16,871 research outputs found
Distributed ARTMAP
Distributed coding at the hidden layer of a multi-layer perceptron (MLP) endows the network with memory compression and noise tolerance capabilities. However, an MLP typically requires slow off-line learning to avoid catastrophic forgetting in an open input environment. An adaptive resonance theory (ART) model is designed to guarantee stable memories even with fast on-line learning. However, ART stability typically requires winner-take-all coding, which may cause category proliferation in a noisy input environment. Distributed ARTMAP (dARTMAP) seeks to combine the computational advantages of MLP and ART systems in a real-time neural network for supervised learning. This system incorporates elements of the unsupervised dART model as well as new features, including a content-addressable memory (CAM) rule. Simulations show that dARTMAP retains fuzzy ARTMAP accuracy while significantly improving memory compression. The model's computational learning rules correspond to paradoxical cortical data.Office of Naval Research (N00014-95-1-0409, N00014-95-1-0657
Revisiting Constraints on Fourth Generation Neutrino Masses
We revisit the current experimental bounds on fourth-generation Majorana
neutrino masses, including the effects of right handed neutrinos. Current
bounds from LEPII are significantly altered by a global analysis. We show that
the current bounds on fourth generation neutrinos decaying to eW and mu W can
be reduced to about 80 GeV (from the current bound of 90 GeV), while a neutrino
decaying to tau W can be as light as 62.1 GeV. The weakened bound opens up a
neutrino decay channel for intermediate mass Higgs, and interesting
multi-particle final states for Higgs and fourth generation lepton decays.Comment: 7 pages 1 fi
CARMA interferometric observations of 2MASS J044427+2512: the first spatially resolved observations of thermal emission of a brown dwarf disk
We present CARMA 1.3 mm continuum data of the disk surrounding the young
brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular
resolution of the CARMA observations (0.16 arcsec) allows us to spatially
resolve for the first time the thermal emission from dust around a brown dwarf.
We analyze the interferometric visibilities and constrain the disk outer radius
adopting disk models with power-law radial profiles of the dust surface
density. In the case of a power-law index equal to or lower than 1, we obtain a
disk radius in the range of about 15 - 30 AU, while larger disks are inferred
for steeper radial profiles. By combining this information on the disk spatial
extent with the sub-mm spectral index of this source we find conclusive
evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss
the implications of our results on the models of dust evolution in
proto-planetary disks and brown dwarf formation.Comment: 14 pages, 3 figures, Accepted for publication in ApJ Letter
dARTMAP: A Neural Network for Fast Distributed Supervised Learning
Distributed coding at the hidden layer of a multi-layer perceptron (MLP) endows the network with memory compression and noise tolerance capabilities. However, an MLP typically requires slow off-line learning to avoid catastrophic forgetting in an open input environment. An adaptive resonance theory (ART) model is designed to guarantee stable memories even with fast on-line learning. However, ART stability typically requires winner-take-all coding, which may cause category proliferation in a noisy input environment. Distributed ARTMAP (dARTMAP) seeks to combine the computational advantages of MLP and ART systems in a real-time neural network for supervised learning, An implementation algorithm here describes one class of dARTMAP networks. This system incorporates elements of the unsupervised dART model as well as new features, including a content-addressable memory (CAM) rule for improved contrast control at the coding field. A dARTMAP system reduces to fuzzy ARTMAP when coding is winner-take-all. Simulations show that dARTMAP retains fuzzy ARTMAP accuracy while significantly improving memory compression.National Science Foundation (IRI-94-01659); Office of Naval Research (N00014-95-1-0409, N00014-95-0657
F-region drift velocities from incoherent-scatter measurements at Millstone Hill
F-region drift velocities measured at Millstone Hill from 1968 to 1974 are presented in tabular form. A brief description of the measurement procedures is also given
Calculation of conductivities and currents in the ionosphere
Formulas and procedures to calculate ionospheric conductivities are summarized. Ionospheric currents are calculated using a semidiurnal E-region neutral wind model and electric fields from measurements at Millstone Hill. The results agree well with ground based magnetogram records for magnetic quiet days
Laser Interferometer Gravitational-Wave Observatory beam tube component and module leak testing
Laser Interferometer Gravitational-Wave Observatory (LIGO) is a joint project of the California Institute of Technology and the Massachusetts Institute of Technology funded by the National Science Foundation. The project is designed to detect gravitational waves from astrophysical sources such as supernova and black holes. The LIGO project constructed observatories at two sites in the U.S. Each site includes two beam tubes (each 4 km long) joined to form an "L" shape. The beam tube is a 1.25 m diam 304 L stainless steel, ultrahigh vacuum tube that will operate at 1×10^–9 Torr or better. The beam tube was manufactured using a custom spiral weld tube mill from material processed to reduce the outgassing rate in order to minimize pumping costs. The integrity of the beam tube was assured by helium mass spectrometer leak testing each component of the beam tube system prior to installation. Each 2 km long, isolatable beam tube module was then leak tested after completion
The Spectroscopically Determined Substellar Mass Function of the Orion Nebula Cluster
We present a spectroscopic study of candidate brown dwarf members of the
Orion Nebula Cluster (ONC). We obtained new J- and/or K-band spectra of ~100
objects within the ONC which are expected to be substellar based on their
K,(H-K) magnitudes and colors. Spectral classification in the near-infrared of
young low mass objects is described, including the effects of surface gravity,
veiling due to circumstellar material, and reddening. From our derived spectral
types and existing near-infrared photometry we construct an HR diagram for the
cluster. Masses are inferred for each object and used to derive the brown dwarf
fraction and assess the mass function for the inner 5.'1 x 5.'1 of the ONC,
down to ~0.02 solar masses. The derived logarithmic mass function rises to a
peak at ~0.2 solar masses, similar to previous IMF determinations derived from
purely photometric methods, but falls off more sharply at the hydrogen-burning
limit before leveling through the substellar regime. We compare the mass
function derived here for the inner ONC to those presented in recent literature
for the sparsely populated Taurus cloud members and the rich cluster IC 348. We
find good agreement between the shapes and peak values of the ONC and IC 348
mass distributions, but little similarity between the ONC and Taurus results.Comment: Accepted for Publication in Apj. Added Erratu
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