142 research outputs found
Secular evolution of a satellite by tidal effect. Application to Triton
Some of the satellites in the Solar System, including the Moon, appear to
have been captured from heliocentric orbits at some point in their past, and
then have evolved to the present configurations. The exact process of how this
trapping occurred is unknown, but the dissociation of a planetesimal binary in
the gravitational field of the planet, gas drag, or a massive collision seem to
be the best candidates. However, all these mechanisms leave the satellites in
elliptical orbits that need to be damped to the present almost circular ones.
Here we give a complete description of the secular tidal evolution of a
satellite just after entering a bounding state with the planet. In particular,
we take into account the spin evolution of the satellite, which has often been
assumed synchronous in previous studies. We apply our model to Triton and
successfully explain some geophysical properties of this satellite, as well as
the main dynamical features observed for the Neptunian system.Comment: 4 pages, 1 figur
Improved resolution and signal-to-noise ratio in laser-ultrasonics by SAFT processing.
Laser-ultrasonics is an emerging nondestructive technique using lasers for the generation and detection of ultrasound which presents numerous advantages for industrial inspection. In this paper, the problem of detection by laser-ultrasonics of small defects within a material is addressed. Experimental results obtained with laser-ultrasonics are processed using the Synthetic Aperture Focusing Technique (SAFT), yielding improved flaw detectability and spatial resolution. Experiments have been performed on an aluminum sample with a contoured back surface and two flat-bottom holes. Practical interest of coupling SAFT to laser-ultrasonics is also discussed
Arithmetic Spacetime Geometry from String Theory
An arithmetic framework to string compactification is described. The approach
is exemplified by formulating a strategy that allows to construct geometric
compactifications from exactly solvable theories at . It is shown that the
conformal field theoretic characters can be derived from the geometry of
spacetime, and that the geometry is uniquely determined by the two-dimensional
field theory on the world sheet. The modular forms that appear in these
constructions admit complex multiplication, and allow an interpretation as
generalized McKay-Thompson series associated to the Mathieu and Conway groups.
This leads to a string motivated notion of arithmetic moonshine.Comment: 36 page
Key components of the eight classes of type IV secretion systems involved in bacterial conjugation or protein secretion
Conjugation of DNA through a type IV secretion system (T4SS) drives horizontal gene transfer. Yet little is known on the diversity of these nanomachines. We previously found that T4SS can be divided in eight classes based on the phylogeny of the only ubiquitous protein of T4SS (VirB4). Here, we use an ab initio approach to identify protein families systematically and specifically associated with VirB4 in each class. We built profiles for these proteins and used them to scan 2262 genomes for the presence of T4SS. Our analysis led to the identification of thousands of occurrences of 116 protein families for a total of 1623 T4SS. Importantly, we could identify almost always in our profiles the essential genes of well-studied T4SS. This allowed us to build a database with the largest number of T4SS described to date. Using profile-profile alignments, we reveal many new cases of homology between components of distant classes of T4SS. We mapped these similarities on the T4SS phylogenetic tree and thus obtained the patterns of acquisition and loss of these protein families in the history of T4SS. The identification of the key VirB4-associated proteins paves the way toward experimental analysis of poorly characterized T4SS classes.Funding. Spanish Ministry of Economy [BFU2011-26608]; European Seventh Framework Program [282004/FP7-HEALTH.2011, 612146/FP7-ICT-2013]; European Research Council Grant [EVOMOBILOME no. 281605].
Source of open access funding: European Research Council grant to the PI
Wave Number of Maximal Growth in Viscous Magnetic Fluids of Arbitrary Depth
An analytical method within the frame of linear stability theory is presented
for the normal field instability in magnetic fluids. It allows to calculate the
maximal growth rate and the corresponding wave number for any combination of
thickness and viscosity of the fluid. Applying this method to magnetic fluids
of finite depth, these results are quantitatively compared to the wave number
of the transient pattern observed experimentally after a jump--like increase of
the field. The wave number grows linearly with increasing induction where the
theoretical and the experimental data agree well. Thereby a long-standing
controversy about the behaviour of the wave number above the critical magnetic
field is tackled.Comment: 19 pages, 15 figures, RevTex; revised version with a new figure and
references added. submitted to Phys Rev
Tate Form and Weak Coupling Limits in F-theory
We consider the weak coupling limit of F-theory in the presence of
non-Abelian gauge groups implemented using the traditional ansatz coming from
Tate's algorithm. We classify the types of singularities that could appear in
the weak coupling limit and explain their resolution. In particular, the weak
coupling limit of SU(n) gauge groups leads to an orientifold theory which
suffers from conifold singulaties that do not admit a crepant resolution
compatible with the orientifold involution. We present a simple resolution to
this problem by introducing a new weak coupling regime that admits
singularities compatible with both a crepant resolution and an orientifold
symmetry. We also comment on possible applications of the new limit to model
building. We finally discuss other unexpected phenomena as for example the
existence of several non-equivalent directions to flow from strong to weak
coupling leading to different gauge groups.Comment: 34 page
Tidal dissipation within hot Jupiters: a new appraisal
Eccentricity or obliquity tides have been proposed as the missing energy
source that may explain the anomalously large radius of some transiting ``hot
Jupiters''. To maintain a non-zero and large obliquity, it was argued that the
planets can be locked in a Cassini state, i.e. a resonance between spin and
orbital precessions. We compute the tidal heating within ``inflated'' close-in
giant planets with a non-zero eccentricity or obliquity. We further inspect
whether the spin of a ``hot Jupiter'' could have been trapped and maintained in
a Cassini state during its early despinning and migration. We estimate the
capture probability in a spin-orbit resonance between 0.5 AU (a distance
where tidal effects become significant) and 0.05 AU for a wide range of secular
orbital frequencies and amplitudes of gravitational perturbations. Numerical
simulations of the spin evolution are performed to explore the influence of
tidal despinning and migration processes on the resonance stability. We find
that tidal heating within a non-synchronous giant planet is about twice larger
than previous estimates based on the hypothesis of synchronization. Chances of
capture in a spin-orbit resonance are very good around 0.5 AU but they decrease
dramatically with the semi-major axis. Furthermore, even if captured, both
tidal despinning and migration processes cause the tidal torque to become large
enough that the obliquity ultimately leaves the resonance and switches to near
. Locking a ``hot Jupiter'' in an isolated spin-orbit resonance is
unlikely at 0.05 AU but could be possible at larger distances. Another
mechanism is then required to maintain a large obliquity and create internal
heating through obliquity tidesComment: 4 pages & 2 Figure
A Digital Repository and Execution Platform for Interactive Scholarly Publications in Neuroscience
The CARMEN Virtual Laboratory (VL) is a cloud-based platform which allows neuroscientists to store, share, develop, execute, reproduce and publicise their work. This paper describes new functionality in the CARMEN VL: an interactive publications repository. This new facility allows users to link data and software to publications. This enables other users to examine data and software associated with the publication and execute the associated software within the VL using the same data as the authors used in the publication. The cloud-based architecture and SaaS (Software as a Service) framework allows vast data sets to be uploaded and analysed using software services. Thus, this new interactive publications facility allows others to build on research results through reuse. This aligns with recent developments by funding agencies, institutions, and publishers with a move to open access research. Open access provides reproducibility and verification of research resources and results. Publications and their associated data and software will be assured of long-term preservation and curation in the repository. Further, analysing research data and the evaluations described in publications frequently requires a number of execution stages many of which are iterative. The VL provides a scientific workflow environment to combine software services into a processing tree. These workflows can also be associated with publications and executed by users. The VL also provides a secure environment where users can decide the access rights for each resource to ensure copyright and privacy restrictions are met
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