82,539 research outputs found
Dynamical Origin of Extrasolar Planet Eccentricity Distribution
We explore the possibility that the observed eccentricity distribution of
extrasolar planets arose through planet-planet interactions, after the initial
stage of planet formation was complete. Our results are based on ~3250
numerical integrations of ensembles of randomly constructed planetary systems,
each lasting 100 Myr. We find that for a remarkably wide range of initial
conditions the eccentricity distributions of dynamically active planetary
systems relax towards a common final equilibrium distribution, well described
by the fitting formula dn ~ e exp[-1/2 (e/0.3)^2] de. This distribution agrees
well with the observed eccentricity distribution for e > 0.2, but predicts too
few planets at lower eccentricities, even when we exclude planets subject to
tidal circularization. These findings suggest that a period of large-scale
dynamical instability has occurred in a significant fraction of newly formed
planetary systems, lasting 1--2 orders of magnitude longer than the ~1 Myr
interval in which gas-giant planets are assembled. This mechanism predicts no
(or weak) correlations between semimajor axis, eccentricity, inclination, and
mass in dynamically relaxed planetary systems. An additional observational
consequence of dynamical relaxation is a significant population of planets
(>10%) that are highly inclined (>25deg) with respect to the initial symmetry
plane of the protoplanetary disk; this population may be detectable in
transiting planets through the Rossiter-McLaughlin effect.Comment: Accepted to ApJ, conclusions updated to reflect the current
observational constraint
A Family of Iterative Gauss-Newton Shooting Methods for Nonlinear Optimal Control
This paper introduces a family of iterative algorithms for unconstrained
nonlinear optimal control. We generalize the well-known iLQR algorithm to
different multiple-shooting variants, combining advantages like
straight-forward initialization and a closed-loop forward integration. All
algorithms have similar computational complexity, i.e. linear complexity in the
time horizon, and can be derived in the same computational framework. We
compare the full-step variants of our algorithms and present several simulation
examples, including a high-dimensional underactuated robot subject to contact
switches. Simulation results show that our multiple-shooting algorithms can
achieve faster convergence, better local contraction rates and much shorter
runtimes than classical iLQR, which makes them a superior choice for nonlinear
model predictive control applications.Comment: 8 page
Royal Road to Coupling Classical and Quantum Dynamics
We present a consistent framework of coupled classical and quantum dynamics.
Our result allows us to overcome severe limitations of previous
phenomenological approaches, like evolutions that do not preserve the
positivity of quantum states or that allow to activate quantum nonlocality for
superluminal signaling. A `hybrid' quantum-classical density is introduced and
its evolution equation derived. The implications and applications of our result
are numerous: it incorporates the back-reaction of quantum on classical
variables, it resolves fundamental problems encountered in standard mean field
theories, and remarkably, also the quantum measurement process, i.e. the most
controversial example of quantum-classical interaction is consistently
described within our approach, leading to a theory of dynamical collapse.Comment: 4 pages, RevTe
Using field theory to construct hybrid particle-continuum simulation schemes with adaptive resolution for soft matter systems
We develop a multiscale hybrid scheme for simulations of soft condensed
matter systems, which allows one to treat the system at the particle level in
selected regions of space, and at the continuum level elsewhere. It is derived
systematically from an underlying particle-based model by field theoretic
methods. Particles in different representation regions can switch
representations on the fly, controlled by a spatially varying tuning function.
As a test case, the hybrid scheme is applied to simulate colloid-polymer
composites with high resolution regions close to the colloids. The hybrid
simulations are significantly faster than reference simulations of a pure
particle-based model, and the results are in good agreement.Comment: 8 pages, 3 figure
A family of droids -- Android malware detection via behavioral modeling: static vs dynamic analysis
Following the increasing popularity of mobile ecosystems, cybercriminals have increasingly targeted them, designing and distributing malicious apps that steal information or cause harm to the device's owner. Aiming to counter them, detection techniques based on either static or dynamic analysis that model Android malware, have been proposed. While the pros and cons of these analysis techniques are known, they are usually compared in the context of their limitations e.g., static analysis is not able to capture runtime behaviors, full code coverage is usually not achieved during dynamic analysis, etc. Whereas, in this paper, we analyze the performance of static and dynamic analysis methods in the detection of Android malware and attempt to compare them in terms of their detection performance, using the same modeling approach. To this end, we build on MaMaDroid, a state-of-the-art detection system that relies on static analysis to create a behavioral model from the sequences of abstracted API calls. Then, aiming to apply the same technique in a dynamic analysis setting, we modify CHIMP, a platform recently proposed to crowdsource human inputs for app testing, in order to extract API calls' sequences from the traces produced while executing the app on a CHIMP virtual device. We call this system AuntieDroid and instantiate it by using both automated (Monkey) and user-generated inputs. We find that combining both static and dynamic analysis yields the best performance, with F-measure reaching 0.92. We also show that static analysis is at least as effective as dynamic analysis, depending on how apps are stimulated during execution, and, finally, investigate the reasons for inconsistent misclassifications across methods.Accepted manuscrip
On the formation of neon-enriched donor stars in ultracompact X-ray binaries
We study the formation of neon-enriched donor stars in ultracompact X-ray
binaries (orbital periods P<80 min) and show that their progenitors have to be
low-mass (0.3 - 0.4 solar mass) ``hybrid'' white dwarfs (with CO cores and
thick helium mantles). Stable mass transfer is possible if in the initial
stages of mass exchange mass is lost from the system, taking away the specific
orbital angular momentum of the accretor (``isotropic re-emission''). The
excess of neon in the transferred matter is due to chemical fractionation of
the white dwarf which has to occur prior to the Roche lobe overflow by the
donor. The estimated lower limit of the orbital periods of the systems with
neon-enriched donors is close to 10 min. We show that the X-ray pulsar 4U
1626-67, which likely also has a neon-enriched companion, may have been formed
via accretion induced collapse of an oxygen-neon white dwarf accretor if the
donor was a hybrid white dwarf.Comment: 6 pages, 3 figures, uses aa.cls 5.1 version class file, accepted for
publication in Astronomy and Astrophysic
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