97,965 research outputs found
On the feedback from super stellar clusters. I. The structure of giant HII regions and HII galaxies
We review the structural properties of giant extragalactic HII regions and
HII galaxies based on 2D hydrodynamic calculations, and propose an evolutionary
sequence that accounts for their observed detailed structure. The model assumes
a massive and young stellar cluster surrounded by a large collection of clouds.
These are thus exposed to the most important star-formation feedback
mechanisms: photoionization and the cluster wind. The models show how the two
feedback mechanisms compete in the disruption of clouds and lead to two
different hydrodynamic solutions: The storage of clouds into a long lasting
ragged shell that inhibits the expansion of the thermalized wind, and the
steady filtering of the shocked wind gas through channels carved within the
cloud stratum. Both solutions are claimed to be concurrently at work in giant
HII regions and HII galaxies, causing their detailed inner structure. This
includes multiple large-scale shells, filled with an X-ray emitting gas, that
evolve to finally merge with each other, giving the appearance of shells within
shells. The models also show how the inner filamentary structure of the giant
superbubbles is largely enhanced with matter ablated from clouds and how cloud
ablation proceeds within the original cloud stratum. The calculations point at
the initial contrast density between the cloud and the intercloud media as the
factor that defines which of the two feedback mechanisms becomes dominant
throughout the evolution. Animated version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.html.Comment: 28 pages, 10 figures, accepted for publication in the ApJ. Animated
version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.htm
SEISMIC: A Self-Exciting Point Process Model for Predicting Tweet Popularity
Social networking websites allow users to create and share content. Big
information cascades of post resharing can form as users of these sites reshare
others' posts with their friends and followers. One of the central challenges
in understanding such cascading behaviors is in forecasting information
outbreaks, where a single post becomes widely popular by being reshared by many
users. In this paper, we focus on predicting the final number of reshares of a
given post. We build on the theory of self-exciting point processes to develop
a statistical model that allows us to make accurate predictions. Our model
requires no training or expensive feature engineering. It results in a simple
and efficiently computable formula that allows us to answer questions, in
real-time, such as: Given a post's resharing history so far, what is our
current estimate of its final number of reshares? Is the post resharing cascade
past the initial stage of explosive growth? And, which posts will be the most
reshared in the future? We validate our model using one month of complete
Twitter data and demonstrate a strong improvement in predictive accuracy over
existing approaches. Our model gives only 15% relative error in predicting
final size of an average information cascade after observing it for just one
hour.Comment: 10 pages, published in KDD 201
Quantum Associative Memory
This paper combines quantum computation with classical neural network theory
to produce a quantum computational learning algorithm. Quantum computation uses
microscopic quantum level effects to perform computational tasks and has
produced results that in some cases are exponentially faster than their
classical counterparts. The unique characteristics of quantum theory may also
be used to create a quantum associative memory with a capacity exponential in
the number of neurons. This paper combines two quantum computational algorithms
to produce such a quantum associative memory. The result is an exponential
increase in the capacity of the memory when compared to traditional associative
memories such as the Hopfield network. The paper covers necessary high-level
quantum mechanical and quantum computational ideas and introduces a quantum
associative memory. Theoretical analysis proves the utility of the memory, and
it is noted that a small version should be physically realizable in the near
future
Optical Synoptic Telescopes: New Science Frontiers
Over the past decade, sky surveys such as the Sloan Digital Sky Survey have
proven the power of large data sets for answering fundamental astrophysical
questions. This observational progress, based on a synergy of advances in
telescope construction, detectors, and information technology, has had a
dramatic impact on nearly all fields of astronomy, and areas of fundamental
physics. The next-generation instruments, and the surveys that will be made
with them, will maintain this revolutionary progress. The hardware and
computational technical challenges and the exciting science opportunities are
attracting scientists and engineers from astronomy, optics, low-light-level
detectors, high-energy physics, statistics, and computer science. The history
of astronomy has taught us repeatedly that there are surprises whenever we view
the sky in a new way. This will be particularly true of discoveries emerging
from a new generation of sky surveys. Imaging data from large ground-based
active optics telescopes with sufficient etendue can address many scientific
missions simultaneously. These new investigations will rely on the statistical
precision obtainable with billions of objects. For the first time, the full sky
will be surveyed deep and fast, opening a new window on a universe of faint
moving and distant exploding objects as well as unraveling the mystery of dark
energy.Comment: 12 pages, 7 figure
Spin-based optical quantum gates via Pauli blocking in semiconductor quantum dots
We present a solid-state implementation of ultrafast conditional quantum
gates. Our proposal for a quantum-computing device is based on the spin degrees
of freedom of electrons confined in semiconductor quantum dots, thus benefiting
from relatively long decoherence times. More specifically, combining Pauli
blocking effects with properly tailored ultrafast laser pulses, we are able to
obtain sub-picosecond spin-dependent switching of the Coulomb interaction,
which is the essence of our conditional phase-gate proposal. This allows us to
realize {\it a fast two qubit gate which does not translate into fast
decoherence times} and paves the road for an all-optical spin-based quantum
computer.Comment: 14 Pages RevTeX, 3 eps figures include
Introduction to Focus Issue: Lagrangian Coherent Structures
The topic of Lagrangian coherent structures (LCS) has been a rapidly growing area of research in nonlinear dynamics for almost a decade. It provides a means to rigorously define and detect transport barriers in dynamical systems with arbitrary time dependence and has a wealth of applications, particularly to fluid flow problems. Here, we give a short introduction to the topic of LCS and review the new work presented in this Focus Issue
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