425 research outputs found
Nonlinear Marine Structures With Random Excitation
Introduction Some of the classical nonlinear and time-varying equations of engineering mathematics appear in the modeling of the dynamic behavior of offshore structures. The dynamics of free-hanging risers, tension leg platforms and suspended loads can be cast in the form of Mathieu's equation; wave excitation causes the time variation of the spring parameter (references Disturbingly large subharmonic resonances or chaotic motions can result if the nonlinear equations (reference [4]) or the spring in the Mathieu equation varies harmonically (reference [1] ). This paper presents physical and mathematical arguments which indicate that these large responses are caused by a phase lock between the motion of the structure and the external excitation, something which is generally unlikely to last for long if a structure is subject to a random excitation. To test these predictions, two typical systems are simulated and randomness is introduced into the previously regular forcing in three different ways; as additive white noise, as frequency wander and as bandwidth spread. The responses are Fourier analyzed and maximum, minimum, mean and rms values are recorded. Random inputs cause the Poincare points to wander in a "Poincare region"; these are displayed as a function of the randomness parameter. The size of the subharmonic motions decays quickly with increasing values of the randomness parameter and they are generally small for realistically random wave forcing signals. Where the motion of a vessel is a significant input to a dynamic system, the filtering action of the vessel's dynamics driven by the wave action can generate a relatively regular motion; Pate
Type Ia Supernovae, Evolution, and the Cosmological Constant
We explore the possible role of evolution in the analysis of data on SNe Ia
at cosmological distances. First, using a variety of simple sleuthing
techniques, we find evidence that the properties of the high and low redshift
SNe Ia observed so far differ from one another. Next, we examine the effects of
including simple phenomenological models for evolution in the analysis. The
result is that cosmological models and evolution are highly degenerate with one
another, so that the incorporation of even very simple models for evolution
makes it virtually impossible to pin down the values of and
, the density parameters for nonrelativistic matter and for the
cosmological constant, respectively. Moreover, we show that if SNe Ia evolve
with time, but evolution is neglected in analyzing data, then, given enough SNe
Ia, the analysis hones in on values of and which
are incorrect. Using Bayesian methods, we show that the probability that the
cosmological constant is nonzero (rather than zero) is unchanged by the SNe Ia
data when one accounts for the possibility of evolution, provided that we do
not discriminate among open, closed and flat cosmologies a priori. The case for
nonzero cosmological constant is stronger if the Universe is presumed to be
flat, but still depends sensitively on the degree to which the peak
luminosities of SNe Ia evolve as a function of redshift. The estimated value of
, however, is only negligibly affected by accounting for possible
evolution.Comment: 45 pages, 15 figures; accepted for publication in The Astrophysical
Journal. Minor revisions and clarifications made including addition of recent
reference
Bayesian Analysis of Two Stellar Populations in Galactic Globular Clusters II: NGC 5024, NGC 5272, and NGC 6352
We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival
ACS Treasury observations of Galactic Globular Clusters to find and
characterize two stellar populations in NGC 5024 (M53), NGC 5272 (M3), and NGC
6352. For these three clusters, both single and double-population analyses are
used to determine a best fit isochrone(s). We employ a sophisticated Bayesian
analysis technique to simultaneously fit the cluster parameters (age, distance,
absorption, and metallicity) that characterize each cluster. For the
two-population analysis, unique population level helium values are also fit to
each distinct population of the cluster and the relative proportions of the
populations are determined. We find differences in helium ranging from
0.05 to 0.11 for these three clusters. Model grids with solar
-element abundances ([/Fe] =0.0) and enhanced -elements
([/Fe]=0.4) are adopted.Comment: ApJ, 21 pages, 14 figures, 7 table
High-Frequency network activity, global increase in Neuronal Activity, and Synchrony Expansion Precede Epileptic Seizures In Vitro
How seizures start is a major question in epilepsy research. Preictal EEG changes occur in both human patients and animal models, but their underlying mechanisms and relationship with seizure initiation remain unknown. Here we demonstrate the existence, in the hippocampal CA1 region, of a preictal state characterized by the progressive and global increase in neuronal activity associated with a widespread buildup of low-amplitude high-frequency activity (HFA) (100 Hz) and reduction in system complexity.HFAis generated by the firing of neurons, mainly pyramidal cells, at much lower frequencies. Individual cycles ofHFAare generated by the near-synchronous (within 5 ms) firing of small numbers of pyramidal cells. The presence of HFA in the low-calcium model implicates nonsynaptic synchronization; the presence of very similar HFA in the high-potassium model shows that it does not depend on an absence of synaptic transmission. Immediately before seizure onset, CA1 is in a state of high sensitivity in which weak depolarizing or synchronizing perturbations can trigger seizures. Transition to seizure is haracterized by a rapid expansion and fusion of the neuronal populations responsible for HFA, associated with a progressive slowing of HFA, leading to a single, massive, hypersynchronous cluster generating the high-amplitude low-frequency activity of the seizure
Photometry of Proxima Centauri and Barnard's Star Using HST Fine Guidance Sensor 3: A Search for Periodic Variations
We have observed Proxima Centauri and Barnard's Star with Hubble Space
Telescope Fine Guidance Sensor 3. Proxima Centauri exhibits small-amplitude,
periodic photometric variations. Once several sources of systematic photometric
error are corrected, we obtain 2 milli-magnitude internal photometric
precision. We identify two distinct behavior modes over the past four years:
higher amplitude, longer period; smaller amplitude, shorter period. Within the
errors one period (P ~ 83d) is twice the other. Barnard's Star shows very weak
evidence for periodicity on a timescale of approximately 130 days. If we
interpret these periodic phenomena as rotational modulation of star spots, we
identify three discrete spots on Proxima Cen and possibly one spot on Barnard's
Star. We find that the disturbances change significantly on time scales as
short as one rotation period.Comment: 39 pages, 17 figure
Interferometric Astrometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: Detection Limits for sub-Stellar Companions
We report on a sub-stellar companion search utilizing interferometric
fringe-tracking astrometry acquired with Fine Guidance Sensor 3 (FGS 3) on the
Hubble Space Telescope. Our targets were Proxima Centauri and Barnard's Star.
We obtain absolute parallax values for Proxima Cen pi_{abs} = 0.7687 arcsecond
and for Barnard's Star pi_{abs} = 0.5454 arcsecond.
Once low-amplitude instrumental systematic errors are identified and removed,
our companion detection sensitivity is less than or equal to one Jupiter mass
for periods longer than 60 days for Proxima Cen. Between the astrometry and the
radial velocity results we exclude all companions with M > 0.8M_{Jup} for the
range of periods 1 < P < 1000 days. For Barnard's Star our companion detection
sensitivity is less than or equal to one Jupiter mass for periods long er than
150 days. Our null results for Barnard's Star are consistent with those of
Gatewood (1995).Comment: 35 pages, 13 figures, to appear in August 1999 A
Triage conducted by lay-staff and emergency training reduces paediatric mortality in the emergency department of a rural hospital in Northern Mozambique
Introduction
The majority of emergency paediatric death in African countries occur within the first 24âŻh of admission. A coloured triage system is widely implemented in high-income countries and the emergency triage and assessment treatment (ETAT) is recommended by the World Health Organization, but not put into practice in Mozambique. We implemented a three-colour triage system in a rural district hospital with lay-staff workers conducting the first triage.
Methods
A retrospective, before and after, mortality analysis was performed using routine patient files from the district hospital between 2014 and 2017. The triage system was implemented in August 2016. Inclusion criteria were children under 15âŻyears of age that entered the emergency centre. Primary outcome was child mortality rate. Secondary outcomes included the percentage agreement between the clinical and non-clinical staff and the duration from triage to first treatment. We used a negative binomial model in STATA 15 to compare mortality rates, and Kappa statistics to estimate the agreement between clinical and non-clinical staff.
Results
4176 admissions were included. The mortality rate ratio (MMR) was 45% lower after the start of the intervention (2016; MRRâŻ=âŻ0.55; 0.38, 0.81; pâŻ=âŻ0.002), compared to before. To estimate the agreement between non-clinical and clinical staff, 548 (of the 671) patient files were included. The agreement was estimated at 88.7% (KappaâŻ=âŻ0.644; pâŻ<âŻ0.001). The median waiting time decreased with urgency of the triage: 2âŻh33 for âgreenâ/least serious (IQR 1âŻh58-3âŻh30), 21âŻmin for yellow/serious (IQR 0âŻh10-0âŻh58) and nine minutes for âredâ/urgent (IQR 2â40âŻmin).
Conclusion
In a rural setting with nurse-led clinical care and non-clinician staff working at the triage reception, implementation of a three-coloured triage system was feasible. Triage and ETAT training was associated with a decrease of 45% of paediatric deaths. The impact on mortality, low cost, and ease of the implementation supports scaling this intervention in similar settings
Observation of light echoes around very young stars
The goal of the paper is to present new results on light echoes from young
stellar objects. Broad band CCD images were obtained over three months at
one-to-two week intervals for the field of NGC 6726, using the large
field-of-view remotely-operated telescope on top of Cerro Burek. We detected
scattered light echoes around two young, low-amplitude, irregular variable
stars. Observations revealed not just one, but multiple light echoes from
brightness pulses of the T Tauri star S CrA and the Herbig Ae/Be star R CrA.
Analysis of S CrA's recurring echoes suggests that the star is located 138 +/-
16 pc from Earth, making these the closest echoes ever detected. The
environment that scatters the stellar light from S CrA is compatible with an
incomplete dust shell or an inclined torus some 10,000 AU in radius and
containing M_{\sun} of dust. The cause of such
concentration at 10,000AU from the star is unknown. It could be the
remnant of the envelope from which the star formed, but the distance of the
cloud is remarkably similar to the nominal distance of the Oort cloud to the
Sun, leading us to also speculate that the dust (or ice) seen around S CrA
might have the same origin as the Solar System Oort cloud.Comment: A&A, in press Received: 16 March 2010 / Accepted: 01 June 201
Rotational velocities of A-type stars I. Measurement of vsini in the southern hemisphere
Within the scope of a Key Programme determining fundamental parameters of
stars observed by HIPPARCOS, spectra of 525 B8 to F2-type stars brighter than
V=8 have been collected at ESO. Fourier transforms of several line profiles in
the range 4200-4500 A are used to derive vsini from the frequency of the first
zero. Statistical analysis of the sample indicates that measurement error is a
function of vsini and this relative error of the rotational velocity is found
to be about 6% on average. The results obtained are compared with data from the
literature. There is a systematic shift from standard values from Slettebak et
al. (1975), which are 10 to 12% lower than our findings. Comparisons with other
independent vsini values tend to prove that those from Slettebak et al. are
underestimated. This effect is attributed to the presence of binaries in the
standard sample of Slettebak et al., and to the model atmosphere they used.Comment: 17 pages, includes 18 figures, accepted in A&
- âŠ