10,925 research outputs found
Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts
Accreting neutron stars (NS) can exhibit high frequency modulations in their
lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The
frequencies can be offset from the spin frequency of the NS by several Hz, and
can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean,
the frequency of which would decrease (in the rotating frame) as the burst
cools, hence explaining the drifts. Most burst oscillations have been observed
during H/He triggered bursts, however there has been one observation of
oscillations during a superburst; hours' long Type I X-ray bursts caused by
unstable carbon burning deeper in the ocean. This paper calculates the
frequency evolution of an oceanic r-mode during a superburst. The rotating
frame frequency varies during the burst from 4-14 Hz, and is sensitive to the
background parameters, in particular the temperature of the ocean and ignition
depth. This calculation is compared to the superburst oscillations observed on
4U-1636-536. The predicted mode frequencies ( 10 Hz) would require a spin
frequency of 592 Hz to match observations; 6 Hz higher than the spin
inferred from an oceanic r-mode model for the H/He triggered burst
oscillations. This model also over-predicts the frequency drift during the
superburst by 90 %.Comment: Accepted for publication in MNRA
PBP4: A New Perspective on Staphylococcus aureus β-Lactam Resistance.
β-lactam antibiotics are excellent drugs for treatment of staphylococcal infections, due to their superior efficacy and safety compared to other drugs. Effectiveness of β-lactams is severely compromised due to resistance, which is widespread among clinical strains of Staphylococcus aureus. β-lactams inhibit bacterial cells by binding to penicillin binding proteins (PBPs), which perform the penultimate steps of bacterial cell wall synthesis. Among PBPs of S. aureus, PBP2a has received the most attention for the past several decades due to its preeminent role in conferring both high-level and broad-spectrum resistance to the entire class of β-lactam drugs. Studies on PBP2a have thus unraveled incredible details of its mechanism of action. We have recently identified that an uncanonical, low molecular weight PBP of S. aureus, PBP4, can also provide high-level and broad-spectrum resistance to the entire class of β-lactam drugs at a level similar to that of PBP2a. The role of PBP4 has typically been considered not so important for β-lactam resistance of S. aureus, and as a result its mode of action remains largely unknown. In this article, we review our current knowledge of PBP4 mediating β-lactam resistance in S. aureus
The onset of low Prandtl number thermal convection in thin spherical shells
This study considers the onset of stress-free Boussinesq thermal convection
in rotating spherical shells with aspect ratio ( and
being the inner and outer radius), Prandtl numbers , and Taylor numbers . We are
particularly interested in the form of the convective cell pattern that
develops, and in its time scales, since this may have observational
consequences. For a fixed and by decreasing from
0.1 to a transition between spiralling columnar (SC) and
equatorially-attached (EA) modes, and a transition between EA and equatorially
antisymmetric or symmetric polar (AP/SP) weakly multicellular modes are found.
The latter modes are preferred at very low . Surprisingly, for the unicellular polar modes become also preferred at
moderate because two new transition curves between EA and
AP/SP and between AP/SP and SC modes are born at a triple-point bifurcation.
The dependence on and of the transitions is studied to
estimate the type of modes, and their critical parameters, preferred at
different stellar regimes.Comment: Accepted for publication in Physical Review Fluids. Contains 17
pages, 8 figures and 3 tables. Added brief erratum correcting values used for
estimates of neutron star ocean viscosit
Polar waves and chaotic flows in thin rotating spherical shells
Convection in rotating spherical geometries is an important physical process
in planetary and stellar systems. Using continuation methods at low Prandtl
number, we find both strong equatorially asymmetric and symmetric polar
nonlinear rotating waves in a model of thermal convection in thin rotating
spherical shells with stress-free boundary conditions. For the symmetric waves
convection is confined to high latitude in both hemispheres but is only
restricted to one hemisphere close to the pole in the case of asymmetric waves.
This is in contrast to what is previously known from studies in the field.
These periodic flows, in which the pattern is rotating steadily in the
azimuthal direction, develop a strong axisymmetric component very close to
onset. Using stability analysis of periodic orbits the regions of stability are
determined and the topology of the stable/unstable oscillatory flows bifurcated
from the branches of rotating waves is described. By means of direct numerical
simulations of these oscillatory chaotic flows, we show that these
three-dimensional convective polar flows exhibit characteristics, such as force
balance or mean physical properties, which are similar to flows occuring in
planetary atmospheres. We show that these results may open a route to
understanding unexplained features of gas giant atmospheres, in particular for
the case of Jupiter. These include the observed equatorial asymmetry with a
pronounced decrease at the equator (the so-called dimple), and the coherent
vortices surrounding the poles recently observed by the Juno mission.Comment: Published in Physical Review Fluids (2019). Contains 2 tables and 8
figure
Children’s semantic and world knowledge overrides fictional information during anticipatory linguistic processing
Using real-time eye-movement measures, we asked how a fantastical discourse context competes with stored representations of semantic and world knowledge to influence children's and adults' moment-by-moment interpretation of a story. Seven-year- olds were less effective at bypassing stored semantic and world knowledge during real-time interpretation than adults. Nevertheless, an effect of discourse context on comprehension was still apparent
Einstein Cluster Alignments Revisited
We have examined whether the major axes of rich galaxy clusters tend to point
toward their nearest neighboring cluster. We have used the data of Ulmer,
McMillan, and Kowalski, who used position angles based on X-ray morphology. We
also studied a subset of this sample with updated positions and distances from
the MX Northern Abell Cluster Survey (for rich clusters () with well
known redshifts). A Kolmogorov-Smirnov (KS) test showed no significant signal
for nonrandom angles on any scale Mpc. However, refining the
null hypothesis with the Wilcoxon rank-sum test, we found a high confidence
signal for alignment. Confidence levels increase to a high of 99.997% as only
near neighbors which are very close are considered. We conclude there is a
strong alignment signal in the data, consistent with gravitational instability
acting on Gaussian perturbations.Comment: Minor revisions. To be published in Ap
How good was the profitability of British Railways, 1870-1912?
This paper provides new estimates of the return on capital employed (ROCE) for major British railway companies. It shows that ROCE was generally below the cost of capital after the mid-1870s and fell till the turn of the century. Addressing issues of cost inefficiency could have restored ROCE to an adequate level in the late 1890s but not in 1910. Declines in ROCE hit share prices and returns to shareholders were negative after 1897. Optimal portfolio analysis shows that, whilst railway securities would have had a substantial weight prior to this date, investors would have been justified in rushing to the exits thereafter
The Short and Long-term Variability of F2 or Stronger (Significant) Tornadoes in the Central Plains
An analysis of the interannual and interdecadal variability of significant tornado events that occurred over a four state region in the central plains was performed over a 53 year period (1950 - 2002) using the Storm Prediction Center (SPC) archives and simple statistical
techniques. A synoptic composite analysis using the National Centers for Environmental
Prediction (NCEP) - National Center for Atmospheric Research (NCAR) re-analyses was
performed in order to determine whether there was support for the statistical relationships through the large-scale composite dynamics. The results showed that when the 53 year raw annual tornado occurrences are used, there was no statistically significant El Nino-related
variability, and El Nino years produced slightly more tornado occurrences. However, when annual tornado occurrences were examined across different phases of the Pacific Decadal Oscillation, there was a likely tendency for more tornado occurrences in El Nino years during PDO2 and no significant interannual variability during PDO1 years. During the 1950 - 1976 period, other studies have found that significant tornado occurrences were overestimated. When
a simple correction factor was applied here and the interannual variability re-examined, the strength of the statistical relationships changed such that there was now a likely tendency for fewer tornado occurrences in El Nino years during the PDO1 period, and no statistically significant relationship for PDO2 years. Finally an examination of the composite dynamics
during the bulk of tornado season revealed that, out of five years examined, the large-scale flows were of similar dynamic character for four of them. The season which produced the most tornadoes qualitatively was shown to be slightly more conducive to severe weather production when using simple empirical severe weather indexes
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