9,457 research outputs found
Quantum Mechanical Corrections to the Schwarzschild Black Hole Metric
Motivated by quantum mechanical corrections to the Newtonian potential, which
can be translated into an -correction to the component of the
Schwarzschild metric, we construct a quantum mechanically corrected metric
assuming . We show how the Bekenstein black hole entropy
receives its logarithmic contribution provided the quantum mechanical
corrections to the metric are negative. In this case the standard horizon at
the Schwarzschild radius increases by small terms proportional to
and a remnant of the order of Planck mass emerges. We contrast these results
with a positive correction to the metric which, apart from a corrected
Schwarzschild horizon, leads to a new purely quantum mechanical horizon.Comment: 14 pages Latex, enlarged version as compared to the published on
The Persistence of Memory, or How the X-Ray Spectrum of SNR 0509-67.5 Reveals the Brightness of its Parent Type Ia Supernova
We examine the dynamics and X-ray spectrum of the young Type Ia supernova
remnant 0509-67.5 in the context of the recent results obtained from the
optical spectroscopy of its light echo. Our goal is to estimate the kinetic
energy of the supernova explosion using Chandra and XMM-Newton observations of
the supernova remnant, thus placing the birth event of 0509-67.5 in the
sequence of dim to bright Type Ia supernovae. We base our analysis on a
standard grid of one-dimensional delayed detonation explosion models, together
with hydrodynamic and X-ray spectral calculations of the supernova remnant
evolution. From the remnant dynamics and the properties of the O, Si, S, and Fe
emission in its X-ray spectrum we conclude that 0509-67.5 was originated ~400
years ago by a bright, highly energetic Type Ia explosion similar to SN 1991T.
Our best model has a kinetic energy of 1.4x10E51 erg and synthesizes 0.97 Msun
of 56Ni. These results are in excellent agreement with the age estimate and
spectroscopy from the light echo. We have thus established the first connection
between a Type Ia supernova and its supernova remnant based on a detailed
quantitative analysis of both objects.Comment: 10 pages, 9 figures, plus an exclusive astro-ph-only Appendix; ApJ in
press, companion paper to Rest et al. 0
Science teachers' views of creating and teaching Big Ideas of science education: experiences from Chile
Background: There is a growing view that âBig Ideas of science educationâ are useful for teaching science but there is not much knowledge of how teachers work with them. / Purpose: This study explores the conceptualisation and practice of the use of Big Ideas of science education by primary and secondary teachers in Chile. / Sample: A total of 63 science teachers (a purposive sample) from pre-school, primary and secondary education in ValparaĂso Region in Chile participated in the study, with 38 of them answering all the questions in the research instrument and 25 answering some of them. / Design and methods: The research instrument was a questionnaire with open-ended questions. / Results: The use of Big Ideas was seen as the ânatural wayâ to teach science, mostly related to the studentsâ daily lives. Many of the teachers had their own understanding of Big Ideas. They were very positive about Big Ideas, seeing them as a possible way of connecting with the daily lives of students and facilitating progression in studentsâ learning of science. The teachers also saw Big Ideas as enabling students to work collaboratively and make links between different parts of the curriculum, helping them to understand how science works, and preferable to having to teach an overloaded science curriculum that lacks such an organising framework. / Conclusion: The teachers were more interested in their own creation and development of Big Ideas rather than simply adopting the existing, official published framework and adhering to what is said in the Chilean curriculum regarding the approach of Big Ideas. These results indicate the need to explore in depth such varied conceptualisations of schoolteachers regarding the approach of Big Ideas. In turn, this can offer empirical insights into the way Big Ideas are treated in policy documents in Chile and elsewhere
Bolometric light curves of supernovae and post-explosion magnetic fields
The various effects leading to diversity in the bolometric light curves of
supernovae are examined: nucleosynthesis, kinematic differences, ejected mass,
degree of mixing, and configuration and intensity of the magnetic field are
discussed. In Type Ia supernovae, a departure in the bolometric light curve
from the full-trapping decline of Co can occur within the two and a half
years after the explosion, depending on the evolutionary path followed by the
WD during the accretion phase. If convection has developed in the WD core
during the presupernova evolution, starting several thousand years before the
explosion, a tangled magnetic field close to the equipartition value should
have grown in the WD. Such an intense magnetic field would confine positrons
where they originate from the Co decays, and preclude a strong departure
from the full-trapping decline, as the supernova expands. This situation is
expected to occur in C+O Chandrasekhar WDs as opposed to edge-lit detonated
sub-Chandrasekhar WDs. If the pre-explosion magnetic field of the WD is less
intense than 10G, a lack of confinement of the positrons emitted in the
Co decay and a departure from full-trapping decline would occur. The
time at which it takes place can provide estimates of the original magnetic
field of the WD, its configuration, and also of the mass of the supernova
ejecta. In SN 1991bg, the bolometric light curve suggests absence of a
significant tangled magnetic field (intensity lower than G).
Chandrasekhar-mass models do not reproduce the bolometric light curve of this
supernova. For SN 1972E, on the contrary, there is evidence for a tangled
configuration of the magnetic field and its light curve is well reproduced by a
Chandrasekhar WD explosion.Comment: 54 pages, including 8 figures. To appear in Ap
Type Ia Supernova Scenarios and the Hubble Sequence
The dependence of the Type Ia supernova (SN Ia) rate on galaxy type is
examined for three currently proposed scenarios: merging of a
Chandrasekhar--mass CO white dwarf (WD) with a CO WD companion, explosion of a
sub--Chandrasekhar mass CO WD induced by accretion of material from a He star
companion, and explosion of a sub--Chandrasekhar CO WD in a symbiotic system.
The variation of the SNe Ia rate and explosion characteristics with time is
derived, and its correlation with parent population age and galaxy redshift is
discussed. Among current scenarios, CO + He star systems should be absent from
E galaxies. Explosion of CO WDs in symbiotic systems could account for the SNe
Ia rate in these galaxies. The same might be true for the CO + CO WD scenario,
depending on the value of the common envelope parameter. A testable prediction
of the sub--Chandrasekhar WD model is that the average brightness and kinetic
energy of the SN Ia events should increase with redshift for a given Hubble
type. Also for this scenario, going along the Hubble sequence from E to Sc
galaxies SNe Ia events should be brighter on average and should show larger
mean velocities of the ejecta. The observational correlations strongly suggest
that the characteristics of the SNe Ia explosion are linked to parent
population age. The scenario in which WDs with masses below the Chandrasekhar
mass explode appears the most promising one to explain the observed variation
of the SN Ia rate with galaxy type together with the luminosity--expansion
velocity trend.Comment: 16 pages uuencoded compressed Postscript, 2 figures included. ApJ
Letters, in pres
Resonant transmission of light through finite chains of subwavelength holes
In this paper we show that the extraordinary optical transmission phenomenon
found before in 2D hole arrays is already present in a linear chain of
subwavelength holes, which can be considered as the basic geometrical unit
showing this property. In order to study this problem we have developed a new
theoretical framework, able to analyze the optical properties of finite
collections of subwavelength apertures and/or dimples (of any shape and placed
in arbitrary positions) drilled in a metallic film.Comment: Accepted for publication in Phys. Rev. Let
Dominant negative phenotype of Bacillus thuringiensis Cry1Ab, Cry11Aa and Cry4Ba mutants suggest hetero-oligomer formation among different Cry toxins.
Background - Bacillus thuringiensis Cry toxins are used worldwide in the control of different insect pests important in agriculture or in human health. The Cry proteins are pore-forming toxins that affect the midgut cell of target insects. It was shown that non-toxic Cry1Ab helix a-4 mutants had a dominant negative (DN) phenotype inhibiting the toxicity of wildtype Cry1Ab when used in equimolar or sub-stoichiometric ratios (1:1, 0.5:1, mutant:wt) indicating that oligomer formation is a key step in toxicity of Cry toxins. Methodology/Principal Findings - The DN Cry1Ab-D136N/T143D mutant that is able to block toxicity of Cry1Ab toxin, was used to analyze its capacity to block the activity against Manduca sexta larvae of other Cry1 toxins, such as Cry1Aa, Cry1Ac, Cry1Ca, Cry1Da, Cry1Ea and Cry1Fa. Cry1Ab-DN mutant inhibited toxicity of Cry1Aa, Cry1Ac and Cry1Fa. In addition, we isolated mutants in helix a-4 of Cry4Ba and Cry11Aa, and demonstrate that Cry4Ba-E159K and Cry11Aa-V142D are inactive and completely block the toxicity against Aedes aegypti of both wildtype toxins, when used at sub-stoichiometric ratios, confirming a DN phenotype. As controls we analyzed Cry1Ab-R99A or Cry11Aa-E97A mutants that are located in helix a-3 and are affected in toxin oligomerization. These mutants do not show a DN phenotype but were able to block toxicity when used in 10:1 or 100:1 ratios (mutant:wt) probably by competition of binding with toxin receptors. Conclusions/Significance - We show that DN phenotype can be observed among different Cry toxins suggesting that may interact in vivo forming hetero-oligomers. The DN phenotype cannot be observed in mutants affected in oligomerization, suggesting that this step is important to inhibit toxicity of other toxin
Composition of the Innermost Core Collapse Supernova Ejecta
With presently known input physics and computer simulations in 1D, a
self-consistent treatment of core collapse supernovae does not yet lead to
successful explosions, while 2D models show some promise. Thus, there are
strong indications that the delayed neutrino mechanism works combined with a
multi-D convection treatment for unstable layers. On the other hand there is a
need to provide correct nucleosynthesis abundances for the progressing field of
galactic evolution and observations of low metallicity stars. The innermost
ejecta is directly affected by the explosion mechanism, i.e. most strongly the
yields of Fe-group nuclei for which an induced piston or thermal bomb treatment
will not provide the correct yields because the effect of neutrino interactions
is not included. We apply parameterized variations to the neutrino scattering
cross sections and alternatively, parameterized variations are applied to the
neutrino absorption cross sections on nucleons in the ``gain region''. We find
that both measures lead to similar results, causing explosions and a Ye>0.5 in
the innermost ejected layers, due to the combined effect of a short weak
interaction time scale and a negligible electron degeneracy, unveiling the
proton-neutron mass difference. We include all weak interactions (electron and
positron capture, beta-decay, neutrino and antineutrino capture on nuclei, and
neutrino and antineutrino capture on nucleons) and present first
nucleosynthesis results for these innermost ejected layers to discuss how they
improve predictions for Fe-group nuclei. The proton-rich environment results in
enhanced abundances of 45Sc, 49Ti, and 64Zn as requested by chemical evolution
studies and observations of low metallicity stars as well as appreciable
production of nuclei in the mass range up to A=80.Comment: 13 pages, 8 figures. Final versio
Are interventions effective at improving driving in older drivers?: A systematic review
Background With the aging of the population, the number of older drivers is on the rise. This poses significant challenges for public health initiatives, as older drivers have a relatively higher risk for collisions. While many studies focus on developing screening tools to identify medically at-risk drivers, little research has been done to develop training programs or interventions to promote, maintain or enhance driving-related abilities among healthy individuals. The purpose of this systematic review is to synopsize the current literature on interventions that are tailored to improve driving in older healthy individuals by working on components of safe driving such as: self-awareness, knowledge, behaviour, skills and/or reducing crash/collision rates in healthy older drivers. Methods Relevant databases such as Scopus and PubMed databases were selected and searched for primary articles published in between January 2007 and December 2017. Articles were identified using MeSH search terms: ("safety" OR "education" OR "training" OR "driving" OR "simulator" OR "program" OR "countermeasures") AND ("older drivers" OR "senior drivers" OR "aged drivers" OR "elderly drivers"). All retrieved abstracts were reviewed, and full texts printed if deemed relevant. Results Twenty-five (25) articles were classified according to: 1) Classroom settings; 2) Computer-based training for cognitive or visual processing; 3) Physical training; 4) In-simulator training; 5) On-road training; and 6) Mixed interventions. Results show that different types of approaches have been successful in improving specific driving skills and/or behaviours. However, there are clear discrepancies on how driving performance/behaviours are evaluated between studies, both in terms of methods or dependent variables, it is therefore difficult to make direct comparisons between these studies. Conclusions This review identified strong study projects, effective at improving older drivers' performance and thus allowed to highlight potential interventions that can b- (undefined
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