1-weighted MR imaging of AD patients
Elastic energy of polyhedral bilayer vesicles
In recent experiments [M. Dubois, B. Dem\'e, T. Gulik-Krzywicki, J.-C.
Dedieu, C. Vautrin, S. D\'esert, E. Perez, and T. Zemb, Nature (London) Vol.
411, 672 (2001)] the spontaneous formation of hollow bilayer vesicles with
polyhedral symmetry has been observed. On the basis of the experimental
phenomenology it was suggested [M. Dubois, V. Lizunov, A. Meister, T.
Gulik-Krzywicki, J. M. Verbavatz, E. Perez, J. Zimmerberg, and T. Zemb, Proc.
Natl. Acad. Sci. U.S.A. Vol. 101, 15082 (2004)] that the mechanism for the
formation of bilayer polyhedra is minimization of elastic bending energy.
Motivated by these experiments, we study the elastic bending energy of
polyhedral bilayer vesicles. In agreement with experiments, and provided that
excess amphiphiles exhibiting spontaneous curvature are present in sufficient
quantity, we find that polyhedral bilayer vesicles can indeed be energetically
favorable compared to spherical bilayer vesicles. Consistent with experimental
observations we also find that the bending energy associated with the vertices
of bilayer polyhedra can be locally reduced through the formation of pores.
However, the stabilization of polyhedral bilayer vesicles over spherical
bilayer vesicles relies crucially on molecular segregation of excess
amphiphiles along the ridges rather than the vertices of bilayer polyhedra.
Furthermore, our analysis implies that, contrary to what has been suggested on
the basis of experiments, the icosahedron does not minimize elastic bending
energy among arbitrary polyhedral shapes and sizes. Instead, we find that, for
large polyhedron sizes, the snub dodecahedron and the snub cube both have lower
total bending energies than the icosahedron
Inference with interference between units in an fMRI experiment of motor inhibition
An experimental unit is an opportunity to randomly apply or withhold a
treatment. There is interference between units if the application of the
treatment to one unit may also affect other units. In cognitive neuroscience, a
common form of experiment presents a sequence of stimuli or requests for
cognitive activity at random to each experimental subject and measures
biological aspects of brain activity that follow these requests. Each subject
is then many experimental units, and interference between units within an
experimental subject is likely, in part because the stimuli follow one another
quickly and in part because human subjects learn or become experienced or
primed or bored as the experiment proceeds. We use a recent fMRI experiment
concerned with the inhibition of motor activity to illustrate and further
develop recently proposed methodology for inference in the presence of
interference. A simulation evaluates the power of competing procedures.Comment: Published by Journal of the American Statistical Association at
http://www.tandfonline.com/doi/full/10.1080/01621459.2012.655954 . R package
cin (Causal Inference for Neuroscience) implementing the proposed method is
freely available on CRAN at https://CRAN.R-project.org/package=ci
Rhythmic dynamics and synchronization via dimensionality reduction : application to human gait
Reliable characterization of locomotor dynamics of human walking is vital to understanding the neuromuscular control of human locomotion and disease diagnosis. However, the inherent oscillation and ubiquity of noise in such non-strictly periodic signals pose great challenges to current methodologies. To this end, we exploit the state-of-the-art technology in pattern recognition and, specifically, dimensionality reduction techniques, and propose to reconstruct and characterize the dynamics accurately on the cycle scale of the signal. This is achieved by deriving a low-dimensional representation of the cycles through global optimization, which effectively preserves the topology of the cycles that are embedded in a high-dimensional Euclidian space. Our approach demonstrates a clear advantage in capturing the intrinsic dynamics and probing the subtle synchronization patterns from uni/bivariate oscillatory signals over traditional methods. Application to human gait data for healthy subjects and diabetics reveals a significant difference in the dynamics of ankle movements and ankle-knee coordination, but not in knee movements. These results indicate that the impaired sensory feedback from the feet due to diabetes does not influence the knee movement in general, and that normal human walking is not critically dependent on the feedback from the peripheral nervous system
On the Cosmological Evolution of the Luminosity Function and the Accretion Rate of Quasars
We consider a class of models for the redshift evolution (between 0\lsim z
\lsim 4) of the observed optical and X-ray quasar luminosity functions (LFs),
with the following assumptions: (i) the mass-function of dark matter halos
follows the Press-Schechter theory, (ii) the black hole (BH) mass scales
linearly with the halo mass, (iii) quasars have a constant universal lifetime,
and (iv) a thin accretion disk provides the optical luminosity of quasars,
while the X-ray/optical flux ratio is calibrated from a sample of observed
quasars. The mass accretion rate onto quasar BHs is a free parameter
of the models, that we constrain using the observed LFs. The accretion rate
inferred from either the optical or X-ray data under these assumptions
generally decreases as a function of cosmic time from to . We find that a comparable accretion rate is inferred from the X-ray and
optical LF only if the X-ray/optical flux ratio decreases with BH mass. Near
, drops to substantially sub-Eddington values at which
advection-dominated accretion flows (ADAFs) exist. Such a decline of ,
possibly followed by a transition to radiatively inefficient ADAFs, could
explain both the absence of bright quasars in the local universe and the
faintness of accreting BHs at the centers of nearby galaxies. We argue that a
decline of the accretion rate of the quasar population is indeed expected in
cosmological structure formation models.Comment: Latex, 23 pages, 9 figures, accepted for publication in Ap
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