15,314 research outputs found
Spitzer spectroscopy of circumstellar disks in the 5 Myr old upper Scorpius OB association
We present mid-infrared spectra between 5.2 and 38 μm for 26 disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early-type (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ~8 μm. In contrast, 10 and 20 μm silicate features or polycyclic aromatic hydrocarbon emission are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral-type dependent, with the most prominent features being associated with K5-M2-type stars. By fitting the spectral energy distributions (SED) of a representative sample of low-mass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU. Complementary high-resolution (R ~ 33,000) optical (λλ4800-9200) spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only seven (all late-type) exhibit definitive signatures of accretion. Mass-accretion rates (M) for these stars were estimated to range from 10^–11 to 10^–8.9 M⊙ yr^–1. Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near- and mid-infrared excess emission and an order of magnitude lower mass-accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time
Recognition of 3-D Objects from Multiple 2-D Views by a Self-Organizing Neural Architecture
The recognition of 3-D objects from sequences of their 2-D views is modeled by a neural architecture, called VIEWNET that uses View Information Encoded With NETworks. VIEWNET illustrates how several types of noise and varialbility in image data can be progressively removed while incornplcte image features are restored and invariant features are discovered using an appropriately designed cascade of processing stages. VIEWNET first processes 2-D views of 3-D objects using the CORT-X 2 filter, which discounts the illuminant, regularizes and completes figural boundaries, and removes noise from the images. Boundary regularization and cornpletion are achieved by the same mechanisms that suppress image noise. A log-polar transform is taken with respect to the centroid of the resulting figure and then re-centered to achieve 2-D scale and rotation invariance. The invariant images are coarse coded to further reduce noise, reduce foreshortening effects, and increase generalization. These compressed codes are input into a supervised learning system based on the fuzzy ARTMAP algorithm. Recognition categories of 2-D views are learned before evidence from sequences of 2-D view categories is accumulated to improve object recognition. Recognition is studied with noisy and clean images using slow and fast learning. VIEWNET is demonstrated on an MIT Lincoln Laboratory database of 2-D views of jet aircraft with and without additive noise. A recognition rate of 90% is achieved with one 2-D view category and of 98.5% correct with three 2-D view categories.National Science Foundation (IRI 90-24877); Office of Naval Research (N00014-91-J-1309, N00014-91-J-4100, N00014-92-J-0499); Air Force Office of Scientific Research (F9620-92-J-0499, 90-0083
Direct UV observations of the circumstellar envelope of alpha Orionis
Observations were made in the IUE LWP camera, low dispersion mode, with alpha Ori being offset various distances from the center of the Long Wavelength Large Aperture along its major axis. Signal was acquired at all offset positions and is comprised of unequal components of background/dark counts, telescope-scattered light, and scattered light emanating from the extended circumstellar shell. The star is known from optical and infrared observations to possess an extended, arc-minute sized, shell of cool material. Attempts to observe this shell with the IUE are described, although the deconvolution of the stellar signal from the telescope scattered light requires further calibration effort
Sequence of phase transitions induced in an array of Josephson junctions by their crossover to pi-state
We show that the transition of Josephson junctions between the conventional
and pi states caused by the decrease in temperature induces in a regular
two-dimensional array of such junctions not just a single phase transition
between two phases with different ordering but a sequence of two, three or four
phase transitions. The corresponding phase diagrams are constructed for the
cases of bipartite (square or honeycomb) and triangular lattices.Comment: 5 pages, v2: as published in EP
Cation occupancy determination in manganese zinc ferrites using Fourier transform infrared spectroscopy
The magnetic and electric properties of ferrites are influenced by the cation distribution within the crystalline spinel lattice. Methods such as extended x-ray-absorption fine structure(EXAFS) have been used to determine cation occupancies within the crystalline structure of materials such as manganesezincferrite (MZFO); however, it is not practical to be used for daily analysis. Fourier transform infrared (FTIR)spectroscopy is another technique which has the potential to determine cation occupancy while offering speed and convenience. In the literature it has been demonstrated that in ferrite systems FTIR data can be correlated to cation percentages when comparing tetrahedral (Td) and octahedral (Oh) sites. FTIRspectra were collected on a series of MZFO nanoparticles in the range from 200 to 600cm−1 and two absorbance peaks were observed. The first absorption region shifted with changing sample composition as calculated from transmission EXAFS experiments and elemental analysis. The data was normalized to the maximum of the peak of interest and the shifts were correlated to cation occupancy
Spin transition in GdN@C, detected by low-temperature on-chip SQUID technique
We present a magnetic study of the GdN@C molecule, consisting of a
Gd-trimer via a Nitrogen atom, encapsulated in a C cage. This molecular
system can be an efficient contrast agent for Magnetic Resonance Imaging (MRI)
applications. We used a low-temperature technique able to detect small magnetic
signals by placing the sample in the vicinity of an on-chip SQUID. The
technique implemented at NHMFL has the particularity to operate in high
magnetic fields of up to 7 T. The GdN@C shows a paramagnetic
behavior and we find a spin transition of the GdN structure at 1.2 K. We
perform quantum mechanical simulations, which indicate that one of the Gd ions
changes from a state () to a state (), likely due to a charge transfer between the C cage and the ion
The K-band spectrum of the Cataclysmic Variable RXJ 0502.8+1624 (Tau 4)
We present the K-band spectrum of the cataclysmic variable RXJ 0502.8+1624
(Tau 4). The spectrum shows a broad, smooth hump, with no absorption lines from
the secondary star visible. This result indicates that the infrared light of
this system is dominated by cyclotron emission, and, in combination with the
optical spectrum and X-ray properties, suggests that Tau 4 is a polar-type
cataclysmic variable (CV).
The system was chosen for study because the broadband JHK colours of Tau 4
are consistent with an L-type dwarf, suggesting that this system might harbour
an elusive sub-stellar secondary star. The result presented here, along with
the recent discovery of cyclotron emission in the cataclysmic variable EF Eri,
suggests that care must be taken when using the broadband JHK colours of CVs
when targeting searches for sub-stellar secondary starsComment: 4 pages, to appear as research note in A&
Brain neurons as quantum computers: {\it in vivo} support of background physics
The question: whether quantum coherent states can sustain decoherence,
heating and dissipation over time scales comparable to the dynamical timescales
of the brain neurons, is actively discussed in the last years. Positive answer
on this question is crucial, in particular, for consideration of brain neurons
as quantum computers. This discussion was mainly based on theoretical
arguments. In present paper nonlinear statistical properties of the Ventral
Tegmental Area (VTA) of genetically depressive limbic brain are studied {\it in
vivo} on the Flinders Sensitive Line of rats (FSL). VTA plays a key role in
generation of pleasure and in development of psychological drug addiction. We
found that the FSL VTA (dopaminergic) neuron signals exhibit multifractal
properties for interspike frequencies on the scales where healthy VTA
dopaminergic neurons exhibit bursting activity. For high moments the observed
multifractal (generalized dimensions) spectrum coincides with the generalized
dimensions spectrum calculated for a spectral measure of a {\it quantum} system
(so-called kicked Harper model, actively used as a model of quantum chaos).
This observation can be considered as a first experimental ({\it in vivo})
indication in the favour of the quantum (at least partially) nature of the
brain neurons activity
Annealing temperature and initial iron valence ratio effects on the structural characteristics of nanoscale nickel zinc ferrite
Nickelzincferrite (NZFO) nanoparticles were synthesized via a reverse micelle method with a nonionic surfactant. Three different initial Fe3+/Fe2+ ratios were employed along with three different firing temperatures (200, 500, 1000 °C) to investigate the effects on the NZFO system. Extended x-rayabsorption fine structure (EXAFS) results reveal zinc loss at high annealing temperatures; at 1000 °C, the loss is nearly total for Fe3+/Fe2+ ratios other than 10:90. Annealing at 500 °C, however, appears necessary for fully incorporating the zinc and nickel into the spinel phase. The best nanoferrite was thus obtained using an initial Fe3+/Fe2+ ratio of 10:90 and a moderate firing temperature of 500 °C. This sample exhibits a room temperature saturation magnetization of 58 emu/g as measured via vibrating sample magnetometry, comparable with bulk values and greater than that of confirmed nano-NZFOs found in the literature. EXAFS also indicates that in all cases in which the elements adopted a spinel structure, the nickel occupies only octahedral sites and the zinc primarily tetrahedral sites
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