869 research outputs found
SSF loads and controllability during assembly
The Orbiter Primary Reaction Control System (PRCS) pulse width and firing frequency is restricted to prevent excessive loads in the Space Station Freedom (SSF). The feasibility of using the SSF Control Moment Gyros (CMG) as a secondary controller for load relief is evaluated. The studies revealed the CMG not only reduced loads but were useful for other SSF functions: vibration suppression and modal excitation. Vibration suppression lowers the g level for the SSF micro-g experiments and damps the low frequency oscillations that cause crew sickness. Modal excitation could be used for the modal identification experiment and health monitoring. The CMG's reduced the peak loads and damped the vibrations. They were found to be an effective multi-purpose ancillary device for SSF operation
Personal Reflections on the Use of Military Force and Its Relevance to National Security Strategy
As I reflect on my personal career in the field of military force and national security, I wish I could offer a simple vision, sharply refined and clarified by decades of experience. But such a vision eludes me
National Interest and Naval Forces in the 1990s
During the past 45 years, the U.S.- Soviet relationship has been the dynamic focus of our foreign and defense policy. Our preoccupation with containing Soviet expansionism-and with deterring nuclear war-has limited to a degree our ability to focus on other national and international problems. Now that relationship is again on the move, but in. a positive, less confrontational direction. What the final outcome will be when the maneuvers are over, and whether our country will retain its position of strength, depend a lot on our wisdom in reacting to these developments in the Soviet Union
Engineering Responses to Pandemics
Focusing on pandemic influenza, this chapter approaches the planning for and
response to such a major worldwide health event as a complex engineering systems problem. Action-oriented analysis of pandemics requires a broad inclusion of academic disciplines since no one domain can cover a significant fraction of the problem. Numerous research papers and action plans have treated pandemics as purely medical happenings, focusing on hospitals, health care professionals, creation and distribution of vaccines and anti-virals, etc. But human behavior with regard to hygiene and social distancing constitutes a first-order partial brake or control of the spread and intensity of infection. Such behavioral options are “non-pharmaceutical interventions.” (NPIs) The chapter employs simple mathematical models to study alternative controls of infection, addressing a well-known parameter in epidemiology, R0, the “reproductive number,” defined as the mean number of new infections generated by an index case. Values of R0 greater than 1.0 usually indicate that the infection begins with exponential growth, the generation-to-generation growth rate being R0. R0 is broken down into constituent parts related to the frequency and intensity of human contacts, both partially under our control. It is suggested that any numerical value for R0 has little meaning outside the social context to which it pertains. Difference equation models are then employed to study the effects of heterogeneity of population social contact rates, the analysis showing that the disease tends to be driven by high frequency individuals. Related analyses show the futility of trying geographically to isolate the disease. Finally, the models are operated under a variety of assumptions related to social distancing and changes in hygienic behavior. The results are promising in terms of potentially reducing the total impact of the pandemic
Stopping Pandemic Flu: Government and Community Interventions in a Multi-Community Model
Focusing on mitigation strategies for global pandemic influenza, we use elementary mathematical models to evaluate the implementation and timing of intervention strategies such as travel restrictions, vaccination, social distancing and improved hygiene. A spreadsheet model of infection spread between several linked heterogeneous communities is based on analytical calculations and Monte Carlo simulations. Since human behavior will likely change during the course of a pandemic, thereby altering the dynamics of the disease, we incorporate a feedback parameter into our model to reflect altered behavior. Our results indicate that while a flu pandemic could be devastating; there are coping methods that when implemented quickly and correctly can significantly mitigate the severity of a global outbreak
The Nature of the Dense Core Population in the Pipe Nebula: Thermal Cores Under Pressure
In this paper we present the results of a systematic investigation of an
entire population of starless dust cores within a single molecular cloud.
Analysis of extinction data shows the cores to be dense objects characterized
by a narrow range of density. Analysis of C18O and NH3 molecular-line
observations reveals very narrow lines. The non-thermal velocity dispersions
measured in both these tracers are found to be subsonic for the large majority
of the cores and show no correlation with core mass (or size). Thermal pressure
is thus the dominate source of internal gas pressure and support for most of
the core population. The total internal gas pressures of the cores are found to
be roughly independent of core mass over the entire range of the core mass
function (CMF) indicating that the cores are in pressure equilibrium with an
external source of pressure. This external pressure is most likely provided by
the weight of the surrounding Pipe cloud within which the cores are embedded.
Most of the cores appear to be pressure confined, gravitationally unbound
entities whose nature, structure and future evolution are determined by only a
few physical factors which include self-gravity, the fundamental processes of
thermal physics and the simple requirement of pressure equilibrium with the
surrounding environment. The observed core properties likely constitute the
initial conditions for star formation in dense gas. The entire core population
is found to be characterized by a single critical Bonnor-Ebert mass. This mass
coincides with the characteristic mass of the Pipe CMF indicating that most
cores formed in the cloud are near critical stability. This suggests that the
mass function of cores (and the IMF) has its origin in the physical process of
thermal fragmentation in a pressurized medium.Comment: To appear in the Astrophysical Journa
Shapes of Molecular Cloud Cores and the Filamentary Mode of Star Formation
Using recent dust continuum data, we generate the intrinsic ellipticity
distribution of dense, starless molecular cloud cores. Under the hypothesis
that the cores are all either oblate or prolate randomly-oriented spheroids, we
show that a satisfactory fit to observations can be obtained with a gaussian
prolate distribution having a mean intrinsic axis ratio of 0.54. Further, we
show that correlations exist between the apparent axis ratio and both the peak
intensity and total flux density of emission from the cores, the sign of which
again favours the prolate hypothesis. The latter result shows that the mass of
a given core depends on its intrinsic ellipticity. Monte Carlo simulations are
performed to find the best-fit power law of this dependence. Finally, we show
how these results are consistent with an evolutionary scenario leading from
filamentary parent clouds to increasingly massive, condensed, and roughly
spherical embedded cores.Comment: 16 pages, incl. 11 Postscript figures. Accepted by Ap
Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map
We present a near-infrared extinction study of the dark globule L694-2, a
starless core that shows strong evidence for inward motions in molecular line
profiles. The J,H, and K band data were taken using the European Southern
Observatory New Technology Telescope. The best fit simple spherical power law
model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled
in extinction. This power law slope is steeper than the value of p=2 for a
singular isothermal sphere, the initial condition of the inside-out model for
protostellar collapse. Including an additional extinction component along the
line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert
sphere results in a super-critical value of the dimensionless radius xi_max=25
+/- 3. The unstable configuration of material may be related to the observed
inward motions. The Bonnor-Ebert model matches the shape of the observed
profile, but significantly underestimates the amount of extinction (by a factor
of ~4). This discrepancy in normalization has also been found for the nearby
protostellar core B335 (Harvey et al. 2001). A cylindrical density model with
scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder
axis provides an equally good radial profile as a power law model, and
reproduces the asymmetry of the core remarkably well. In addition, this model
provides a basis for understanding the discrepancy in the normalization of the
Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full
extent (mass) of the core being missed by assuming symmetry between the
profiles in the plane of the sky and along the line-of-sight. If the core is
sufficiently magnetized then fragmentation may be avoided, and later evolution
might produce a protostar similar to B335.Comment: 38 pages, 7 figures, accepted to Astrophysical Journa
Vocal Responses to Perturbations in Voice Auditory Feedback in Individuals with Parkinson's Disease
One of the most common symptoms of speech deficits in individuals with Parkinson's disease (PD) is significantly reduced vocal loudness and pitch range. The present study investigated whether abnormal vocalizations in individuals with PD are related to sensory processing of voice auditory feedback. Perturbations in loudness or pitch of voice auditory feedback are known to elicit short latency, compensatory responses in voice amplitude or fundamental frequency.Twelve individuals with Parkinson's disease and 13 age- and sex-matched healthy control subjects sustained a vowel sound (/α/) and received unexpected, brief (200 ms) perturbations in voice loudness (±3 or 6 dB) or pitch (±100 cents) auditory feedback. Results showed that, while all subjects produced compensatory responses in their voice amplitude or fundamental frequency, individuals with PD exhibited larger response magnitudes than the control subjects. Furthermore, for loudness-shifted feedback, upward stimuli resulted in shorter response latencies than downward stimuli in the control subjects but not in individuals with PD.The larger response magnitudes in individuals with PD compared with the control subjects suggest that processing of voice auditory feedback is abnormal in PD. Although the precise mechanisms of the voice feedback processing are unknown, results of this study suggest that abnormal voice control in individuals with PD may be related to dysfunctional mechanisms of error detection or correction in sensory feedback processing
Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models
We use 3D numerical MHD simulations to follow the evolution of cold,
turbulent, gaseous systems with parameters representing GMC conditions. We
study three cloud simulations with varying mean magnetic fields, but identical
initial velocity fields. We show that turbulent energy is reduced by a factor
two after 0.4-0.8 flow crossing times (2-4 Myr), and that the magnetically
supercritical cloud models collapse after ~6 Myr, while the subcritical cloud
does not collapse. We compare density, velocity, and magnetic field structure
in three sets of snapshots with matched Mach numbers. The volume and column
densities are both log-normally distributed, with mean volume density a factor
3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4
times the unperturbed value. We use a binning algorithm to investigate the
dependence of kinetic quantities on spatial scale for regions of column density
contrast (ROCs). The average velocity dispersion for the ROCs is only weakly
correlated with scale, similar to the mean size-linewidth relation for clumps
within GMCs. ROCs are often superpositions of spatially unconnected regions
that cannot easily be separated using velocity information; the same difficulty
may affect observed GMC clumps. We analyze magnetic field structure, and show
that in the high density regime, total magnetic field strengths increase with
density with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic field
strengths vary widely across a projected cloud, and do not correlate with
column density. We compute simulated interstellar polarization maps at varying
orientations, and determine that the Chandrasekhar-Fermi formula multiplied by
a factor ~0.5 yields a good estimate of the plane-of sky magnetic field
strength provided the dispersion in polarization angles is < 25 degrees.Comment: 56 pages, 25 figures; Ap.J., accepte
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