3,083 research outputs found
Optical Modeling Activities for NASA's James Webb Space Telescope (JWST): V. Operational Alignment Updates
This paper is part five of a series on the ongoing optical modeling activities for the James Webb Space Telescope (JWST). The first two papers discussed modeling JWST on-orbit performance using wavefront sensitivities to predict line of sight motion induced blur, and stability during thermal transients. The third paper investigates the aberrations resulting from alignment and figure compensation of the controllable degrees of freedom (primary and secondary mirrors), which may be encountered during ground alignment and on-orbit commissioning of the observatory, and the fourth introduced the software toolkits used to perform much of the optical analysis for JWST. The work here models observatory operations by simulating line-of-sight image motion and alignment drifts over a two-week period. Alignment updates are then simulated using wavefront sensing and control processes to calculate and perform the corrections. A single model environment in Matlab is used for evaluating the predicted performance of the observatory during these operations
Constraints, Histones, and the 30 Nanometer Spiral
We investigate the mechanical stability of a segment of DNA wrapped around a
histone in the nucleosome configuration. The assumption underlying this
investigation is that the proper model for this packaging arrangement is that
of an elastic rod that is free to twist and that writhes subject to mechanical
constraints. We find that the number of constraints required to stabilize the
nuclesome configuration is determined by the length of the segment, the number
of times the DNA wraps around the histone spool, and the specific constraints
utilized. While it can be shown that four constraints suffice, in principle, to
insure stability of the nucleosome, a proper choice must be made to guarantee
the effectiveness of this minimal number. The optimal choice of constraints
appears to bear a relation to the existence of a spiral ridge on the surface of
the histone octamer. The particular configuration that we investigate is
related to the 30 nanometer spiral, a higher-order organization of DNA in
chromatin.Comment: ReVTeX, 15 pages, 18 figure
Invariants of the Haldane-Shastry Chain
Using a formalism developed by Polychronakos, we explicitly construct a set
of invariants of the motion for the Haldane-Shastry chain.Comment: 11 pages, UVA-92-0
Correctly validating results from single molecule data: the case of stretched exponential decay in the catalytic activity of single lipase B molecules
The question of how to validate and interpret correctly the waiting time
probability density functions (WT-PDFs) from single molecule data is addressed.
It is shown by simulation that when a stretched exponential WT-PDF, with a
stretched exponent alfa and a time scale parameter tau, generates the off
periods of a two-state trajectory, a reliable recovery of the input WT-PDF from
the trajectory is obtained even when the bin size used to define the
trajectory, dt, is much larger than the parameter tau. This holds true as long
as the first moment of the WT-PDF is much larger than dt. Our results validate
the results in an earlier study of the activity of single Lipase B molecules
and disprove recent related critique
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Chromatin dysregulation and DNA methylation at transcription start sites associated with transcriptional repression in cancers.
Although promoter-associated CpG islands have been established as targets of DNA methylation changes in cancer, previous studies suggest that epigenetic dysregulation outside the promoter region may be more closely associated with transcriptional changes. Here we examine DNA methylation, chromatin marks, and transcriptional alterations to define the relationship between transcriptional modulation and spatial changes in chromatin structure. Using human papillomavirus-related oropharyngeal carcinoma as a model, we show aberrant enrichment of repressive H3K9me3 at the transcriptional start site (TSS) with methylation-associated, tumor-specific gene silencing. Further analysis identifies a hypermethylated subtype which shows a functional convergence on MYC targets and association with CREBBP/EP300 mutation. The tumor-specific shift to transcriptional repression associated with DNA methylation at TSSs was confirmed in multiple tumor types. Our data may show a common underlying epigenetic dysregulation in cancer associated with broad enrichment of repressive chromatin marks and aberrant DNA hypermethylation at TSSs in combination with MYC network activation
Generality of shear thickening in suspensions
Suspensions are of wide interest and form the basis for many smart fluids.
For most suspensions, the viscosity decreases with increasing shear rate, i.e.
they shear thin. Few are reported to do the opposite, i.e. shear thicken,
despite the longstanding expectation that shear thickening is a generic type of
suspension behavior. Here we resolve this apparent contradiction. We
demonstrate that shear thickening can be masked by a yield stress and can be
recovered when the yield stress is decreased below a threshold. We show the
generality of this argument and quantify the threshold in rheology experiments
where we control yield stresses arising from a variety of sources, such as
attractions from particle surface interactions, induced dipoles from applied
electric and magnetic fields, as well as confinement of hard particles at high
packing fractions. These findings open up possibilities for the design of smart
suspensions that combine shear thickening with electro- or magnetorheological
response.Comment: 11 pages, 9 figures, accepted for publication in Nature Material
Should the existing science of teams be applied to fluid teams? An exploration of fluid team effectiveness within the context of healthcare simulation
IntroductionFluid teams have become increasingly prevalent and necessary for modern-day issues, yet they differ from more traditional teams, on which much of the current teams literature is based. For example, fluid teams are often comprised of members from different disciplines or organizational divisions who do not have a shared history or future, as they come together to perform a critical, time-sensitive task, and then disband. For these reasons, the mechanisms through which they function and perform may differ from those of more traditional teams, and research is needed to better understand these differences.MethodsTo this end, this study utilized critical incident techniques and thematic analysis to examine fluid teams within healthcare, one of the primary contexts in which they are prevalent. Interdisciplinary faculty and students in the medical field who encounter fluid teams within simulation-based education were prompted to reflect on key factors that facilitate or hinder fluid team effectiveness.ResultsPrimary themes extracted pertained to the conditions fluid teams operate within (e.g., high-stress), the behaviors and emergent states that contribute to their success (e.g., communication), and the KSAO’s of value for members of fluid teams to possess (e.g., readiness). These themes were then compared to existing literature, yielding the identification of some similarities but also many important differences between fluid and traditional teams.DiscussionA series of practical recommendations for how to promote fluid team effectiveness is then presented
Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
The nearby extrasolar planet GJ 436b--which has been labelled as a 'hot
Neptune'--reveals itself by the dimming of light as it crosses in front of and
behind its parent star as seen from Earth. Respectively known as the primary
transit and secondary eclipse, the former constrains the planet's radius and
mass, and the latter constrains the planet's temperature and, with measurements
at multiple wavelengths, its atmospheric composition. Previous work using
transmission spectroscopy failed to detect the 1.4-\mu m water vapour band,
leaving the planet's atmospheric composition poorly constrained. Here we report
the detection of planetary thermal emission from the dayside of GJ 436b at
multiple infrared wavelengths during the secondary eclipse. The best-fit
compositional models contain a high CO abundance and a substantial methane
(CH4) deficiency relative to thermochemical equilibrium models for the
predicted hydrogen-dominated atmosphere. Moreover, we report the presence of
some H2O and traces of CO2. Because CH4 is expected to be the dominant
carbon-bearing species, disequilibrium processes such as vertical mixing and
polymerization of methane into substances such as ethylene may be required to
explain the hot Neptune's small CH4-to-CO ratio, which is at least 10^5 times
smaller than predicted
Response Inhibition and Error Monitoring during a Visual Go/No-Go Task in Inuit Children Exposed to Lead, Polychlorinated Biphenyls, and Methylmercury
Background: Lead (Pb) and polychlorinated biphenyls (PCBs) are neurotoxic contaminants that have been related to impairment in response inhibition
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