1,646 research outputs found
Asteroseismic effects in close binary stars
Turbulent processes in the convective envelopes of the sun and stars have
been shown to be a source of internal acoustic excitations. In single stars,
acoustic waves having frequencies below a certain cutoff frequency propagate
nearly adiabatically and are effectively trapped below the photosphere where
they are internally reflected. This reflection essentially occurs where the
local wavelength becomes comparable to the pressure scale height. In close
binary stars, the sound speed is a constant on equipotentials, while the
pressure scale height, which depends on the local effective gravity, varies on
equipotentials and may be much greater near the inner Lagrangian point (L_1).
As a result, waves reaching the vicinity of L_1 may propagate unimpeded into
low density regions, where they tend to dissipate quickly due to non-linear and
radiative effects. We study the three dimensional propagation and enhanced
damping of such waves inside a set of close binary stellar models using a WKB
approximation of the acoustic field. We find that these waves can have much
higher damping rates in close binaries, compared to their non-binary
counterparts. We also find that the relative distribution of acoustic energy
density at the visible surface of close binaries develops a ring-like feature
at specific acoustic frequencies and binary separations
Strongly interacting photons in one-dimensional continuum
Photon-photon scattering in vacuum is extremely weak. However, strong
effective interactions between single photons can be realized by employing
strong light-matter coupling. These interactions are a fundamental building
block for quantum optics, bringing many-body physics to the photonic world and
providing important resources for quantum photonic devices and for optical
metrology. In this Colloquium, we review the physics of strongly-interacting
photons in one-dimensional systems with no optical confinement along the
propagation direction. We focus on two recently-demonstrated experimental
realizations: superconducting qubits coupled to open transmission lines, and
interacting Rydberg atoms in a cold gas. Advancements in the theoretical
understanding of these systems are presented in complementary formalisms and
compared to experimental results. The experimental achievements are summarized
alongside a description of the quantum optical effects and quantum devices
emerging from them.Comment: Updated version, accepted for publication in Reviews of Modern
Physic
A Citation-Based Ranking of Strategic Management Journals
Rankings of strategy journals are important for authors, readers, and promotion and tenure committees. We present several rankings, based either on the number of articles that cited the journal or the per-article impact. Our analyses cover various periods between 1991 and 2006, for most of which the Strategic Management Journal was in first place and Journal of Economics & Management Strategy (JEMS) second, although JEMS ranked first in certain instances. Long Range Planning and Technology Analysis & Strategic Management also achieve a top position. Strategic Organization makes an impressive entry and achieves a top position in 2003-2006.Journal rankings; Citation analysis; Strategic Management; Academic impact; Strategy
Fidelity Between Unitary Operators and the Generation of Gates Robust Against Off-Resonance Perturbations
We perform a functional expansion of the fidelity between two unitary
matrices in order to find the necessary conditions for the robust
implementation of a target gate. Comparison of these conditions with those
obtained from the Magnus expansion and Dyson series shows that they are
equivalent in first order. By exploiting techniques from robust design
optimization, we account for issues of experimental feasibility by introducing
an additional criterion to the search for control pulses. This search is
accomplished by exploring the competition between the multiple objectives in
the implementation of the NOT gate by means of evolutionary multi-objective
optimization
Non-Equipartition of Energy, Masses of Nova Ejecta, and Type Ia Supernovae
The total masses ejected during classical nova eruptions are needed to answer
two questions with broad astrophysical implications: Can accreting white dwarfs
be pushed towards the Chandrasekhar mass limit to yield type Ia supernovae? Are
Ultra-luminous red variables a new kind of astrophysical phenomenon, or merely
extreme classical novae? We review the methods used to determine nova ejecta
masses. Except for the unique case of BT Mon (nova 1939), all nova ejecta mass
determinations depend on untested assumptions and multi-parameter modeling. The
remarkably simple assumption of equipartition between kinetic and radiated
energy (E_kin and E_rad, respectively) in nova ejecta has been invoked as a way
around this conundrum for the ultra-luminous red variable in M31. The deduced
mass is far larger than that produced by any classical nova model. Our nova
eruption simulations show that radiation and kinetic energy in nova ejecta are
very far from being in energy equipartition, with variations of four orders of
magnitude in the ratio E_kin/E_rad being commonplace. The assumption of
equipartition must not be used to deduce nova ejecta masses; any such
"determinations" can be overestimates by a factor of up to 10,000. We
data-mined our extensive series of nova simulations to search for correlations
that could yield nova ejecta masses. Remarkably, the mass ejected during a nova
eruption is dependent only on (and is directly proportional to) E_rad. If we
measure the distance to an erupting nova and its bolometric light curve then
E_rad and hence the mass ejected can be directly measured.Comment: 9 pages, 4 figures, awaiting publication in ApJ
ANNz: estimating photometric redshifts using artificial neural networks
We introduce ANNz, a freely available software package for photometric
redshift estimation using Artificial Neural Networks. ANNz learns the relation
between photometry and redshift from an appropriate training set of galaxies
for which the redshift is already known. Where a large and representative
training set is available ANNz is a highly competitive tool when compared with
traditional template-fitting methods.
The ANNz package is demonstrated on the Sloan Digital Sky Survey Data Release
1, and for this particular data set the r.m.s. redshift error in the range 0 <
z < 0.7 is 0.023. Non-ideal conditions (spectroscopic sets which are small, or
which are brighter than the photometric set for which redshifts are required)
are simulated and the impact on the photometric redshift accuracy assessed.Comment: 6 pages, 6 figures. Replaced to match version accepted by PASP (minor
changes to original submission). The ANNz package may be obtained from
http://www.ast.cam.ac.uk/~aa
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