1,269 research outputs found
Model of light collimation by photonic crystal surface modes
We propose a quantitative model explaining the mechanism of light collimation
by leaky surface modes that propagate on a corrugated surface around the output
of a photonic crystal waveguide. The dispersion relation of these modes is
determined for a number of surface terminations. Analytical results obtained on
the basis of the model are compared to those of rigorous numerical simulations.
Maximum collimation is shown to occur at frequency values corresponding to
excitation of surface modes whose wave number retains a nonzero real part.Comment: 6 pages, 7 figures. Version 2: corrected sign of k_x' (sections 4-6,
fig. 2), minor clarifications in section 2. Version 3: significant changes,
including reformulation of the model using the theory of aperture antennas,
as well as extended discussion of the accuracy of the mode
Supernova Remnants in the Magellanic Clouds. VI. The DEML316 Supernova Remnants
The DEML316 system contains two shells, both with the characteristic
signatures of supernova remnants (SNRs). We analyze Chandra and XMM-Newton data
for DEML316, investigating its spatial and spectral X-ray features. Our Chandra
observations resolve the structure of the northeastern SNR (Shell A) as a
bright inner ring and a set of "arcs" surrounded by fainter diffuse emission.
The spectrum is well fit by a thermal plasma model with temperature ~1.4 keV;
we do not find significant spectral differences for different regions of this
SNR. The southwestern SNR (Shell B) exhibits an irregular X-ray outline, with a
brighter interior ring of emission including a bright knot of emission. Overall
the emission of the SNR is well described by a thermal plasma of temperature
~0.6 keV. The Bright Knot, however, is spectrally distinct from the rest of the
SNR, requiring the addition of a high-energy spectral component consistent with
a power-law spectrum of photon index 1.6--1.8.
We confirm the findings of Nishiuchi et al. (2001) that the spectra of these
shells are notably different, with Shell A requiring a high iron abundance for
a good spectral fit, implying a Type Ia origin. We further explicitly compare
abundance ratios to model predictions for Type Ia and Type II supernovae. The
low ratios for Shell A (O/Fe of 1.5 and Ne/Fe of 0.2) and the high ratios for
Shell B (O/Fe of 30--130 and Ne/Fe of 8--16) are consistent with Type Ia and
Type II origins, respectively. The difference between the SNR progenitor types
casts some doubt on the suggestion that these SNRs are interacting with one
another.Comment: Accepted for ApJ v. 635 (December issue
A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud
We present a study of the magnetic field of the Small Magellanic Cloud (SMC),
carried out using radio Faraday rotation and optical starlight polarization
data. Consistent negative rotation measures (RMs) across the SMC indicate that
the line-of-sight magnetic field is directed uniformly away from us with a
strength 0.19 +/- 0.06 microGauss. Applying the Chandrasekhar-Fermi method to
starlight polarization data yields an ordered magnetic field in the plane of
the sky of strength 1.6 +/- 0.4 microGauss oriented at a position angle 4 +/-
12 degs, measured counter-clockwise from the great circle on the sky joining
the SMC to the Large Magellanic Cloud (LMC). We construct a three-dimensional
magnetic field model of the SMC, under the assumption that the RMs and
starlight polarization probe the same underlying large-scale field. The vector
defining the overall orientation of the SMC magnetic field shows a potential
alignment with the vector joining the center of the SMC to the center of the
LMC, suggesting the possibility of a "pan-Magellanic'' magnetic field. A
cosmic-ray driven dynamo is the most viable explanation of the observed field
geometry, but has difficulties accounting for the observed uni-directional
field lines. A study of Faraday rotation through the Magellanic Bridge is
needed to further test the pan-Magellanic field hypothesis.Comment: 28 pages, 6 figures, accepted for publication in Ap
MACHO 96-LMC-2: Lensing of a Binary Source in the LMC and Constraints on the Lensing Object
We present photometry and analysis of the microlensing alert MACHO 96-LMC-2.
The ~3% photometry provided by the Global Microlensing Alert Network follow--up
effort reveals a periodic modulation in the lightcurve. We attribute this to
binarity of the lensed source. Microlensing fits to a rotating binary source
magnified by a single lens converge on two minima, separated by delta chi^2 ~
1. The most significant fit X1 predicts a primary which contributes ~100% of
the light, a dark secondary, and an orbital period (T) of 9.2 days. The second
fit X2 yields a binary source with two stars of roughly equal mass and
luminosity, and T = 21.2 days.
The lensed object appears to lie on the upper LMC main sequence. We estimate
the mass of the primary component of the binary system, M ~2 M_sun. For the
preferred model X1, we explore the range of dark companions by assuming 0.1
M_sun and 1.4 M_sun objects in models X1a and X1b, respectively. We find lens
velocities projected to the LMC in these models of v^hat_X1a = 18.3 +/- 3.1
km/s and v^hat_X1b = 188 +/- 32 k/ms. In both these cases, a likelihood
analysis suggests an LMC lens is preferred over a Galactic halo lens, although
only marginally so in model X1b. We also find v^hat_X2 = 39.6 +/- 6.1 k/ms,
where the likelihood for the lens location is strongly dominated by the LMC
disk. In all cases, the lens mass is consistent with that of an M-dwarf. The
LMC self-lensing rate contributed by 96-LMC-2 is consistent with model
self-lensing rates. (Abridged)Comment: 23 pages, including 3 tables and 6 figures; Accepted for publication
in The Astrophysical Journa
An evaluative baseline for geo-semantic relatedness and similarity
In geographic information science and semantics, the computation of semantic similarity is widely recognised as key to supporting a vast number of tasks in information integration and retrieval. By contrast, the role of geo-semantic relatedness has been largely ignored. In natural language processing, semantic relatedness is often confused with the more specific semantic similarity. In this article, we discuss a notion of geo-semantic relatedness based on Lehrerâs semantic fields, and we compare it with geo-semantic similarity. We then describe and validate the Geo Relatedness and Similarity Dataset (GeReSiD), a new open dataset designed to evaluate computational measures of geo-semantic relatedness and similarity. This dataset is larger than existing datasets of this kind, and includes 97 geographic terms combined into 50 term pairs rated by 203 human subjects. GeReSiD is available online and can be used as an evaluation baseline to determine empirically to what degree a given computational model approximates geo-semantic relatedness and similarity
Extracting invariant characteristics of sketch maps: Towards place query-by-sketch
In geography, invariant aspects of sketches are essential to study because they reflect the human perception of real-world places. A person's perception of a place can be ex-pressed in sketches. In this article, we quantitatively and qualitatively analyzed the characteristics of single objects and characteristics among objects in sketches and the real world to find reliable invariants that can be used to establish references/correspondences between sketch and world in a matching process. These characteristics include category, shape, name, and relative size of each object. Moreover, quantity and spatial relationshipsâsuch as topological, or-dering, and location relationshipsâamong all objects are also analyzed to assess consistency between sketched and actual places. The approach presented in this study extracts the reliable invariants for query-by-sketch and prioritizes their relevance for a sketch-map matching process
The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wavebands
We present the initial results from the Spitzer Survey of the Small
Magellanic Cloud (S3MC), which imaged the star-forming body of the Small
Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the
F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and
takes a wide range of values that are unrelated to metallicity but
anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that
photodestruction is primarily responsible for the low abundance of PAHs
observed in star-forming low-metallicity galaxies. We use the S3MC images to
compile a photometric catalog of ~400,000 mid- and far-infrared point sources
in the SMC. The sources detected at the longest wavelengths fall into four main
categories: 1) bright 5.8 um sources with very faint optical counterparts and
very red mid-infrared colors ([5.8]-[8.0]>1.2), which we identify as YSOs. 2)
Bright mid-infrared sources with mildly red colors (0.16<[5.8]-[8.0]<0.6),
identified as carbon stars. 3) Bright mid-infrared sources with neutral colors
and bright optical counterparts, corresponding to oxygen-rich evolved stars.
And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This
excess is reminiscent of debris disks, and is detected in only a small fraction
of these stars (<5%). The majority of the brightest infrared point sources in
the SMC fall into groups one to three. We use this photometric information to
produce a catalog of 282 bright YSOs in the SMC with a very low level of
contamination (~7%).Comment: Accepted for publication in The Astrophysical Journal. Given the
draconian figure file-size limits implemented in astro-ph, readers are
encouraged to download the manuscript with full quality images from
http://celestial.berkeley.edu/spitzer/publications/s3mcsurvey.pd
A Survey of Volunteered Open Geo-Knowledge Bases in the Semantic Web
Over the past decade, rapid advances in web technologies, coupled with
innovative models of spatial data collection and consumption, have generated a
robust growth in geo-referenced information, resulting in spatial information
overload. Increasing 'geographic intelligence' in traditional text-based
information retrieval has become a prominent approach to respond to this issue
and to fulfill users' spatial information needs. Numerous efforts in the
Semantic Geospatial Web, Volunteered Geographic Information (VGI), and the
Linking Open Data initiative have converged in a constellation of open
knowledge bases, freely available online. In this article, we survey these open
knowledge bases, focusing on their geospatial dimension. Particular attention
is devoted to the crucial issue of the quality of geo-knowledge bases, as well
as of crowdsourced data. A new knowledge base, the OpenStreetMap Semantic
Network, is outlined as our contribution to this area. Research directions in
information integration and Geographic Information Retrieval (GIR) are then
reviewed, with a critical discussion of their current limitations and future
prospects
Spitzer SAGE survey of the Large Magellanic Cloud II: Evolved Stars and Infrared Color Magnitude Diagrams
Color-magnitude diagrams (CMDs) are presented for the Spitzer SAGE (Surveying
the Agents of a Galaxy's Evolution) survey of the Large Magellanic Cloud (LMC).
IRAC and MIPS 24 um epoch one data are presented. These data represent the
deepest, widest mid-infrared CMDs of their kind ever produced in the LMC.
Combined with the 2MASS survey, the diagrams are used to delineate the evolved
stellar populations in the Large Magellanic Cloud as well as Galactic
foreground and extragalactic background populations. Some 32000 evolved stars
brighter than the tip of the red giant branch are identified. Of these,
approximately 17500 are classified as oxygen-rich, 7000 carbon-rich, and
another 1200 as ``extreme'' asymptotic giant branch (AGB) stars. Brighter
members of the latter group have been called ``obscured'' AGB stars in the
literature owing to their dusty circumstellar envelopes. A large number (1200)
of luminous oxygen--rich AGB stars/M supergiants are also identified. Finally,
there is strong evidence from the 24 um MIPS channel that previously
unexplored, lower luminosity oxygen-rich AGB stars contribute significantly to
the mass loss budget of the LMC (1200 such sources are identified).Comment: LaTex, 31 pages, 10 figures. Accepted for publication in the
Astronomical Journa
Study of Inclusive J/psi Production in Two-Photon Collisions at LEP II with the DELPHI Detector
Inclusive J/psi production in photon-photon collisions has been observed at
LEP II beam energies. A clear signal from the reaction gamma gamma -> J/psi+X
is seen. The number of observed N(J/psi -> mu+mu-) events is 36 +/- 7 for an
integrated luminosity of 617 pb^{-1}, yielding a cross-section of
sigma(J/psi+X) = 45 +/- 9 (stat) +/- 17 (syst) pb. Based on a study of the
event shapes of different types of gamma gamma processes in the PYTHIA program,
we conclude that (74 +/- 22)% of the observed J/psi events are due to
`resolved' photons, the dominant contribution of which is most probably due to
the gluon content of the photon.Comment: 13 pages, 8 figures, Accepted by Phys. Lett.
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