7,690 research outputs found
CCD photometry of 2060 Chiron, 1991 January
Observations of 2060 Chiron was performed on 7 to 8 Jan. 1991 with the Mt. Palomar 1.52 m telescope in the Gunn-R passband. On-chip field stars were used to perform differential reductions. The repeatability of the 5.9 hour light curve was excellent, both within a night and from night to night. No evidence for short-term secular variations similar to those seen last year by both Luu and Jewitt (1990) and Buratti and Dunbar (1991) is seen in the new light curve. Chiron's rotational light curve appears strikingly similar to that obtained a year earlier by Luu and Jewitt (1990), both in amplitude and shape. Both light curves show strongly correlated changes over a timescale of perhaps 15 minutes. These same features were marginally visible in the 1986 light curve. Such behavior is believed to be evidence that Chiron may be more aspherical than the 4 percent intensity variation might otherwise indicate, and favors a viewing geometry where the subearth latitude is rather low. Chiron was much fainter in 1985, when a partial light curve was obtained by Marcialis. Due to the lower sampling rate of these early data, no conclusions can be made regarding the high-frequency light curve structure back then. All three of these light curves differ significantly from that obtained by Buratti and Dunbar (1991), one week before the observations of Luu and Jewitt. The Chiron field was calibrated using Landolt standards on Ut 15 Mar. 1991. A mean R-magnitude of 15.6 + or - 0.1 was found. Variability of 2060 Chiron was demonstrated over timescales of minutes, hours, and years. An intense campaign was urged to monitor the photometric behavior of Chiron throughout the 1990s
Extracellular ATP inhibits twitching motility-mediated biofilm expansion by Pseudomonas aeruginosa
© 2015 Nolan et al.; licensee BioMed Central. Background: Pseudomonas aeruginosa is an opportunistic pathogen that exploits damaged epithelia to cause infection. Type IV pili (tfp) are polarly located filamentous structures which are the major adhesins for attachment of P. aeruginosa to epithelial cells. The extension and retraction of tfp powers a mode of surface translocation termed twitching motility that is involved in biofilm development and also mediates the active expansion of biofilms across surfaces. Extracellular adenosine triphosphate (eATP) is a key "danger" signalling molecule that is released by damaged epithelial cells to alert the immune system to the potential presence of pathogens. As P. aeruginosa has a propensity for infecting damaged epithelial tissues we have explored the influence of eATP on tfp biogenesis and twitching motility-mediated biofilm expansion by P. aeruginosa. Results: In this study we have found that eATP inhibits P. aeruginosa twitching motility-mediated expansion of interstitial biofilms at levels that are not inhibitory to growth. We have determined that eATP does not inhibit expression of the tfp major subunit, PilA, but reduces the levels of surface assembled tfp. We have also determined that the active twitching zone of expanding P. aeruginosa interstitial biofilms contain large quantities of eATP which may serve as a signalling molecule to co-ordinate cell movements in the expanding biofilm. The inhibition of twitching motility-mediated interstitial biofilm expansion requires eATP hydrolysis and does not appear to be mediated by the Chp chemosensory system. Conclusions: Endogenous eATP produced by P. aeruginosa serves as a signalling molecule to co-ordinate complex multicellular behaviours of this pathogen. Given the propensity for P. aeruginosa to infect damaged epithelial tissues, our observations suggest that eATP released by damaged cells may provide a cue to reduce twitching motility of P. aeruginosa in order to establish infection at the site of damage. Furthermore, eATP produced by P. aeruginosa biofilms and by damaged epithelial cells may play a role in P. aeruginosa pathogenesis by inducing inflammatory damage and fibrosis. Our findings have significant implications in the development and pathogenesis of P. aeruginosa biofilm infections
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Approval Ratings of Inpatient Coercive Interventions in a National Sample of Mental Health Service Users and Staff in England
Objective: To ascertain the degree of approval amongst service users and staff for various coercive measures commonly used in acute mental health care.
Methods: A cross-sectional design was adopted. The Attitudes to Containment Measures Questionnaire (ACMQ) was completed by 1,361 service users and 1,226 staff in acute care mental health services from three regions of England. This provided evaluation of eleven coercive measures (e.g. seclusion) on six dimensions of approval (e.g. indignity, safety) in a large national sample. Comparisons between groups were tested using independent samples t-tests, χ2 or Spearman correlations.
Results: Both service users and staff disapproved strongly of mechanical restraint and expressed a relative preference for compulsory intramuscular medication and seclusion. Male staff, older service users and staff who had been involved in implementing coercion expressed greater approval of coercive measures.
Conclusion: Mechanical restraint remains highly objectionable to staff and service users in English mental health services despite its widespread acceptance elsewhere in the world
The Gamma Ray Pulsar Population
We apply a likelihood analysis to pulsar detections, pulsar upper limits, and
diffuse background measurements from the OSSE and EGRET instruments on the
Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray
pulsars and some properties of the gamma-ray pulsar population. We find that
the dependence of luminosity on spin period and dipole magnetic field is much
steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively),
suggesting that different emission mechanisms are responsible for low- and
high-energy gamma-ray emission. Incorporating a spin-down model and assuming a
pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray
background due to unidentified pulsars and find that pulsars may be an
important component of the OSSE diffuse flux, but are most likely not important
at EGRET energies. Using measurements of the diffuse background flux from these
instruments, we are able to place constraints on the braking index, initial
spin period, and magnetic field of the Galactic pulsar population. We are also
able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr).
Our results are based on a large gamma-ray beam, but they do not scale in a
simple way with beam size. With our assumed beam size, the implied gamma-ray
efficiency for the EGRET detections is no more than 20%. We estimate that about
20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use
our model to predict the pulsar population that will be seen by future
gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray
pulsars as steady sources, only 120 of which are currently known radio pulsars.Comment: 32 pages, including figures. submitted to Ap
Bulk and surface energetics of lithium hydride crystal: benchmarks from quantum Monte Carlo and quantum chemistry
We show how accurate benchmark values of the surface formation energy of
crystalline lithium hydride can be computed by the complementary techniques of
quantum Monte Carlo (QMC) and wavefunction-based molecular quantum chemistry.
To demonstrate the high accuracy of the QMC techniques, we present a detailed
study of the energetics of the bulk LiH crystal, using both pseudopotential and
all-electron approaches. We show that the equilibrium lattice parameter agrees
with experiment to within 0.03 %, which is around the experimental uncertainty,
and the cohesive energy agrees to within around 10 meV per formula unit. QMC in
periodic slab geometry is used to compute the formation energy of the LiH (001)
surface, and we show that the value can be accurately converged with respect to
slab thickness and other technical parameters. The quantum chemistry
calculations build on the recently developed hierarchical scheme for computing
the correlation energy of a crystal to high precision. We show that the
hierarchical scheme allows the accurate calculation of the surface formation
energy, and we present results that are well converged with respect to basis
set and with respect to the level of correlation treatment. The QMC and
hierarchical results for the surface formation energy agree to within about 1
%.Comment: 16 pages, 4 figure
Solving the time-dependent Schr\"odinger equation with absorbing boundary conditions and source terms in Mathematica 6.0
In recent decades a lot of research has been done on the numerical solution
of the time-dependent Schr\"odinger equation. On the one hand, some of the
proposed numerical methods do not need any kind of matrix inversion, but source
terms cannot be easily implemented into this schemes; on the other, some
methods involving matrix inversion can implement source terms in a natural way,
but are not easy to implement into some computational software programs widely
used by non-experts in programming (e.g. Mathematica). We present a simple
method to solve the time-dependent Schr\"odinger equation by using a standard
Crank-Nicholson method together with a Cayley's form for the finite-difference
representation of evolution operator. Here, such standard numerical scheme has
been simplified by inverting analytically the matrix of the evolution operator
in position representation. The analytical inversion of the N x N matrix let us
easily and fully implement the numerical method, with or without source terms,
into Mathematica or even into any numerical computing language or computational
software used for scientific computing.Comment: 15 pages, 7 figure
The Central Singularity in Spherical Collapse
The gravitational strength of the central singularity in spherically
symmetric space-times is investigated. Necessary conditions for the singularity
to be gravitationally weak are derived and it is shown that these are violated
in a wide variety of circumstances. These conditions allow conclusions to be
drawn about the nature of the singularity without having to integrate the
geodesic equations. In particular, any geodesic with a non-zero amount of
angular momentum which impinges on the singularity terminates in a strong
curvature singularity.Comment: 17 pages; revised and corrected with improved result
A Characterisation of Strong Wave Tails in Curved Space-Times
A characterisation of when wave tails are strong is proposed. The existence
of a curvature induced tail (i.e. a Green's function term whose support
includes the interior of the light-cone) is commonly understood to cause
backscattering of the field governed by the relevant wave equation. Strong
tails are characterised as those for which the purely radiative part of the
field is backscattered. With this definition, it is shown that electromagnetic
waves in asymptotically flat space-times and fields governed by tail-free
propagation have weak tails, but minimally coupled scalar fields in a
cosmological scenario have strong tails.Comment: 17 pages, Revtex, to appear in Classical and Quantum Gravit
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