4,333 research outputs found
Visualizing classification of natural video sequences using sparse, hierarchical models of cortex.
Recent work on hierarchical models of visual cortex has reported state-of-the-art accuracy on whole-scene labeling using natural still imagery. This raises the question of whether the reported accuracy may be due to the sophisticated, non-biological back-end supervised classifiers typically used (support vector machines) and/or the limited number of images used in these experiments. In particular, is the model classifying features from the object or the background? Previous work (Landecker, Brumby, et al., COSYNE 2010) proposed tracing the spatial support of a classifier’s decision back through a hierarchical cortical model to determine which parts of the image contributed to the classification, compared to the positions of objects in the scene. In this way, we can go beyond standard measures of accuracy to provide tools for visualizing and analyzing high-level object classification. We now describe new work exploring the extension of these ideas to detection of objects in video sequences of natural scenes
New Brown Dwarfs and an Updated Initial Mass Function in Taurus
I have performed a search for young low-mass stars and brown dwarfs (BDs) in
2 regions encompassing a total area of 4 deg^2 in the Taurus star-forming
region, discovering 15 new members of Taurus. In addition, I present 7 new
members outside of these areas from the initial stage of a survey of all of
Taurus. These 22 objects exhibit spectral types of M4.5-M9.25 and masses of
0.3-0.015 M_sun according to the theoretical evolutionary models of Baraffe and
Chabrier, 7 of which are likely to be BDs. Emission in H(alpha), He I, Ca II,
[O I], and [S II] and excess emission in optical and near-IR bands among some
of these objects suggest the presence of accretion, outflows, and circumstellar
disks. The results from the 4 deg^2 survey have been combined with previous
studies of Taurus to arrive at an IMF for a total area of 12.4 deg^2. As in the
previous IMFs for Taurus, the updated IMF peaks at a higher mass (0.8 M_sun)
than the mass functions in IC 348 and Orion (0.1-0.2 M_sun). Meanwhile, the
deficit of BDs in Taurus appears to be less significant (x1.4-1.8) than found
in earlier studies (x2) because of a slightly higher BD fraction in the new IMF
for Taurus and a lower BD fraction in the new spectroscopic IMF for the
Trapezium from Slesnick and coworkers. The spatial distribution of the low-mass
stars and BDs discovered in the two new survey areas closely matches that of
the more massive members. Thus, on the degree size scales (~3 pc) probed to
date, there is no indication that BDs form through ejection.Comment: 35 pages, The Astrophysical Journal, 2004, v617 (December 20
Discrete complex analysis on planar quad-graphs
We develop a linear theory of discrete complex analysis on general
quad-graphs, continuing and extending previous work of Duffin, Mercat, Kenyon,
Chelkak and Smirnov on discrete complex analysis on rhombic quad-graphs. Our
approach based on the medial graph yields more instructive proofs of discrete
analogs of several classical theorems and even new results. We provide discrete
counterparts of fundamental concepts in complex analysis such as holomorphic
functions, derivatives, the Laplacian, and exterior calculus. Also, we discuss
discrete versions of important basic theorems such as Green's identities and
Cauchy's integral formulae. For the first time, we discretize Green's first
identity and Cauchy's integral formula for the derivative of a holomorphic
function. In this paper, we focus on planar quad-graphs, but we would like to
mention that many notions and theorems can be adapted to discrete Riemann
surfaces in a straightforward way.
In the case of planar parallelogram-graphs with bounded interior angles and
bounded ratio of side lengths, we construct a discrete Green's function and
discrete Cauchy's kernels with asymptotics comparable to the smooth case.
Further restricting to the integer lattice of a two-dimensional skew coordinate
system yields appropriate discrete Cauchy's integral formulae for higher order
derivatives.Comment: 49 pages, 8 figure
Dimers on two-dimensional lattices
We consider close-packed dimers, or perfect matchings, on two-dimensional
regular lattices. We review known results and derive new expressions for the
free energy, entropy, and the molecular freedom of dimers for a number of
lattices including the simple-quartic (4^4), honeycomb (6^3), triangular (3^6),
kagome (3.6.3.6), 3-12 (3.12^2) and its dual [3.12^2], and 4-8 (4.8^2) and its
dual Union Jack [4.8^2] Archimedean tilings. The occurrence and nature of phase
transitions are also analyzed and discussed.Comment: Typos corrections in Eqs. (28), (32) and (43
A note on dimer models and McKay quivers
We give one formulation of an algorithm of Hanany and Vegh which takes a
lattice polygon as an input and produces a set of isoradial dimer models. We
study the case of lattice triangles in detail and discuss the relation with
coamoebas following Feng, He, Kennaway and Vafa.Comment: 25 pages, 35 figures. v3:completely rewritte
The TAOS Project: Upper Bounds on the Population of Small KBOs and Tests of Models of Formation and Evolution of the Outer Solar System
We have analyzed the first 3.75 years of data from TAOS, the Taiwanese
American Occultation Survey. TAOS monitors bright stars to search for
occultations by Kuiper Belt Objects (KBOs). This dataset comprises 5e5
star-hours of multi-telescope photometric data taken at 4 or 5 Hz. No events
consistent with KBO occultations were found in this dataset. We compute the
number of events expected for the Kuiper Belt formation and evolution models of
Pan & Sari (2005), Kenyon & Bromley (2004), Benavidez & Campo Bagatin (2009),
and Fraser (2009). A comparison with the upper limits we derive from our data
constrains the parameter space of these models. This is the first detailed
comparison of models of the KBO size distribution with data from an occultation
survey. Our results suggest that the KBO population is comprised of objects
with low internal strength and that planetary migration played a role in the
shaping of the size distribution.Comment: 18 pages, 16 figures, Aj submitte
Losses in luminescent solar concentrators unveiled
A novel experimental method is presented to determine the optical efficiency and the loss channels of a luminescent solar concentrator (LSC). Despite strong promise, LSCs have not yet reached their full potential due to various mechanisms affecting the device's optical efficiency. Among those loss channels, escape cone and non-unity quantum yield losses are generally the most dominant. To further advance the field of LSCs, it is vital to understand the impact of each independently. So far, researchers have only characterized the total loss in LSCs. Here, an experimental method is proposed to separate the contribution from each individual loss channel. The experimental apparatus is the same as used for quantum yield measurements of fluorophores in solid samples. Therefore, the setup is commonly available to research groups already involved in LSC research. The accuracy of this method is demonstrated by comparing the experimental results with Monte-Carlo ray tracing. Our experimental method can have a strong impact on LSC research as it offers a means to unveil the loss channels of LSCs in addition to the optical efficiency
Planet formation around low mass stars: the moving snow line and super-Earths
We develop a semi-analytic model for planet formation during the pre-main
sequence contraction phase of a low mass star. During this evolution, the
stellar magnetosphere maintains a fixed ratio between the inner disk radius and
the stellar radius. As the star contracts at constant effective temperature,
the `snow line', which separates regions of rocky planet formation from regions
of icy planet formation, moves inward. This process enables rapid formation of
icy protoplanets that collide and merge into super-Earths before the star
reaches the main sequence. The masses and orbits of these super-Earths are
consistent with super-Earths detected in recent microlensing experiments.Comment: accepted by ApJ Letter
The interaction of a gap with a free boundary in a two dimensional dimer system
Let be a fixed vertical lattice line of the unit triangular lattice in
the plane, and let \Cal H be the half plane to the left of . We
consider lozenge tilings of \Cal H that have a triangular gap of side-length
two and in which is a free boundary - i.e., tiles are allowed to
protrude out half-way across . We prove that the correlation function of
this gap near the free boundary has asymptotics ,
, where is the distance from the gap to the free boundary. This
parallels the electrostatic phenomenon by which the field of an electric charge
near a conductor can be obtained by the method of images.Comment: 34 pages, AmS-Te
A Spectroscopic Survey of Subarcsecond Binaries in the Taurus-Auriga Dark Cloud with the Hubble Space Telescope
We report the results of a spectroscopic survey of 20 close T Tauri binaries
in the Taurus-Auriga dark cloud where the separations between primaries and
their secondaries are less than the typical size of a circumstellar disk around
a young star. Analysis of low-resolution and medium-resolution STIS spectra
yields the stellar luminosities, reddenings, ages, masses, mass accretion
rates, IR excesses, and emission line luminosities for each star in each pair.
We examine the ability of IR color excesses, H-alpha equivalent widths, [O I]
emission, and veiling to distinguish between weak emission and classical T
Tauri stars. Four pairs have one cTTs and one wTTs; the cTTs is the primary in
three of these systems. This frequency of mixed pairs among the close T Tauri
binaries is similar to the frequency of mixed pairs in wider young binaries.
Extinctions within pairs are usually similar; however, the secondary is more
heavily reddened than the primary in some systems, where it may be viewed
through the primary's disk. Mass accretion rates of primaries and secondaries
are strongly correlated, and H-alpha luminosities, IR excesses, and ages also
correlate within pairs. Primaries tend to have somewhat larger accretion rates
than their secondaries do, and are typically slightly older than their
secondaries according to three different sets of modern pre-main-sequence
evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in
reflection nebulae, are striking; the secondary in each pair is less massive
but more luminous than the primary. The stellar masses of the UY Aur and GG Tau
binaries measured from their rotating molecular disks are about 30% larger than
the masses inferred from the spectra and evolutionary tracks
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