49,027 research outputs found
DNA nano-mechanics: how proteins deform the double helix
It is a standard exercise in mechanical engineering to infer the external
forces and torques on a body from its static shape and known elastic
properties. Here we apply this kind of analysis to distorted double-helical DNA
in complexes with proteins. We extract the local mean forces and torques acting
on each base-pair of bound DNA from high-resolution complex structures. Our
method relies on known elastic potentials and a careful choice of coordinates
of the well-established rigid base-pair model of DNA. The results are robust
with respect to parameter and conformation uncertainty. They reveal the complex
nano-mechanical patterns of interaction between proteins and DNA. Being
non-trivially and non-locally related to observed DNA conformations, base-pair
forces and torques provide a new view on DNA-protein binding that complements
structural analysis.Comment: accepted for publication in JCP; some minor changes in response to
review 18 pages, 5 figure + supplement: 4 pages, 3 figure
Search for magnetic fields in particle-accelerating colliding-wind binaries
Some colliding-wind massive binaries, called particle-accelerating
colliding-wind binaries (PACWB), exhibit synchrotron radio emission, which is
assumed to be generated by a stellar magnetic field. However, no measurement of
magnetic fields in these stars has ever been performed. We aim at quantifying
the possible stellar magnetic fields present in PACWB to provide constraints
for models. We gathered 21 high-resolution spectropolarimetric observations of
9 PACWB available in the ESPaDOnS, Narval and HarpsPol archives. We analysed
these observations with the Least Squares Deconvolution method. We separated
the binary spectral components when possible. No magnetic signature is detected
in any of the 9 PACWB stars and all longitudinal field measurements are
compatible with 0 G. We derived the upper field strength of a possible field
that could have remained hidden in the noise of the data. While the data are
not very constraining for some stars, for several stars we could derive an
upper limit of the polar field strength of the order of 200 G. We can therefore
exclude the presence of strong or moderate stellar magnetic fields in PACWB,
typical of the ones present in magnetic massive stars. Weak magnetic fields
could however be present in these objects. These observational results provide
the first quantitative constraints for future models of PACWB.Comment: Accepted in A&
A Note on Flux Induced Superpotentials in String Theory
Non-vanishing fluxes in M-theory and string theory compactifications induce a
superpotential in the lower dimensional theory. Gukov has conjectured the
explicit form of this superpotential. We check this conjecture for the
heterotic string compactified on a Calabi-Yau three-fold as well as for warped
M-theory compactifications on Spin(7) holonomy manifolds, by performing a
Kaluza-Klein reduction.Comment: 19 pages, no figure
Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature?
In this study we focus on the sea level trend pattern observed by satellite altimetry in the tropical Pacific over the 1993–2009 time span (i.e. 17 yr). Our objective is to investigate whether this 17-yr-long trend pattern was different before the altimetry era, what was its spatio-temporal variability and what have been its main drivers. We try to discriminate the respective roles of the internal variability of the climate system and of external forcing factors, in particular anthropogenic emissions (greenhouse gases and aerosols). On the basis of a 2-D past sea level reconstruction over 1950–2009 (based on a combination of observations and ocean modelling) and multi-century control runs (i.e. with constant, preindustrial external forcing) from eight coupled climate models, we have investigated how the observed 17-yr sea level trend pattern evolved during the last decades and centuries, and try to estimate the characteristic time scales of its variability. For that purpose, we have computed sea level trend patterns over successive 17-yr windows (i.e. the length of the altimetry record), both for the 60-yr long reconstructed sea level and the model runs. We find that the 2-D sea level reconstruction shows spatial trend patterns similar to the one observed during the altimetry era. The pattern appears to have fluctuated with time with a characteristic time scale of the order of 25–30 yr. The same behaviour is found in multi-centennial control runs of the coupled climate models. A similar analysis is performed with 20th century coupled climate model runs with complete external forcing (i.e. solar plus volcanic variability and changes in anthropogenic forcing). Results suggest that in the tropical Pacific, sea level trend fluctuations are dominated by the internal variability of the ocean–atmosphere coupled system. While our analysis cannot rule out any influence of anthropogenic forcing, it concludes that the latter effect in that particular region is stillhardly detectable
Shear viscosity of a highly excited string and the black hole membrane paradigm
Black hole membrane paradigm states that a certain viscous membrane seems to
be sitting on a stretched horizon of a black hole from the viewpoint of a
distant observer. We show that the shear viscosity of the fictitious membrane
can be reproduced by a highly excited string covering the stretched horizon
except for a numerical coefficient.Comment: 22 pages, no figure, minor correction
Quantum Gravity Corrections for Schwarzschild Black Holes
We consider the Matrix theory proposal describing eleven-dimensional
Schwarzschild black holes. We argue that the Newtonian potential between two
black holes receives a genuine long range quantum gravity correction, which is
finite and can be computed from the supergravity point of view. The result
agrees with Matrix theory up to a numerical factor which we have not computed.Comment: 14 pages, Tex, no figure
The long period eccentric orbit of the particle accelerator HD167971 revealed by long baseline interferometry
Using optical long baseline interferometry, we resolved for the first time
the two wide components of HD167971, a candidate hierarchical triple system
known to efficiently accelerate particles. Our multi-epoch VLTI observations
provide direct evidence for a gravitational link between the O8 supergiant and
the close eclipsing O + O binary. The separation varies from 8 to 15 mas over
the three-year baseline of our observations, suggesting that the components
evolve on a wide and very eccentric orbit (most probably e>0.5). These results
provide evidence that the wide orbit revealed by our study is not coplanar with
the orbit of the inner eclipsing binary. From our measurements of the
near-infrared luminosity ratio, we constrain the spectral classification of the
components in the close binary to be O6-O7, and confirm that these stars are
likely main-sequence objects. Our results are discussed in the context of the
bright non-thermal radio emission already reported for this system, and we
provide arguments in favour of a maximum radio emission coincident with
periastron passage. HD167971 turns out to be an efficient O-type particle
accelerator that constitutes a valuable target for future high angular
resolution radio imaging using VLBI facilities.Comment: 8 pages, including 4 figures, accepted by Monthly Notices of the
Royal Astronomical Societ
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