174 research outputs found
Half-spectral unidirectional invisibility in non-Hermitian periodic optical structures
The phenomenon of half-spectral unidirectional invisibility is introduced for
one-dimensional periodic optical structures with tailored real and imaginary
refractive index distributions in a non--symmetric configuration.
The effect refers to the property that the optical medium appears to be
invisible, both in reflection and transmission, below the Bragg frequency when
probed from one side, and above the Bragg frequency when probed from the
opposite side. Half-spectral invisibility is obtained by a combination of
in-phase index and gain gratings whose spatial amplitudes are related each
other by a Hilbert transform.Comment: 6 pages, 6 figure
Compression and Conditional Emulation of Climate Model Output
Numerical climate model simulations run at high spatial and temporal
resolutions generate massive quantities of data. As our computing capabilities
continue to increase, storing all of the data is not sustainable, and thus it
is important to develop methods for representing the full datasets by smaller
compressed versions. We propose a statistical compression and decompression
algorithm based on storing a set of summary statistics as well as a statistical
model describing the conditional distribution of the full dataset given the
summary statistics. The statistical model can be used to generate realizations
representing the full dataset, along with characterizations of the
uncertainties in the generated data. Thus, the methods are capable of both
compression and conditional emulation of the climate models. Considerable
attention is paid to accurately modeling the original dataset--one year of
daily mean temperature data--particularly with regard to the inherent spatial
nonstationarity in global fields, and to determining the statistics to be
stored, so that the variation in the original data can be closely captured,
while allowing for fast decompression and conditional emulation on modest
computers
The Initial Mass Function of the Orion Nebula Cluster across the H-burning limit
We present a new census of the Orion Nebula Cluster (ONC) over a large field
of view (>30'x30'), significantly increasing the known population of stellar
and substellar cluster members with precisely determined properties. We develop
and exploit a technique to determine stellar effective temperatures from
optical colors, nearly doubling the previously available number of objects with
effective temperature determinations in this benchmark cluster. Our technique
utilizes colors from deep photometry in the I-band and in two medium-band
filters at lambda~753 and 770nm, which accurately measure the depth of a
molecular feature present in the spectra of cool stars. From these colors we
can derive effective temperatures with a precision corresponding to better than
one-half spectral subtype, and importantly this precision is independent of the
extinction to the individual stars. Also, because this technique utilizes only
photometry redward of 750nm, the results are only mildly sensitive to optical
veiling produced by accretion. Completing our census with previously available
data, we place some 1750 sources in the Hertzsprung-Russel diagram and assign
masses and ages down to 0.02 solar masses. At faint luminosities, we detect a
large population of background sources which is easily separated in our
photometry from the bona fide cluster members. The resulting initial mass
function of the cluster has good completeness well into the substellar mass
range, and we find that it declines steeply with decreasing mass. This suggests
a deficiency of newly formed brown dwarfs in the cluster compared to the
Galactic disk population.Comment: 16 pages, 18 figures. Accepted for publication in The Astrophysical
Journa
Non-supersymmetric D-branes with Vanishing Cylinder Amplitudes in Asymmetric Orbifolds
We study the type II string vacua with chiral space-time SUSY constructed as
asymmetric orbifolds of torus and compactifications. Despite the fact
that all the D-branes are non-BPS in any chiral SUSY vacua, we show that the
relevant non-geometric vacua of asymmetric orbifolds allow rather generally
configurations of D-branes which lead to vanishing cylinder amplitudes,
implying the bose-fermi cancellation at each mass level of the open string
spectrum. After working on simple models of toroidal asymmetric orbifolds, we
focus on the asymmetric orbifolds of , where is described by a general SCFT with defined by the
Gepner construction for . Even when the modular invariant partition
functions in the bulk remain unchanged, the spectra of such non-BPS D-branes
with the bose-fermi cancellation can vary significantly according to the choice
of orbifolding.Comment: 32 pages, no figures; (v2) comments and discussion added on
properties of D-brane
Non-Critical String Duals of N=1 Quiver Theories
We construct N=1 non-critical strings in four dimensions dual to strongly
coupled N=1 quiver gauge theories in the Coulomb phase, generalizing the string
duals of Argyres-Douglas points in N=2 gauge theories. They are the first
examples of superstrings vacua with an exact worldsheet description dual to
chiral N=1 theories. We identify the dual of the non-critical superstring using
a brane setup describing the field theory in the classical limit. We analyze
the spectrum of chiral operators in the strongly coupled regime and show how
worldsheet instanton effects give non-perturbative information about the gauge
theory. We also consider aspects of D-branes relevant for the holographic
duality.Comment: JHEP style; 40 pages, 3 figures; v2: minor corrections, refs added,
version to appear in JHE
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