8,572 research outputs found
Search for Dust Emission from (24) Themis Using the Gemini-North Telescope
We report the results of a search for a dust trail aligned with the orbit
plane of the large main-belt asteroid (24) Themis, which has been reported to
have water ice frost on its surface. Observations were obtained with the GMOS
instrument on the Gemini-North Observatory in imaging mode, where we used a
chip gap to block much of the light from the asteroid, allowing us to take long
exposures while avoiding saturation by the object. No dust trail is detected
within 2' of Themis to a 3-sigma limiting surface brightness magnitude of 29.7
mag/arcsec^2, as measured along the expected direction of the dust trail.
Detailed consideration of dust ejection physics indicates that particles large
enough to form a detectable dust trail were unlikely to be ejected as a result
of sublimation from an object as large as Themis. We nonetheless demonstrate
that our observations would have been capable of detecting faint dust emission
as close as 20" from the object, even in a crowded star field. This approach
could be used to conduct future searches for sublimation-generated dust
emission from Themis or other large asteroids closer to perihelion than was
done in this work. It would also be useful for deep imaging of collisionally
generated dust emission from large asteroids at times when the visibility of
dust features are expected to be maximized, such as during orbit plane
crossings, during close approaches to the Earth, or following detected impact
events.Comment: 11 pages, 4 figures, accepted for publication in PAS
Effects of Kinks on DNA Elasticity
We study the elastic response of a worm-like polymer chain with reversible
kink-like structural defects. This is a generic model for (a) the
double-stranded DNA with sharp bends induced by binding of certain proteins,
and (b) effects of trans-gauche rotations in the backbone of the
single-stranded DNA. The problem is solved both analytically and numerically by
generalizing the well-known analogy to the Quantum Rotator. In the small
stretching force regime, we find that the persistence length is renormalized
due to the presence of the kinks. In the opposite regime, the response to the
strong stretching is determined solely by the bare persistence length with
exponential corrections due to the ``ideal gas of kinks''. This high-force
behavior changes significantly in the limit of high bending rigidity of the
chain. In that case, the leading corrections to the mechanical response are
likely to be due to the formation of multi-kink structures, such as kink pairs.Comment: v1: 16 pages, 7 figures, LaTeX; submitted to Physical Review E; v2: a
new subsection on soft kinks added to section Theory, sections Introduction
and Conclusions expanded, references added, other minor changes; v3: a
reference adde
NOMAD: Non-locking, stOchastic Multi-machine algorithm for Asynchronous and Decentralized matrix completion
We develop an efficient parallel distributed algorithm for matrix completion,
named NOMAD (Non-locking, stOchastic Multi-machine algorithm for Asynchronous
and Decentralized matrix completion). NOMAD is a decentralized algorithm with
non-blocking communication between processors. One of the key features of NOMAD
is that the ownership of a variable is asynchronously transferred between
processors in a decentralized fashion. As a consequence it is a lock-free
parallel algorithm. In spite of being an asynchronous algorithm, the variable
updates of NOMAD are serializable, that is, there is an equivalent update
ordering in a serial implementation. NOMAD outperforms synchronous algorithms
which require explicit bulk synchronization after every iteration: our
extensive empirical evaluation shows that not only does our algorithm perform
well in distributed setting on commodity hardware, but also outperforms
state-of-the-art algorithms on a HPC cluster both in multi-core and distributed
memory settings
Gate Voltage Controllable Non-Equilibrium and Non-Ohmic Behavior in Suspended Carbon Nanotubes
In this work, we measure the electrical conductance and temperature of individual, suspended quasi-metallic single-walled carbon nanotubes under high voltage biases using Raman spectroscopy, while varying the doping conditions with an applied gate voltage. By applying a gate voltage, the high-bias conductance can be switched dramatically between linear (Ohmic) behavior and nonlinear behavior exhibiting negative differential conductance (NDC). Phonon populations are observed to be in thermal equilibrium under Ohmic conditions but switch to nonequilibrium under NDC conditions. A typical Landauer transport model assuming zero bandgap is found to be inadequate to describe the experimental data. A more detailed model is presented, which incorporates the doping dependence in order to fit this data
The role of controllability in optimizing quantum dynamics
This paper discusses the important role of controllability played on the
complexity of optimizing quantum mechanical control systems. The study is based
on a topology analysis of the corresponding quantum control landscape, which is
referred to as the optimization objective as a functional of control fields. We
find that the degree of controllability is closely relevant with the ruggedness
of the landscape, which determines the search efficiency for global optima.
This effect is demonstrated via the gate fidelity control landscape of a system
whose controllability is restricted on a SU(2) dynamic symmetry group. We show
that multiple local false traps (i.e., non-global suboptima) exist even if the
target gate is realizable and that the number of these traps is increased by
the loss of controllability, while the controllable systems are always devoid
of false traps.Comment: 13 pages, 3 figure
HIV with contact-tracing: a case study in Approximate Bayesian Computation
Missing data is a recurrent issue in epidemiology where the infection process
may be partially observed. Approximate Bayesian Computation, an alternative to
data imputation methods such as Markov Chain Monte Carlo integration, is
proposed for making inference in epidemiological models. It is a
likelihood-free method that relies exclusively on numerical simulations. ABC
consists in computing a distance between simulated and observed summary
statistics and weighting the simulations according to this distance. We propose
an original extension of ABC to path-valued summary statistics, corresponding
to the cumulated number of detections as a function of time. For a standard
compartmental model with Suceptible, Infectious and Recovered individuals
(SIR), we show that the posterior distributions obtained with ABC and MCMC are
similar. In a refined SIR model well-suited to the HIV contact-tracing data in
Cuba, we perform a comparison between ABC with full and binned detection times.
For the Cuban data, we evaluate the efficiency of the detection system and
predict the evolution of the HIV-AIDS disease. In particular, the percentage of
undetected infectious individuals is found to be of the order of 40%
Processes in KaffeOS: lsolation, resource management, and sharing in Java
Journal ArticleSingle-language runtime systems, in the form of Java virtual machines, are widely deployed platforms for executing untrusted mobile code. These runtimes provide some of the features that operating systems provide: inter-application memory protection and basic system services. They do not. however, provide the ability to isolate applications from each other, or limit their resource consumption. This paper describes KaffeOS, a system that provides these features for a Java runtime. The KaffeOS architecture take many lessons from operating from operating system design, such as the use of a user/kernel boundary
Pair Analysis of Field Galaxies from the Red-Sequence Cluster Survey
We study the evolution of the number of close companions of similar
luminosities per galaxy (Nc) by choosing a volume-limited subset of the
photometric redshift catalog from the Red-Sequence Cluster Survey (RCS-1). The
sample contains over 157,000 objects with a moderate redshift range of 0.25 < z
< 0.8 and absolute magnitude in Rc (M_Rc) < -20. This is the largest sample
used for pair evolution analysis, providing data over 9 redshift bins with
about 17,500 galaxies in each. After applying incompleteness and projection
corrections, Nc shows a clear evolution with redshift. The Nc value for the
whole sample grows with redshift as (1+z)^m, where m = 2.83 +/- 0.33 in good
agreement with N-body simulations in a LCDM cosmology. We also separate the
sample into two different absolute magnitude bins: -25 < M_Rc < -21 and -21 <
M_Rc < -20, and find that the brighter the absolute magnitude, the smaller the
m value. Furthermore, we study the evolution of the pair fraction for different
projected separation bins and different luminosities. We find that the m value
becomes smaller for larger separation, and the pair fraction for the fainter
luminosity bin has stronger evolution. We derive the major merger remnant
fraction f_rem = 0.06, which implies that about 6% of galaxies with -25 < M_Rc
< -20 have undergone major mergers since z = 0.8.Comment: ApJ, in pres
A Nonlinear Super-Exponential Rational Model of Speculative Financial Bubbles
Keeping a basic tenet of economic theory, rational expectations, we model the
nonlinear positive feedback between agents in the stock market as an interplay
between nonlinearity and multiplicative noise. The derived hyperbolic
stochastic finite-time singularity formula transforms a Gaussian white noise
into a rich time series possessing all the stylized facts of empirical prices,
as well as accelerated speculative bubbles preceding crashes. We use the
formula to invert the two years of price history prior to the recent crash on
the Nasdaq (april 2000) and prior to the crash in the Hong Kong market
associated with the Asian crisis in early 1994. These complex price dynamics
are captured using only one exponent controlling the explosion, the variance
and mean of the underlying random walk. This offers a new and powerful
detection tool of speculative bubbles and herding behavior.Comment: Latex document of 24 pages including 5 eps figure
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