6,377 research outputs found
Blocking a wave: Frequency band gaps in ice shelves with periodic crevasses
We assess how the propagation of high-frequency elastic-flexural waves
through an ice shelf is modified by the presence of spatially periodic
crevasses. Analysis of the normal modes supported by the ice shelf with and
without crevasses reveals that a periodic crevasse distribution qualitatively
changes the mechanical response. The normal modes of an ice shelf free of
crevasses are evenly distributed as a function of frequency. In contrast, the
normal modes of a crevasse-ridden ice shelf are distributed unevenly. There are
"band gaps", frequency ranges over which no eigenmodes exist. A model ice shelf
that is 50 km in lateral extent and 300 m thick with crevasses spaced 500 m
apart has a band gap from 0.2 to 0.38 Hz. This is a frequency range relevant
for ocean wave/ice-shelf interactions. When the outermost edge of the crevassed
ice shelf is oscillated at a frequency within the band gap, the ice shelf
responds very differently from a crevasse-free ice shelf. The flexural motion
of the crevassed ice shelf is confined to a small region near the outermost
edge of the ice shelf and effectively "blocked" from reaching the interior.Comment: 6 pages, 4 figures, accepted to Annals of Glaciolog
Vibronic coupling in the superoxide anion: The vibrational dependence of the photoelectron angular distribution
We present a comprehensive photoelectron imaging study of the O₂(X³Σg⁻,v′=0–6)←O₂⁻(X²Πg,v′′=0) and O₂(a¹Δg,v′=0–4)←O₂⁻(X²Πg,v′′=0)photodetachment bands at wavelengths between 900 and 455 nm, examining the effect of vibronic coupling on the photoelectron angular distribution (PAD). This work extends the v′=1–4 data for detachment into the ground electronic state, presented in a recent communication [R. Mabbs, F. Mbaiwa, J. Wei, M. Van Duzor, S. T. Gibson, S. J. Cavanagh, and B. R. Lewis, Phys. Rev. A82, 011401–R (2010)]. Measured vibronic intensities are compared to Franck–Condon predictions and used as supporting evidence of vibronic coupling. The results are analyzed within the context of the one-electron, zero core contribution (ZCC) model [R. M. Stehman and S. B. Woo, Phys. Rev. A23, 2866 (1981)]. For both bands, the photoelectron anisotropy parameter variation with electron kinetic energy,β(E), displays the characteristics of photodetachment from a d-like orbital, consistent with the π∗g 2p highest occupied molecular orbital of O₂⁻. However, differences exist between the β(E) trends for detachment into different vibrational levels of the X³Σg⁻ and a ¹Δg electronic states of O₂. The ZCC model invokes vibrational channel specific “detachment orbitals” and attributes this behavior to coupling of the electronic and nuclear motion in the parent anion. The spatial extent of the model detachment orbital is dependent on the final state of O₂: the higher the neutral vibrational excitation, the larger the electron binding energy. Although vibronic coupling is ignored in most theoretical treatments of PADs in the direct photodetachment of molecular anions, the present findings clearly show that it can be important. These results represent a benchmark data set for a relatively simple system, upon which to base rigorous tests of more sophisticated models.The authors gratefully acknowledge support by the
National Science Foundation Grant No. CHE-0748738 and
ANU ARC Discovery Projects under Grant Nos. DP0666267
and DP0880850
The Formation of Crystalline Dust in AGB Winds from Binary Induced Spiral Shocks
As stars evolve along the Asymptotic Giant Branch, strong winds are driven
from the outer envelope. These winds form a shell, which may ultimately become
a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at
the presence of a binary companion. Some post-Asymptotic Giant Branch objects
are surrounded by torii of crystalline dust, but there is no generally accepted
mechanism for annealing the amorphous grains in the wind to crystals. In this
Letter, we show that the shaping of the wind by a binary companion is likely to
lead to the formation of crystalline dust in the orbital plane of the binary.Comment: Submitted to ApJ
Dynamic real-time risk analytics of uncontrollable states in complex internet of things systems, cyber risk at the edge
The Internet of Things (IoT) triggers new types of cyber risks. Therefore,
the integration of new IoT devices and services requires a self-assessment of
IoT cyber security posture. By security posture this article refers to the
cybersecurity strength of an organisation to predict, prevent and respond to
cyberthreats. At present, there is a gap in the state of the art, because there
are no self-assessment methods for quantifying IoT cyber risk posture. To
address this gap, an empirical analysis is performed of 12 cyber risk
assessment approaches. The results and the main findings from the analysis is
presented as the current and a target risk state for IoT systems, followed by
conclusions and recommendations on a transformation roadmap, describing how IoT
systems can achieve the target state with a new goal-oriented dependency model.
By target state, we refer to the cyber security target that matches the generic
security requirements of an organisation. The research paper studies and adapts
four alternatives for IoT risk assessment and identifies the goal-oriented
dependency modelling as a dominant approach among the risk assessment models
studied. The new goal-oriented dependency model in this article enables the
assessment of uncontrollable risk states in complex IoT systems and can be used
for a quantitative self-assessment of IoT cyber risk posture
Kepler Observations of the Three Pre-Launch Exoplanet Candidates: Discover of Two Eclipsing Binaries and a New Exoplanet
Three transiting exoplanet candidate stars were discovered in a ground-based photometric survey prior to the launch of NASA's Kepler mission. Kepler observations of them were obtained during Quarter 1 of the Kepler mission. All three stars are faint by radial velocity follow-up standards, so we have examined these candidates with regard to eliminating false positives and providing high confidence exoplanet selection. We present a first attempt to exclude false positives for this set of faint stars without high-resolution radial velocity analysis. This method of exoplanet confirmation will form a large part of the Kepler mission follow-up for Jupiter-sized exoplanet candidates orbiting faint stars. Using the Kepler light curves and pixel data, as well as medium-resolution reconnaissance spectroscopy and speckle imaging, we find that two of our candidates are binary stars. One consists of a late-F star with an early M companion, while the other is a K0 star plus a late M-dwarf/brown dwarf in a 19 day elliptical orbit. The third candidate (BOKS-1) is an r = 15 G8V star hosting a newly discovered exoplanet with a radius of 1.12 R_(Jupiter) in a 3.9 day orbit
Improving formaldehyde consumption drives methanol assimilation in engineered E. coli
Due to volatile sugar prices, the food vs fuel debate, and recent increases in the supply of natural gas, methanol has emerged as a promising feedstock for the bio-based economy. However, attempts to engineer Escherichia coli to metabolize methanol have achieved limited success. Here, we provide a rigorous systematic analysis of several potential pathway bottlenecks. We show that regeneration of ribulose 5-phosphate in E. coli is insufficient to sustain methanol assimilation, and overcome this by activating the sedoheptulose bisphosphatase variant of the ribulose monophosphate pathway. By leveraging the kinetic isotope effect associated with deuterated methanol as a chemical probe, we further demonstrate that under these conditions overall pathway flux is kinetically limited by methanol dehydrogenase. Finally, we identify NADH as a potent kinetic inhibitor of this enzyme. These results provide direction for future engineering strategies to improve methanol utilization, and underscore the value of chemical biology methodologies in metabolic engineering
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